Chronology: Venus 1.0

Digital elevation model of one hemisphere of Venus based on Magellan radar mapper data. Blue and purple signify low elevations, shades of green signify intermediate elevations, and red, pink, and tan signify high elevations. The tallest mountain on Venus, Skadi Mons, is part of Maxwell Montes, the light colored "tadpole" feature near the top of the image. Image credit: NASA.

Chronology is the exoskeleton of history; without its supporting structure, events collapse in an unrecognizable heap. Because this blog presents historical spaceflight plans and their context in random order, without the benefit of an overarching chronology, I periodically write a post which places in chronological order posts in this blog that cover a specific subject area.

This time around, the subject area is Venus. Until the early 1960s, many scientists held out hope that Venus might support life. Even before Mariner II flew past it (14 December 1962), however, scientists had begun to suspect that close examination would undermine their visions of a clement Venus. The cloudy planet soon became an object lesson in the importance of greenhouse gases in planetary atmospheres.

Among the planets, no world has received more visitors than Venus. From the 1960s until the 1980s, Venus was the main planetary exploration target of the Soviet Union; no country placed more spacecraft on the Venusian surface.

Mariner 10 was the first spacecraft to fly by Venus and use it as a gravity-assist way station (5 February 1974); that is, it used the planet's gravity and orbital momentum to change its course and speed, enabling it to conduct three Mercury flybys in 1974-1975. The twin Soviet Vega spacecraft each used a Venus gravity-assist in 1985 to gain enough energy to reach Comet Halley in 1986; during their Venus flybys, they released combination lander/balloon payloads.

Venus helped to rescue the NASA robotic exploration program in the late 1980s. The U.S. space agency had intended to launch the Galileo Jupiter orbiter and probe into low-Earth orbit in May 1986 attached to a powerful Centaur G-prime upper stage in the payload pay of a Space Shuttle Orbiter. Astronauts would have released the stage and spacecraft, then the former would have ignited to boost the latter directly to Jupiter, with arrival in December 1988.

After the Challenger Space Shuttle failure (28 January 1986), however, Centaur G-prime, which burned liquid hydrogen fuel with liquid oxygen oxidizer, was judged to be too volatile to carry on board a piloted spacecraft. In its place, NASA opted for a solid-propellant upper stage and a complex Venus-Earth-Earth Gravity Assist (VEEGA) trajectory. Following launch on board the Shuttle Orbiter Atlantis (18 October 1989), a Venus flyby (10 February 1990) put Galileo on course for Earth gravity-assist flybys in December 1990 and December 1992 with arrival at Jupiter in December 1995.

Galileo had been expected to be the first U.S. planetary spacecraft launched since Pioneer Venus Multiprobe (PVM) left Earth in August 1978; its new reliance on the VEEGA trajectory meant, however, that NASA had to shuffle its planetary mission schedule. Because Galileo needed to use the October 1989 launch window for a direct flight to Venus, the Magellan Venus radar mapper lifted off on board Atlantis (4 May 1989), orbited the Sun one-and-a-half times, and entered Venus polar orbit (10 August 1990). Missions to Venus thus bracketed a nearly 11-year drought in U.S. planetary mission launches.

In recent years, we have seen proposals for piloted Venus orbiter and atmosphere missions. These mark a renewal of interest that began in the 1950s and continued through the 1960s. Had those early plans gone ahead, NASA might have launched astronauts on Venus flyby and orbiter missions in the 1970s and early 1980s. Recent proposals and 1960s proposals have in common reliance on robots to explore the harsh Venusian surface; no humans would land there.

Venus is mentioned with (perhaps surprising) frequency throughout this blog. What follows is a chronological list of links to posts with a significant Venus exploration component.

Centaurs, Soviets, and Seltzer Seas: Mariner 2's Venusian Adventure (1962)

EMPIRE Building: Ford Aeronutronic's 1962 Plan for Piloted Mars/Venus Flybys

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

Venus as Proving Ground: A 1967 Proposal for a Piloted Venus Orbiter

Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)

Apollo Ends at Venus: A 1967 Proposal for Single-Launch Piloted Venus Flybys in 1972, 1973, and 1975

Floaters, Armored Landers, Radar Orbiters, and Drop Sondes: Automated Probes for Piloted Venus Flybys (1967-1968)

Things to Do During a Venus-Mars-Venus Piloted Flyby Mission (1968)

Two for the Price of One: 1980s Piloted Missions with Stopovers at Mars and Venus (1969)

After Venus: Pioneer Mars Orbiter with Penetrators (1974)

High Noon on the Moon (1991)

Apollo 16 Commander John Young leaps in the Moon's low gravity and salutes Old Glory. The bright morning Sun shines from the left (east) in this image, causing the Lunar Module Orion in the background to cast a west-pointing shadow. All Apollo landings took place during lunar morning. Image credit: NASA.

"Why does the Moon change shape?" It's a question astronomy educators hear often. The answer is that our planet's natural satellite does not change shape; it is, of course, always spherical. What changes is how the Sun illuminates the side of the Moon we can see.

The Moon, like most other Solar System moons, is a synchronous rotator; that is, the period of time it needs to rotate on its axis once is essentially equal to the period of time it needs to orbit its primary once. For the Moon, the time required for both one rotation about its axis and one revolution about the Earth is about 28 days.

That is why humans on Earth see only the Moon's Nearside hemisphere. The Farside hemisphere, always turned away from Earth, remained mysterious until 1959, when humans glimpsed it for the first time courtesy of the Soviet Union's Luna III spacecraft.

This adaptation of a NASA diagram by Bill Dunford displays the parts of the Moon that are lit by the Sun as viewed from a vantage point above the terrestrial and lunar north poles. Nothing about it is to scale. The Sun is out of frame to the right. Numbers are called out in the text below. 

We call the shape of the lighted part of the Moon as viewed from Earth its "phase." Traditionally, however, the first phase of the lunar day/night cycle is new Moon (1 on the lunar phase diagram), when Nearside has no lighted part. When new, the Moon is situated between the Earth and the Sun. In addition to being unlit, the Nearside is lost in the Sun's glare.

Occasionally the Moon crosses over the Sun; new Moon is the time of partial, annular, and total solar eclipses. An eclipse does not occur every time the Moon is new because the Moon's orbit about the Earth is inclined slightly (about 5.1°) relative to Earth's orbit about the Sun.

The disc of the Moon is about 0.5° wide as viewed from Earth. The Sun, though 400 times farther away than the Moon (about 149,600,000 kilometers versus 363,100 kilometers) is also 400 times bigger than the Moon (about 1,392,000 kilometers versus 3475 kilometers), so it appears to be the same size as the Moon (0.5° wide) in Earth's sky. This means that, during total eclipses, the Sun's ghostly corona becomes visible as the Moon blocks the bright disk of the Sun.

The Moon is, however, moving away from Earth at a rate of about four meters per century. In just a few million years, it will no longer be wide enough in Earth's sky to cause total solar eclipses. All eclipses after that will be either partial or annular.

About two days past new Moon, people on Earth can look west in evening twilight and glimpse a slender crescent Moon (2 on the lunar phase diagram). It is called a waxing ("growing") crescent. The horns of the waxing crescent point toward the east, away from the setting Sun. If one looks carefully, one might observe that the part of the Nearside that is not yet lit by the Sun is visible.

This is probably a good place to mention that the Earth seems to change shape as viewed from the Nearside hemisphere. When the Moon is new, the Earth is full. In fact, Earth phases are always the opposite of Nearside phases.

As seen from the Nearside, the full Earth is about four times larger than the full Moon (2° wide in the black lunar sky) and reflects about 75 times as much light. When the Moon is a waxing crescent, Earth is mostly full. This means that sunlight reflected off Earth can light the part of the Nearside that direct sunlight does not yet reach.

As on Earth, the Sun rises in the east on the Moon. The line between light and darkness — the terminator — advances westward a little faster than a typical human can comfortably jog. High mountains and crater rims catch the morning Sun's bright rays first. Viewed through even a modest-size telescope, they appear as isolated islands of light. As the Sun climbs higher, light fills in the plains, lowlands, and crater floors.

Seven days past new, the Nearside is half lit for observers on Earth (3 on the lunar phase diagram). This phase is called first quarter. The first-quarter Moon rises in the east as the Sun stands at noon, reaches its highest point at sunset, and sets in the west at midnight.

About 10 days past new, the Nearside is halfway between first quarter and full (4 on the lunar phase diagram). We call this phase waxing gibbous ("gibbous" means convex — the term refers to the shape of the advancing dawn terminator).

Fourteen days past new, the Nearside is fully lit by the Sun as viewed from Earth (5 on the lunar phase diagram). The full Moon is visible all night; it rises in the east as the Sun sets in the west, stands highest at midnight, and sets in the west as the Sun rises in the east. When the nearside is full, the Earth is new as viewed from the Moon.

When the Moon is full, the Earth stands between it and the Sun. For this reason, full Moon is when partial and total lunar eclipses — during which the shadow of the Earth falls on the Moon — can occur. As with solar eclipses, lunar eclipses do not occur at every full Moon because the Moon's orbit is tilted relative to Earth's orbit about the Sun.

Newcomers to the pleasures of amateur astronomy often turn their first telescope toward the Moon for the first time at full Moon. If one can stand the bright glare from the fully lit Nearside, one can examine many contrasting light and dark areas through a small telescope; many such albedo features (as they are known) are, in fact, best seen when the Nearside is fully lit. Of particular interest are the Nearside-spanning whitish-gray rays of the large impact crater Tycho.

All things considered, however, the fully lit Nearside appears bland; crater rims and mountains cast no shadows, so all sense of surface relief is absent. The Moon might as well be a painted billiard ball. Viewing the Moon when it is less than full — and focusing on the terminator — is, in my opinion, much more rewarding.

About 18 days past new, the Moon has reached waning (shrinking) gibbous phase (6 on the lunar phase diagram). It rises between dusk and midnight and is visible in clear skies in the west until mid-morning the next day. The terminator line, formerly the line of lunar dawn, becomes the line of lunar dusk. Darkness advances from east to west as light advanced two weeks before.

Twenty-one days past new, night reclaims the Nearside's eastern half. This phase is called last quarter (7 on the lunar phase diagram). For people on Earth, the Moon rises at midnight, stands highest at dawn, and sets at noon.

About 25 days past new, the crescent Moon — called the waning ("shrinking") crescent — rises in the east just before the Sun (8 on the lunar phase diagram). Its horns point westward, away from the Sun. The dark part of the Nearside is again lit by sunlight reflected off a nearly full Earth. A relatively small telescope reveals the advance of the sunset terminator; crater bottoms, lowlands, and plains grow dark, then mountains and crater rims slowly shrink and finally vanish in darkness.

If you look through a telescope at the crescent Moon before dawn, take care not to look at the Sun when it peeks above the horizon; eye damage will result. Instead, attempt to keep sight of the crescent Moon as fades into the blue sky of earthly day.

The end of Day 28 sees a new lunar day-night cycle begin (1 on the lunar phase diagram). The Moon stands between the Sun and Earth, lost in the Sun's glare, and it is again midnight at the center of the Nearside hemisphere.

The small basaltic plain Sinus Medii — Latin for "Central Bay" — marks the Nearside's center. Equatorial Sinus Medii was an early Apollo Program landing site candidate, but no Lunar Module (LM) spacecraft landed there. When it is midnight in Sinus Medii, it is high noon at the center of the rugged Farside hemisphere. The Farside's center is located on the lunar equator north of the impact crater Daedalus.

Changes in orbital geometry and lighting angles in the Earth-Moon system are today mainly of interest to stargazers amateur and professional, but a half-century ago it was different. Apollo missions were leaving Cape Kennedy, Florida, every few months bound for the Moon, and lighting conditions were a critical part of landing site selection and mission timing.

Conservative Apollo mission rules dictated that the LM should land only between 12 and 48 hours after sunrise at its target landing site, when the Sun would stand between 5° and 20° above the eastern horizon. At the appointed time, the Apollo mission Commander (CDR) and Lunar Module Pilot (LMP) would ignite the descent engine of their spindly-legged spacecraft over the Farside to slow it so that its orbit would intersect the lunar surface at its Nearside landing site.

As it approached its pre-planned landing site from the east, the LM would pitch up to point its descent engine and four round foot pads at the lunar surface. As the landing site became visible outside the twin triangular LM windows, the Sun would shine from behind the spacecraft. This would prevent it from shining into the astronauts' eyes. The shadow of the LM would then become visible on the surface, enabling the astronauts to gauge the size of lunar surface features to help them pick out a spot for a safe landing.

Because of limited supplies of avionics cooling water, battery power, and breathing oxygen, the longest an Apollo lunar surface mission could last was about 72 hours. The period during which Apollo explorers could gain experience working in lunar lighting conditions thus only spanned from 12 hours (the earliest permitted landing time) to five days (the latest permitted landing time of two days plus the maximum stay-time of three days) after dawn at the landing site.

In 1991, Dean Eppler, a geologist in the NASA Johnson Space Center (JSC) Lunar & Mars Exploration Program Office (LMEPO) with an interest in lunar geologic fieldwork, conducted a study of the effects on lunar surface operations of the whole range of lunar lighting conditions in support of Space Exploration Initiative (SEI) planning. SEI, launched amid great fanfare by President George H. W. Bush on 20 July 1989, aimed to complete Space Station Freedom, return American astronauts to the Moon to stay, and then launch humans to Mars. "To stay" implied that astronauts would need to land, drive, walk, and work on the Moon throughout its day-night cycle at multiple locations all over the Moon.

Eppler had help from a spaceflight legend. John Young (1930-2018) joined NASA in 1962 as a member of the second Astronaut Class ("the New Nine") and was a veteran of six space missions (Gemini III, Gemini X, Apollo 10, Apollo 16, STS-1, and STS-9), four of which he commanded. He was Chief of the Astronaut Office at JSC from 1974 until 5 May 1987, when he was made JSC Director Aaron Cohen's Special Assistant for Engineering, Operations, and Safety.

Apollo 16 Commander John Young (left) with Command Module Pilot Kenneth Mattingly (center) and Lunar Module Pilot Charles Duke (right). Image credit: NASA.

Though his new job was widely seen as punishment for candid views he expressed in the aftermath of the 28 January 1986 Challenger accident, Young tackled it with gusto. He delved into a wide range of technical and safety issues and distributed throughout NASA hundreds of memoranda offering advice. Young also made himself available to people such as Eppler (and, incidentally, to this author); that is, to individuals eager to learn from and commit to record Young's unique body of experience and knowledge.

Young first had an opportunity to observe the Moon's surface from lunar orbit when he served as Apollo 10 Command Module Pilot (CMP) in May 1969. He told Eppler that, viewed from a spacecraft in lunar orbit, the transition from the sunlit part of the Moon to the earth-lit part was sudden and that the eye adjusted almost immediately to the reduced light level. Features on the lunar surface remained almost as visible as they had been under direct sunlight, and it was even possible to pick out features within shadows in earth-lit areas.

Young reported that the change from the earth-lit part of the Moon to unlit portions of the Farside, out of reach of light from both Sun and Earth, was "dramatic." Nothing could be seen of the Moon's surface even at an orbital altitude of only a few tens of kilometers. The horizon was discernible only because stars were visible above it but not below it.

As Apollo 16 CDR in April 1972, Young piloted the LM Orion to a landing at Descartes, the only Apollo site entirely within the lunar highlands. The highlands, which cover about 80% of the Moon's surface, are lighter in hue than basaltic plains like Sinus Medii.

Young told Eppler that, in his opinion, landing a spacecraft equivalent to the Apollo LM would be possible at a site lit only by light reflected off the Earth. Landing in earthlight at a prepared site — that is, one with flashing strobes and electronic landing aids — would be easier than landing a helicopter at night on Earth, he added.

Young experienced the challenges of getting about on the lunar surface under low-angle sunlight soon after climbing down Orion's ladder at Descartes. Moving toward the Sun (eastward) was difficult because of its fierce glare, and moving away from the Sun (westward) was treacherous because shadows disappeared behind the rocks and crater rims that cast them. This created a washed-out landscape where obstacles were hard to see and avoid.

Moving north or south meant reduced glare and visible shadows. This is one reason why the first two Apollo flights that included a Lunar Roving Vehicle (LRV), Apollo 15 and Apollo 16, had pre-planned lunar traverses that were oriented generally toward north and south.

Image credit: NASA.

Image credit: NASA.

Image credit: NASA.

The photographs above, taken from the same location within the space of a few minutes by Apollo 16 LMP Charles Duke, give some sense of the difficulties posed by these lunar-surface lighting phenomena. The reader should bear in mind that early 1970s photographic film was less capable of capturing surface topography in challenging circumstances than were astronaut eyes.

The top image shows the view toward the glaring low-angle Sun. The middle image shows John Young at work near the Apollo 16 LRV. He is facing north. As the orientation of the shadows indicates, the Sun is located to the right of the field of view, so surface feature visibility is near optimum. Rocks, footprints, and LRV tracks are obvious.

The bottom image, taken facing west directly away from the low Sun, looks very different, but in reality displays a rocky landscape similar to that shown in the top and middle images. Apart from rocks close to Duke (and Duke's own helmet), however, surface features obscure their own shadows and thus are almost invisible.

Based on Young's observations and his own calculations, Eppler proposed schedules for operations at various lunar surface locations. He determined that in Sinus Medii the period from local dawn until 5.5 days after local sunrise would be optimal for walking, driving, and landing.

From 5.5 days to nine days after sunrise at Sinus Medii the Sun would hang within 20° of local vertical, with noon taking place on day seven. The near-vertical lighting angle would mean that terrain features would cast no shadows, making walking and driving difficult. A descending lander would cast a shadow, but only directly beneath the lander, where it would most likely not be visible to the pilot. Eppler advised that only "restricted surface operations" should occur during the near-noon period. Landings should take place only at prepared sites.

The period from nine to 28 days after sunrise at Sinus Medii would be optimal for surface activity, Eppler found, though lighting conditions would vary greatly over that span of time. Between nine and 14 days after sunrise, the Sun would lower toward the west and would again cast visible shadows (except toward the east, away from the Sun). A lunar lander approaching an outpost landing field from the east would have to contend with both direct solar glare and absence of a handy lander shadow. Sunset would occur on day 14, with a half-lit Earth shining high in the sky.

On day 21 — midnight at Sinus Medii — full Earth would light the landscape. Seven days later, with a half-Earth high in the sky, the Sun would rise again in the east. Surface activity could thus take place at Sinus Medii without break or restrictions for 24.5 days of the 28-day lunar day/night cycle; that is, from day nine after sunrise to day 5.5 after sunrise.

At the center of the Farside, the lighting situation would be very different. Starting 14 days after local dawn, the Sun would set and — with no Earth in the sky — the landscape would become lost in darkness. Only by using artificial lighting could astronauts find their way. Landings would be prohibited throughout Farside night except at prepared sites.

Eppler also examined lighting on the east and west lunar limbs (that is, on the edges of the Nearside hemisphere near the equator) and at the Moon's poles. The western limb would see the Sun set in the west 14 days after local sunrise with a full Earth on the eastern horizon. The lighted fraction of the Earth would decrease as night progressed.

Between day 23 and day 28 after sunrise at the western limb site, Earth would provide too little light for surface operations without artificial lights. It would become completely invisible at western limb sunrise — which would, of course, occur in the east.

The eastern limb would experience sunset while Earth was new, so would become very dark immediately. Eppler expected that a fat crescent Earth, located just above the western horizon, would provide adequate lighting for surface operations starting on day 19 after local sunrise. On day 21, Earth would be half lit, and it would be full on day 28, when the Sun would once again rise in the east.

The lunar poles would see Earth phases like those at Sinus Medii. Earth would hover, bobbing and tilting slightly, near the Nearside southern horizon for north pole sites and near the Nearside northern horizon for south pole sites.

The Sun would circle the horizon at a polar site, never setting. Astronauts would need to note its position on the horizon and take care not to turn directly toward it without adequate eye protection. In addition, local mountains and crater rims would occasionally block the Sun or Earth and some areas — mainly deep crater bottoms — would forever lie in cold shadow.

Sources

Lighting Constraints on Lunar Surface Operations, NASA Technical Memorandum 4271, Dean B. Eppler, NASA Johnson Space Center, May 1991.

Forever Young: A Life of Adventure in Air and Space, John W. Young with James R. Hansen, University Press of Florida, 2012.

More Information

What If an Apollo Lunar Module Ran Low on Fuel and Aborted its Landing? (1966)

Keep My Memory Green: Skill Retention During Long-Duration Spaceflight (1968)

Log of a Moon Expedition (1969)

Chronology: Apollo-to-Shuttle Transition 2.0

Three years ago I published on this blog the first of my "Chronology" compilations of links to posts with a common theme. That first chronological compilation brought together links to posts on the transition from Apollo to the Space Shuttle. The aim was to impose chronology on posts that do not occur in chronological order in this blog as an aid to reader understanding.

This, my fourth "Chronology" compilation, updates that first compilation. I've added links to three posts dating from 1968, 1970, and 1972; that is, near the start, at the middle, and near the end of the planning phase of the Apollo-Shuttle transition.

"A True Gateway": Robert Gilruth's June 1968 Space Station Presentation

Series Development: A 1969 Plan to Merge Shuttle and Saturn V to Spread Out Space Program Cost (December 1969)

Think Big: A 1970 Flight Schedule for NASA's 1969 Integrated Program Plan (June 1970)

McDonnell Douglas Phase B Space Station (June 1970)

From Monolithic to Modular: NASA Establishes a Baseline Configuration for a Shuttle-Launched Space Station (July 1970)

An Alternate Station/Shuttle Evolution: The Spirit of '76 (August 1970)

Apollo's End: NASA Cancels Apollo 15 & Apollo 19 to Save Station/Shuttle (August-September 1970)

The Last Days of the Nuclear Shuttle (February 1971)

A Bridge From Skylab to Station/Shuttle: Interim Space Station Program (April 1971)

Where to Launch and Land the Space Shuttle? (April 1972)

"Still Under Active Consideration": Five Proposed Earth-Orbital Apollo Missions for the 1970s (August 1972)

The First Voyager (1967)

Artist concept of a Voyager spacecraft. Although the spacecraft design is correctly depicted, some liberties are taken with the planets (Mars and Earth are never this close together). The robotic Voyager includes a main engine derived from the Apollo Lunar Module Descent Propulsion System; a body-mounted, ring-shaped solar array; a skeletal high-gain antenna (it points toward Earth); a cylindrical body with rectangular thermal-control louver ports; and a Mars landing capsule with a conical heat shield sealed inside a "sterilization canister." The spacecraft is larger than it might appear: at this point in its mission, it would weigh 10.25 tons and measure 6.1 meters in diameter. Image credit: NASA/Jet Propulsion Laboratory.

In 1961, the Pasadena, California-based Jet Propulsion Laboratory (JPL), a spaceflight engineering laboratory managed by California Institute of Technology on contract to NASA, began study of Voyager, a robotic spacecraft program for exploring Mars and Venus in the late 1960s and 1970s. NASA Headquarters formally approved Project Voyager in 1964. Cuts in NASA's space science budget and debate over how Voyager should be managed and launched delayed NASA's push for a formal "new start" until January 1967, when President Lyndon Johnson's Fiscal Year (FY) 1968 NASA budget called for $71.5 million for the new program.

In January 1967, NASA's Office of Space Science and Applications published a 26-page brochure as part of its efforts to move Voyager from planning to development. The brochure was an introduction (and sales pitch) aimed at members of Congress and other individuals who would need to support Voyager if it was to become part of NASA's approved planetary exploration program for the 1970s.

In the foreword to the brochure, Homer Newell, NASA Associate Administrator for Space Science and Applications, explained that Voyager's chosen launch vehicle was the "awe-inspiring" Saturn V. One three-stage Saturn V rocket would launch two 12-ton Voyager spacecraft to Mars. For comparison, the Mariner IV Mars flyby spacecraft, launched on an Atlas-Agena D rocket in November 1964, had weighed only 260.4 kilograms. Newell wrote that

[s]uccesses already achieved in the 1960s with unmanned spacecraft of limited weight and power. . .foretell the great work of exploration that lies ahead. . .With Voyager, the U.S. capability for planetary exploration will grow by several orders of magnitude. . .Voyager could well be the means by which man first learns of extraterrestrial life.

NASA, the brochure explained, favored Mars over Venus as Voyager's first exploration target because "the high surface temperatures on Venus make the existence of extraterrestrial life less likely than on Mars" and because "the thin, normally transparent Martian atmosphere is conducive to detailed scanning of its surface features from orbit." In addition, "manned landings on Mars will someday be possible. . .[but] they may not be possible on Venus."

The brochure placed Voyager within an evolutionary robotic exploration program designed to take advantage of low-energy Earth-Mars transfer opportunities that occur every 26 months. It retroactively made Mariner IV, which had flown by Mars on 14-15 July 1965, the first mission in its program. Inclusion of Mariner IV, the first successful Mars explorer, is somewhat ironic, for its discoveries had helped to undermine support for Voyager.

In addition to recording for slow playback 21 black-and-white images that took in about 1% of the martian surface, Mariner IV had enabled Earth-bound scientists to measure martian atmospheric pressure by transmitting its feeble radio signal through the atmosphere as it passed behind the planet as viewed from Earth. Based on the degree of refraction of the signal, scientists had determined that surface pressure on Mars is not, as expected, about 10% of Earth sea-level pressure; it is, in fact, less than 1% of Earth sea-level pressure.

The Saturn IB with Centaur third stage was the first planned Voyager launch vehicle. Never fully developed, this booster would have launched a single Voyager Orbiter/Lander combination with a mass of up to 5440 kilograms toward Mars or Venus. The third stage includes the cylindrical lower part of the Payload Shroud. The Interstage, linking the S-IB first stage and the S-IVB second stage and covering the S-IVB's single J-2 engine, is generally considered to be part of the S-IVB stage. The IU is the Instrument Unit, which houses the guidance system for the rocket's first two stages. Image credit: NASA/heroicrelics.org/DSFPortree.

The brochure acknowledged that the new atmosphere data had forced a redesign of the Voyager landing system. The new design replaced lightweight parachutes with heavier landing rockets. According to historians Edward Clinton Ezell and Linda Neumann Ezell, writing in their 1984 NASA-published history On Mars: Exploration of the Red Planet, 1958-1978, the redesign bumped Voyager's projected cost above the psychologically significant $1 billion mark. Adoption of the Saturn V launch vehicle in place of the Saturn IB with a Centaur upper stage — a move designed to justify continuation of the Saturn V assembly line after Apollo and to provide flexibility in the event that Voyager redesigns significantly boosted its mass and heat shield diameter — pushed the program's price-tag past $2 billion.

The brochure called for new Mariner Mars flybys in 1969 and 1971. In 1969, a Mariner spacecraft would photograph the entire visible disk of Mars during approach and return detailed images of 10% of the planet. During the 1971 flyby, a Mariner would release a small sterilized probe into the martian atmosphere to measure pressure, density, temperature, and composition as it plummeted toward surface impact and destruction. The flyby spacecraft would act as a relay for probe signals and would image 10% of Mars at high resolution.

The first Voyager missions would take place in 1973. A battery-powered Voyager Lander with a mass of up to 390 kilograms would seek life and observe changes at the landing site over several days, and a solar-powered Voyager Orbiter would observe seasonal changes on a planet-wide scale for months.

The Voyager 1975 orbiters and landers would rely on Radioisotope Thermoelectric Generators (RTGs) for electricity. This would allow the landers to survive on Mars for one martian year (about two Earth years); that is, long enough for them to observe seasonal changes at their landing sites. Voyager could land up to 499 kilograms on Mars in the 1975 opportunity. The 1977 and 1979 Voyager missions would see introduction of a lander-deployed Mars surface rover and biological experiments specially designed to study any living things found in 1973 and 1975. A Voyager lander could deliver up to 680.4 kilograms to the surface of Mars in 1977 and 1979.

The brochure then detailed the 1973 Voyager Mars mission, which it described as typical. Voyagers would lift off from the Kennedy Space Center Complex 39 launch pads NASA built for the Apollo Saturn V launches. The 1970s Mars launch windows would last at least 25 days and would include daily one-hour launch opportunities. Voyager Saturn V rockets would be identical to Apollo lunar Saturn Vs; that is, each would comprise an S-IC first stage with five F-1 engines, an S-II second stage with five J-2 engines, and an S-IVB third stage with one J-2.

Voyager Saturn V rockets would have included the three stages and ring-shaped Instrument Unit of the Apollo Saturn V rocket. A cylindrical segmented shroud with a conical top containing twin Voyager spacecraft would, however, have replaced the Apollo spacecraft, which included the  Command and Service Module (CSM) and Lunar Module (LM) spacecraft, the Spacecraft Launch Adapter shroud containing the LM, and the pencil-shaped Launch Escape System tower. Image credit: NASA. 

The twin Voyager lander/orbiter combinations would be stacked atop the S-IVB third stage within a protective launch shroud. The first stage would burn for 2.5 minutes and fall away at an altitude of 62.8 kilometers, then the second stage would burn for 6.5 minutes and fall away at an altitude of 182.5 kilometers. The third stage would fire briefly to place itself, the twin Voyagers, and their launch shroud into Earth parking orbit.

Voyager's launch shroud would measure 6.7 meters in diameter — the same diameter as the S-IVB stage — and would have a mass of 4.7 tons. Once in Earth orbit, the shroud's conical top section would jettison, exposing the upper Voyager to space. The S-IVB stage would then ignite a second time to push the Voyagers out of Earth orbit toward Mars. After S-IVB shutdown, the upper Voyager would separate. The shroud's cylindrical central portion would then jettison to expose the lower Voyager, which would separate from the S-IVB a short time later. In the 1973 opportunity, each Voyager would have a mass of 10.25 tons after separation.

A model made in 1967 displays how two Voyager spacecraft would be stacked within the shroud atop the Saturn V S-IVB third stage. The conical, streamlined nose cone at the top of this shroud/spacecraft stack is absent. Image credit: NASA.

During the months-long interplanetary cruise, the twin Voyagers would turn their ring-shaped body-mounted solar arrays toward the Sun. They would use course-correction engines based on the Minuteman missile second-stage engine to place themselves on precise paths to Mars. The S-IVB stage trailing them would make no course adjustments, so would miss the planet by a wide margin. Because the Voyagers would perform their course corrections at different times, they would arrive at Mars up to 10 days apart.

As each Voyager neared Mars, it would fire its main rocket engine to slow down so that the planet's gravity could capture it into an elliptical orbit. Initial orbit periapsis (low point) would be about 1127 kilometers above the planet, while apoapsis (high point) would occur beyond the orbit of Deimos, the outer martian moon, which orbits at a mean altitude of 22,660 kilometers. The brochure noted that the leading Voyager main engine candidate was a modified Apollo Lunar Module descent engine. The complete Voyager Orbiter propulsion system fully loaded with propellants would weigh 6.5 tons.

After orbit insertion, the Orbiter's instruments would be turned toward Mars to image candidate sites for the first Voyager landing. After scientists and engineers on Earth settled on a site, the 2.5-ton Voyager landing capsule would eject its sterilization canister, separate from the Orbiter beyond Deimos, and fire a 188.2-kilogram solid-propellant deorbit rocket to change its path so that at periapsis it would intersect the martian atmosphere. The deorbit rocket would then detach.

The Voyager landing capsule would enter the martian atmosphere moving at between two and three miles per second. Aerodynamic braking using the 6.1-meter-diameter conical heat shield would cut speed to between 122 and 305 meters per second by the time the capsule fell to within 4570 meters of the surface. The heat shield would eject, then the Lander would fire its descent engines and deploy a supplemental parachute.

Voyager hardware heritage. Note the Voyager capsule and Lander configurations. Image credit: NASA.

During descent, the Lander would image the surface and collect atmospheric data. It would release the parachute, then slow to a hover three meters above Mars. Its descent engines would then shut off, allowing it to drop to a gentle touchdown on three legs.

The 1973 Lander would include 136.1 kilograms of science equipment. Over several days, it would search for water and life, measure cosmic and solar radiation, and study the atmosphere — it would, for example, measure the quantity of dust in the martian air.

The 1973 Orbiter, for its part, would include 181.4 kilograms of scientific instrumentation, which it would use to map Mars in detail and search for surface changes over time, determine surface composition, and measure solar and cosmic radiation. The Orbiter would also act as a martian weather satellite. It would, the brochure explained, use its main engine to change the altitude and inclination of its orbit several times during its two-year operational lifetime, allowing detailed study of much of Mars.

Congress refused to fund Voyager in FY 1968, in part because it had come to be seen as a lead-in to a costly post-Apollo piloted Mars/Venus flyby program, and also because the Apollo 1 fire (27 January 1967) undermined confidence in NASA. The U.S. civilian space agency formally abandoned its Voyager plans in September 1967.

In 1968, however, Congress agreed to fund the Viking program in FY 1969. Like Voyager, Viking would emphasize the search for life and would use twin spacecraft, each including a lander and an orbiter. Unlike its ill-starred progenitor, however, Viking made no claim to be a precursor for a piloted Mars mission. In addition, Viking would be managed by NASA's Langley Research Center, not JPL, though the later would build the Viking orbiters. Many interpreted assignment of Viking management to Langley as a congressional rebuke to JPL for its independent mindset; efforts to preserve NASA centers as Apollo spending began to wind down probably also played a role.

Twin flyby Mariners 6 and 7 flew by Mars in 1969, and Mariner 9 orbited the planet in 1971-1972. After skipping the 1973 Mars launch opportunity, NASA launched Viking 1 on a Titan-IIIE rocket with a Centaur upper stage on 20 August 1975. Viking 1's Mariner-based, solar-powered orbiter and RTG-powered lander together weighed about 2.56 tons at launch. After deploying the lander in Mars orbit, the Viking 1 orbiter weighed about 898.1 kilograms.

The Viking 1 lander became the first spacecraft to land successfully on Mars on 20 July 1976, seven years to the day after Apollo 11 became the first manned lunar lander. The lander had a mass of about 598.7 kilograms after touchdown; of this, about 42.2 kilograms comprised scientific instrumentation. Viking 2 launched from Earth on 9 September 1975, and its lander touched down on 3 September 1976. At about three meters wide, the Viking landers were about half the size of the planned Voyager landers.

NASA and JPL recycled the Voyager name in 1977, applying it to twin Mariner-derived Jupiter-Saturn flyby spacecraft (the mission was originally called Mariner Jupiter-Saturn 77). Voyager 2 left Earth first, on 20 August 1977, atop a Titan III-E/Centaur. Voyager 1 launched 16 days later, on 5 September. Voyager 1 passed Voyager 2 on 19 December 1977, as the twin spacecraft traversed the Asteroid Belt between Mars and Jupiter.

Beyond Mars: artist concept of the flight path of a Mariner Jupiter-Saturn 77 spacecraft. Image credit: NASA.

At each planet, the Voyagers performed a gravity-assist maneuver; that is, they used the target planet's gravity and orbital momentum to speed themselves onward to their next destination. Voyager 1 flew by Jupiter at a distance of 349,000 kilometers on 5 March 1979; Voyager 2 followed on 9 July 1979, passing the giant planet at a distance of 570,000 kilometers. Voyager 1 then flew by Saturn, its last planned target, at a distance of 124,000 kilometers on 12 November 1980. Voyager 2 passed Saturn at a distance of 101,000 kilometers on 25 August 1981.

By that time, the decision had been made to add Uranus and Neptune to Voyager 2's list of targets. The intrepid spacecraft flew by the former at a distance of 81,500 kilometers on 24 January 1986, and passed the latter at a distance of just 4951 kilometers on 25 August 1989.

The Voyagers continue to transmit data on space conditions beyond the planets. At this writing, Voyager 1 is 144.7 Astronomical Units (AU) from the Sun (one AU is defined as the mean distance from Earth to the Sun, or about 149.6 million kilometers - for comparison, the most distant planet in the Solar System, Neptune, is on average 30.1 AU from the Sun). Radio signals traveling at the speed of light (299,792 kilometers per second) need more than 20 hours to reach it. Voyager 2, which dove below the plane of the Solar System after departing Neptune, is 119.8 AU from the Sun; radio signals need about 16 hours, 42 minutes to reach it.

Voyager 1 became the first spacecraft to pass beyond the heliosphere, the bubble of space where solar particles and fields are dominant, in August 2012. Voyager 2 joined it at the edge of interstellar space in November 2018.

Sources

Summary of the Voyager Program, NASA Office of Space Science and Applications, January 1967.

On Mars: Exploration of the Red Planet 1958-1978, NASA SP-4212, Edward Clinton Ezell and Linda Neumann Ezell, NASA, 1984, pp. 85-86, 101-103, 117-118.

Voyager: Mission Status (https://voyager.jpl.nasa.gov/mission/status/#where_are_they_now - accessed 4 February 2019).

Saturn IB/Centaur (http://heroicrelics.org/info/saturn-i-and-ib/saturn-ib-centaur.html - accessed 6 February 2019).

More Information

Centaurs, Soviets, and Seltzer Seas: Mariner 2's Venusian Adventure (1962)

Things To Do During a Venus/Mars/Venus Piloted Flyby Mission (1968)

Lunar Viking (1970)

The Challenge of the Planets, Part Three: Gravity

Purple Pigeon: Mars Multi-Rover Mission (1977)

Image credit: JPL/NASA.

Planetary scientist Bruce Murray became director of the Jet Propulsion Laboratory (JPL) in April 1976, just three months before Viking 1 was due to land on the northern plains of Mars. Though NASA's Langley Research Center managed Project Viking, JPL included Viking Mission Control. When Viking 1 landed, JPL could expect to play host to hundreds of journalists from all over the Earth.

According to his 1989 memoir Journey into Space: The First Thirty Years of Space Exploration, Murray saw this as an opportunity. He quickly assembled a group of six engineers to propose planetary missions that he could pitch to the journalists and, through them, to U.S. taxpayers.

The missions, which Murray dubbed "Purple Pigeons," were intended to include both "high science content" and "excitement and drama [that would] garner public support." They were called Purple Pigeons to differentiate them from "Gray Mice," unexciting and timid missions which Murray felt would help to ensure that JPL had no future in the space exploration business. By August 1976, the Purple Pigeons flock included a solar sail mission to Halley's Comet, a Mars Surface Sample Return (MSSR), a Venus radar mapper, a Saturn/Titan orbiter/lander, a Ganymede lander, an asteroid tour, and an automated lunar base.

Bruce Murray, JPL director from April 1976 until June 1982. Image creditI JPL/Caltech.

The Purple Pigeons effort continued even after Viking 2 landed (3 September 1976) and all the journalists went home. In a February 1977 JPL report, for example, JPL engineers described a Purple Pigeon mission that would explore Mars with up to four rovers simultaneously. The Viking-based multi-rover mission would include a pair of identical 4800-kilogram spacecraft, each consisting of a Viking-type orbiter and a 1578-kilogram Mars lander bearing twin 222.4-kilogram rovers. The rovers would, the report stated, perform traverses to "regions difficult to reach by direct landings." This would, it added, fill the gap between "detailed information" from MSSR missions and "global information" from Mars orbiters.

The image at the top of this post shows a somewhat different (probably later) multi-rover mission design. Its four six-wheel, multi-cab rovers (two of which are operating out of view over the horizon) rely on a single Viking orbiter-type spacecraft to relay radio signals to and from Earth. In principle, however, it is identical to the early multi-rover mission design described in this post.

Most MSSR plans of the 1970s assumed a "grab" sample; that is, that the stationary MSSR lander would return to Earth a sample of whatever rocks and soil happened to be within reach of its robotic sample scoop. The report suggested that the rovers of the multi-rover mission might enhance a follow-on MSSR mission by collecting and storing samples as they roved across the planet. After the MSSR lander arrived on Mars, the rovers would rendezvous with it and hand over their samples for return to Earth. The report contended that its multi-rover/MSSR strategy would be "an enormous advance over even multiple grab samples" collected by MSSR landers at widely scattered sites.

At the time the Purple Pigeons team proposed the multi-rover mission, NASA intended to launch all payloads, including interplanetary spacecraft, on board reusable Space Shuttles. The Shuttle orbiter would be able to climb no higher than about 500 kilometers, so launching payloads to higher Earth orbits or interplanetary destinations would demand an upper stage. The powerful liquid-propellant Centaur upper stage would not be ready in time for the opening of the Mars multi-rover launch window, which spanned from 11 December 1983 to 20 January 1984, so JPL tapped a three-stage solid-propellant Interim Upper Stage (IUS) to push its Purple Pigeon out of Earth orbit toward Mars.

After an Earth-Mars cruise lasting about nine months, the twin multi-rover spacecraft would arrive at Mars a week or two apart between 16 September and 27 October 1984. They would each fire their main engines to slow down so that Mars gravity could capture them into an elliptical orbit with a periapsis (low point) of 500 kilometers, a five-day period, and an inclination of 35° relative to the martian equator.

The multi-rover landers would then separate and each fire a solid-propellant de-orbit rocket at the apoapsis (high point) of its orbit to begin descent to the surface. Landing sites between 50° north latitude and the south pole would in theory be accessible, though the need for a direct Earth-to-rover radio link would in practice prevent landings below 55° south.

The landers would each be encased within an aeroshell with a heat shield for protection during the fiery descent through the martian atmosphere. The aeroshell would have the same 3.5-meter diameter as its Viking predecessor, though its afterbody would be modified to make room for the large cooling vanes of the twin rovers' electricity-producing Radioisotope Thermal Generators (RTGs).

JPL's dual rovers packed inside their modified Viking-type aeroshell. Image credit: JPL.

After the landers touched down, the orbiters would maneuver to areosynchronous orbit. In such an orbit, 17,058 kilometers above the martian equator, only minor orbital corrections would enable a spacecraft to "hover" indefinitely over one spot on the equator. Each orbiter would position itself over a spot on the equator near its lander's longitude so that it could relay radio signals between its rovers on Mars and operators on Earth.

The multi-rover lander, which would serve no purpose beyond rover delivery, would constitute a radical departure from the triangular Viking lander design, though it would use Viking technology where possible to save development costs. It would comprise a rectangular frame to which would be attached three uprated Viking-type terminal descent engines, two spherical propellant tanks, and three beefed-up Viking-type landing legs.

Multi-rover lander. Image credit: JPL.

The 1.5-meter-long rovers would be mounted on the lander frame with their four 0.5-meter-diameter wire wheels compressed. Releasing a latching mechanism would permit the wheels to expand, raising the rover off four stabilizing "taper pins." The pins and one terminal descent engine would then swing out of the way, ramps would deploy, and the first rover would roll onto the rocky martian surface. The second rover would then ride a motor-driven "dolly" to the first rover's initial position before unlatching and joining its twin on the ground.

JPL envisioned that its four-wheeled rovers would each deploy a one-meter-tall boom holding a still-image camera, a floodlight, a strobe light, a weather station, and a pointable horn-shaped radio antenna. The camera/antenna boom, the tallest part of the rover, would stand about two meters above the surface. Controllers on Earth would then put the rovers through an initial checkout lasting at least two weeks. The checkout would culminate in slow "manual" (Earth-controlled) and faster semi-autonomous (Earth-directed but rover-controlled) traverses.

JPL's nuclear-powered rover viewed from above (top) and from the side. Image credit: JPL.

In semi-autonomous mode, operators would plan traverse routes and science targets using stereo images from the rover camera taken from terrain "high points," then would command the rover to proceed. The rovers might assist each other in traverse planning; for example, "high point" pictures from one might fill in blind spots in the other's field of view. "After the first few kilometers of traverse," the JPL engineers assumed, operators on Earth would "begin to build an intuitive feeling for the Martian geography and its impact on the rover capabilities, allowing them to plan better paths." The rovers would also photograph each other to enhance the mission's "general public appeal."

The rover mobility system would include one electric drive motor per wheel, eight proximity sensors for obstacle detection, inclinometers to monitor rover tilt, motor temperature sensors to judge wheel traction, a gyrocompass/odometer, a laser rangefinder with a 30-meter range, and an "8-bit word, 16k active, 64k bulk, floating point arithmetic and 16-bit accuracy" computer. The JPL engineers judged that their rovers would be capable of moving at up to 50 meters per hour over terrain similar to that seen at the Viking 1 landing site.

Dusk at the Viking 1 landing site in Chryse Planitia. Image credit: NASA.

Alpha-scattering X-ray fluorescence and gamma-ray spectrometers would collect data while the rovers were in motion, but all other science, including imaging and sample collection, would occur only while they were parked. Each rover would gather samples using an "articulated arm" with an "electromechanical hand."

In order to avoid "an overabundance of data from a single track," the rovers would travel slightly different routes and rendezvous at the end of each leg of their traverse. They would, however, travel close enough together that each could aid the other in the event of trouble. If one rover became stuck in loose dirt, for example, its companion could use its articulated arm to place rocks under its wheels to improve traction. If one rover of a pair failed, the report maintained, the other would continue to yield "good, solid science."

The rovers would be designed to operate for at least one martian year (about two Earth years) to help ensure that at least one of the four could successfully rendezvous with the follow-on MSSR mission, which would leave Earth in 1986. Estimates of rover traverse distances in 1970s and 1980s studies were typically highly optimistic, and the multi-rover mission was no exception: each of the mission's four rovers was expected to travel up to 1000 kilometers. The JPL engineers concluded their report by calling for new technology development to ensure that adequate power and mobility systems would become available by the time their Purple Pigeon was due to fly.

Sources

Journey into Space: The First Thirty Years of Space Exploration, Bruce Murray, W. W. Norton & Co., 1989.

Feasibility of a Mars Multi-Rover Mission, JPL 760-160, Jet Propulsion Laboratory, 28 February 1977.

More Information

Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)

Prelude to Mars Sample Return: The Mars 1984 Mission (1977)

Making Propellants from Martian Air (1978)

Exploring Mars from Pole to Pole: MESUR Network (1991)

Pioneer Venus 2 releases its three small Venus atmosphere entry probes. Through artist license, the large probe is visible against the clouds of Venus; it would not in fact have been visible at the time the small probes were released. Image credit: NASA.

On 8 August 1978, NASA launched Pioneer Venus 2 (PV2) on an Atlas-Centaur rocket. The 904-kilogram spacecraft, known also as Pioneer Venus Multiprobe, released a 1.5-meter-diameter battery-powered atmosphere entry probe on 16 November and three 76-centimeter-diameter probes on 20 November.

On 9 December 1978, the five parts of PV2 entered the thick, hot Venusian atmosphere. The drum-shaped probe carrier burned up as planned at an altitude of 110 kilometers. Sturdy conical heat shields protected the spherical instrumented probes from aerodynamic heating. As drag slowed it, the large probe deployed a parachute.

Two of the small probes, which did not include parachutes, exceeded all expectations by surviving landing and transmitting data from the hellish Venusian surface. One, the Day Probe, transmitted for 67.5 minutes before succumbing to heat, pressure, and battery failure, setting a new world record for spacecraft endurance on Venus.

PV2 was the last U.S. planetary mission launched until 1989. NASA Ames Research Center (ARC), located near San Francisco, California, managed PV2 and its sister spacecraft, PV1 (the Pioneer Venus Orbiter).

In July 1991, ARC proposed a multiprobe system outwardly not too different from PV2, but intended to create a long-lived network of low-cost science stations on Mars. According to ARC's report on the concept, its network would reflect a design philosophy with "unique characteristics . . . derived from the Pioneer Project corporate memory."

Mars networks were first proposed in the early 1970s. Scientific advisory groups endorsed the network concept repeatedly in the following two decades as the best way to obtain global-scale weather and seismic data. In the late 1980s, at the behest of the NASA Headquarters Solar System Exploration Division (SSED), the Jet Propulsion Laboratory (JPL) Precursor Task Team included a network in its program of precursor robotic missions for paving the way for astronauts on Mars. In common with previous Mars network plans, the 1989 plan invoked spear-shaped penetrators to hard-land stations at low cost.

NASA ARC's Mars Environment Survey (MESUR - pronounced "measure"), on the other hand, invoked cheap rough-landing landers, or "stations," that would deploy protective airbags seconds before landing. MESUR would build up a "pole-to-pole" network of 16 stations during the 1999, 2001, and 2003 minimum-energy Mars launch opportunities.

Each 158.5-kilogram MESUR lander would leave Earth attached toa Mars atmosphere entry deceleration system and a simple cruise stage. Upon arrival at Mars, each would cast off its cruise stage and enter the atmosphere directly from its Earth-Mars trajectory at up to seven kilometers per second. The ARC report compared this with the Viking landers, which entered from Mars orbit at only 4.4 kilometers per second. The lander's heat shield, a two-meter-diameter flattened cone, would be designed to withstand atmosphere entry during planet-wide dust storms, when suspended dust particles might exacerbate shield erosion.

Partial cutaway of a MESUR station on the surface of Mars. Image credit: NASA Ames Research Center.

The ARC report acknowledged that the disk-shaped lander might bounce to rest on Mars in either "heads" or "tails" orientation, but rejected as costly and risky a mechanical system for tipping it upright. The ARC engineers opted instead for circular ports that would enable controllers to deploy instruments from either side of the station. Instruments might include imagers, an atmospheric structure experiment, gas analyzers, a weather station, a spectrometer, and a seismometer.

The report explained that solar cells were initially ARC's preferred MESUR power system, but analysis had shown that the number of cells that could be mounted on the lander's small surface would not generate enough electricity to drive its science instruments unless landings were limited to sites within 30° of the martian equator. This limitation was deemed unacceptable by the MESUR Science Definition Team, so engineers opted for a small (nine-kilogram) General Purpose Heat Source (GPHS) Radioisotope Thermal Generator (RTG) "brick" based on Ulysses solar polar orbiter/Galileo Jupiter orbiter RTG technology.

Sixteen MESUR landers would need 16 GPHS bricks over six years. The report noted that the entire MESUR Network would need less than half as much plutonium as the Cassini Saturn orbiter, which would carry two RTGs with 18 GPHS bricks each.

Cutaway of the MESUR Network launch shroud showing four MESUR landers (one is mostly obscured behind the lander support structure) and the solid-propellant Mars transfer orbit injection stage. Image: NASA Ames Research Center.

The MESUR mission would begin in 1999 with the launch of a single Delta II 7925 rocket from Cape Canaveral, Florida, with four MESUR landers mounted on a framework within its 9.5-foot-diameter streamlined launch shroud. After a solid-propellant upper stage placed them on course for Mars, the landers would separate from the framework to travel on "independent free-flyer trajectories" that would permit precise Mars landing site targeting. Three side-mounted landers would tumble after separation, but sloshing propellants in their cruise stages would gradually damp their gyrations.

The landers would discard their cruise stages 125 kilometers above Mars. Ten kilometers above the planet, each would deploy a pilot parachute, then cast off its heat shield and open its single main parachute. The landers would image the surface and collect atmospheric structure data during the final eight kilometers of descent.

Just two meters above the landing site, each lander would release its main parachute and inflate its airbags. A small rocket on the parachute would ignite to prevent it from settling over the lander.

The MESUR lander design would permit landings at sites up to six kilometers above the base datum, the martian equivalent of Earth's sea level. The base datum, referenced to the minimum Mars atmospheric pressure required for liquid water to exist on the surface, was established after Mariner 9 mapped the planet from orbit in 1971-1972. (In 2001, a new system referenced to the mean radius of Mars as measured by Mars Global Surveyor's MOLA instrument replaced the base datum.)

Though all 16 MESUR landers would carry the same suite of instruments, their individual landing sites would be selected to cater to different science requirements. The report advised that weather stations should be spaced widely over the planet, while seismic stations should form closely spaced "triads." These conflicting requirements forced a "compromise network design."

MESUR Network Stations 1 and 2 would land near each other on the north rim of Valles Marineris to form a "seismic pair." Station 3, at the foot of Olympus Mons in Tharsis, would also emphasize seismic research. Station 4 would aim to extend the weather record for Chryse Planitia, where Viking 1 accumulated data from 1976 to 1983.

The Tharsis hemisphere of Mars showing proposed positions of MESUR stations. See text for explanation. Image credit: NASA.

In 2001, two Delta II 7925s would launch 20 days apart bearing four more MESUR landers and a communications relay orbiter, respectively. The latter payload, based on an existing Earth-orbital comsat design, would serve as radio relay for the expanding network, enabling MESUR stations to return data from sites all over the martian surface.

It would reach Mars in 10 months on a slow "Type II" trajectory to reduce the amount of propellant it would need to slow down so that the planet's gravity could capture it. Launch of the communications orbiter would be delayed until 2001 in order to spread its cost over a longer period.

With the successful arrival of the four 2001 stations, a "minimal network" would be in place on Mars. Station 5, on the Marineris north rim, would create a "seismic triad" with Stations 1 and 2, while Station 6, northwest of Olympus Mons, would create a seismic pair with Station 3. Station 7, east of Solis Planum ("a region of known dust storm activity"), and Station 8, in western Acidalia Planum, would expand martian meteorological coverage.

The final two MESUR Delta II 7925 launches in 2003 would boost four landers each on course for Mars. Stations 9 and 10 would be located near the north and south poles, respectively, while Station 11 would report weather conditions in Aonia Terra, southwest of the great Argyre basin. Stations 12 (northwest Hellas), 13 (Elysium Planitia), and 14 (Deuteronilus Mensae) would further extend martian meteorological coverage.

Station 15 (Sirenum Terra) would form a Tharsis seismic triad with Stations 3 and 6. Station 16, in Syrtis Major on the side of Mars opposite Olympus Mons, would create a seismic pair with Station 13 and, with the Tharsis triad, enable the size of Mars's core to be determined.

The Syrtis Major hemisphere of Mars showing proposed positions of MESUR stations. See text for explanation. Image credit: NASA.

The entire 16-station network and its communications orbiter would function for at least a martian year (a little more than two Earth years). This would mean that the 1999 stations would have to endure for three martian years (6.5 Earth years), while the 2001 stations and communications orbiter would need to function for two martian years (4.3 Earth years).

In its 1991 strategic plan, published the same month as ARC's MESUR report, the SSED dubbed MESUR its "baseline plan" for a Mars network mission. In November 1991, NASA elected to move MESUR Phase A development to JPL, where the project was split into two parts.

MESUR Network would be preceded by MESUR Pathfinder, a single-spacecraft mission for technology testing. Pathfinder was built larger than the the planned MESUR landers so that it could deliver to Mars a six-wheeled "microrover." JPL also opted for solar power in place of NASA ARC's RTG bricks and a petal system to permit it to flip itself upright and release the rover instead of small instrument deployment ports.

In 1994, in the wake of the Mars Observer failure, NASA funded the Mars Surveyor Program in place of MESUR Network. Work continued on Pathfinder under the auspices of NASA's low-cost Discovery Program, however, and it landed successfully on Mars on 4 July 1997.

Mars Pathfinder Lander (background) and Sojourner rover. Image credit: NASA.

Sources

Mars Environmental Survey (MESUR) Science Objectives and Mission Description, NASA Ames Research Center, 19 July 1991.

Solar System Exploration Division Strategic Plan: Preparing the Way to the New Frontier of the 21st Century, Special Studies Office, Space Telescope Science Institute, July 1991.

More Information

Pioneer Mars Orbiter with Penetrators (1974)

Prelude to Mars Sample Return: The Mars 1984 Mission (1977)