15 March 2019

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 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 line between light and dark — 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, has become the line of lunar dusk. Darkness advances from east to west as light advanced two weeks before.

Twenty-one days past new, night has reclaimed 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 maria 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, Young 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

The Eighth Continent

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)

12 March 2019

Chronology: Apollo-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)

04 February 2019

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 planet's 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 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 consist of 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 its 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 manned 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 motion 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)

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

"Assuming That Everything Goes Perfectly Well in the Apollo Program. . ." (1967)

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

Flyby's Last Gasp: North American Rockwell's S-IIB Interplanetary Booster (1968)

Lunar Viking (1970)

The Challenge of the Planets, Part Three: Gravity