|The Apollo-shaped Boeing Starliner capsule approaches the International Space Station for the first time during Orbital Flight Test-2 on 20 May 2022. Image credit: NASA|
|The crew of a piloted flyby spacecraft prepares to retrieve the upper stage of a Mars Surface Sample Returner probe. Image credit: NASA.|
An Unfortunate Condition: A 1967-1968 Pitch to Launch a Comet Halley Rendezvous Mission in the Late 1970s
|Comet Halley's last visit before the space age: a photographic plate captured at Yerkes Observatory on 6 June 1910. Image credit: Yerkes Observatory.|
Herman Michielsen was a Senior Staff Scientist at Lockheed Missiles & Space Company's Palo Alto Research Laboratory in California in August 1967, when he presented a paper on possible missions to Comet Halley to an American Institute of Aeronautics and Astronautics (AIAA) conference in Huntsville, Alabama. His paper was the earliest serious work describing options for exploring Comet Halley using spacecraft during its 1985-1986 apparition, the first that would take place since the advent of spaceflight in 1957.
Lockheed funded Michielsen's Comet Halley research under its Independent Research Program, which gave its scientific staff opportunities to perform studies on company time outside their normal range of work. At the time he presented his Comet Halley paper, much of Michielsen's work had focused on calculating lunar and planetary ephemerides using advanced computers and on Earth satellite tracking. He was an important figure in the Independent Tracking Coordination Program, which aimed to supplement the limited number of professional Earth satellite visual observations with those of skilled amateurs around the world.
Comet Halley requires little introduction; it is the one recurrent comet the name of which is widely known to non-astronomers. Observations of Comet Halley were recorded in China as early as 240 BC. Not until the 18th century, however, was it understood that Comet Halley follows an elliptical Sun-centered path that brings it to a perihelion (closest point in its orbit about the Sun) between the orbits of Venus and Mercury about every 76 years.
The comet is named for Edmond Halley, the English astronomer who wrote in 1705 that comets observed in 1531, 1607, and 1682 were in fact a single comet. Halley successfully predicted that the comet would return in 1758, though he did not live to see its return.
Michielsen noted that short-period comets — that is, any comet with a period of 200 years or less — are typically visible only using telescopes and barely show a tail. Comet Halley is a short-period comet but bucks this tendency, making it an object of interest for future exploration using robot probes. The Lockheed scientist predicted that its return in 1985-1986 would become "a culmination point in the field of cometary probes."
Comet Halley is, however, not an ideal target for a spacecraft because it follows a retrograde path around the Sun. The great majority of Solar System bodies orbit their primary — the Sun, a planet, or any of the various categories of small body — in a prograde direction, which is to say counterclockwise. For its part, Comet Halley orbits the Sun clockwise. Michielsen called this "an unfortunate condition."
Michielsen calculated that spacecraft on a prograde intercept path would encounter Comet Halley at Earth's distance from the Sun (one Astronomical Unit, or AU) moving at about 60 kilometers per second (km/sec) relative to the comet; at Comet Halley's perihelion distance, 0.59 AU from the Sun, the relative intercept speed would exceed 90 km/sec. High encounter speeds near and at perihelion would mean that a probe could view the comet's nucleus, which was expected to measure at most a few tens of kilometers across, for only a very short time, making impossible any in-depth observations when the comet was most active.
At the time Michielsen presented his work, most comet scientists favored astronomer Fred Whipple's "dirty snowball" model of the structure of the comet nucleus. It should be noted, however, that in 1967-1968 rival models had supporters. Confirming the nature of the nucleus was among the most important justifications for comet exploration until the 1980s.
Michielsen proposed that an effort be made in time for the 1985-1986 apparition to place a robot probe into a retrograde Sun-centered orbit that would enable it to rendezvous with and travel beside Comet Halley for weeks or months. He wrote that a rendezvous mission would permit "a return of useful data many orders of magnitude greater than that from even a number of high-speed intercepts." A rendezvous would, however, be extremely challenging in terms of propulsive energy required.
A Comet Halley rendezvous might approach feasibility, he wrote, if the rendezvous probe were first launched into an elliptical Sun-centered orbit with an aphelion (farthest point in its orbit about the Sun) at about seven AU (that is, between the orbits of Jupiter and Saturn, which orbit the Sun at 5.2 AU and 9.5 AU, respectively). He proposed a launch in 1978, with the probe approaching aphelion in 1982.
Near aphelion, the spacecraft would move relatively slowly, so could place itself into a retrograde orbit using a propulsive maneuver (an "aphelion pulse") that changed its speed by only about 9.3 km/sec. Combined with Earth-departure and fine-targeting maneuvers, the total propulsive velocity change required to carry out a Comet Halley rendezvous in 1985 would amount to about 31 km/sec.
|Diagram of Comet Halley and rendezvous spacecraft paths during Michielsen's aphelion-pulse mission. Please click on image to enlarge. Image credit: DSFPortree.|
Other options would enable a Halley rendezvous with even less propulsive velocity change, Michielsen added. Departing Earth in 1973 would, for example, trim the aphelion pulse velocity change by 2.5 km/sec. The 12-year flight time from Earth launch to Halley rendezvous might, however, be seen as excessive.
In the early-to-mid-1960s, many planners considered the possibilities of propellant-saving gravity-assist maneuvers. Michielsen explained that a spacecraft launched on 13 September 1977 that passed in front of Jupiter on 16 September 1978 would be slowed and its course bent onto a retrograde path that would permit a rendezvous with Comet Halley on 27 May 1985, 254 days before its predicted perihelion on 5 February 1986. He also described a mission launched from Earth on 16 October 1978 that would encounter Jupiter on 14 October 1979 and rendezvous with Comet Halley on 10 September 1985, 148 days ahead of predicted perihelion.
Jupiter would be better positioned for the gravity-assist flyby in the 1977 opportunity, Michielsen added, thus reducing the required Earth-departure velocity and the velocity at which the spacecraft would approach Comet Halley. The propulsive velocity change from Earth departure through Halley rendezvous would total 24.6 km/sec for the mission launched in 1977 and 25.6 km/sec for the 1978 mission.
Michielsen then briefly explored the possibility of a Saturn gravity-assist flyby, which he said was suggested at the August 1967 AIAA meeting by Maxwell Hunter, who was a National Space Council member from 1962 until he joined Lockheed in 1965. A Saturn flyby Comet Halley rendezvous mission launched from Earth on 30 August 1973 would require a total propulsive velocity change of 22.2 km/sec; one launched on 14 September 1974 would need 22.9 km/sec. Saturn flyby would occur on 19 January 1976 for the 1973 launch and on 14 January 1977 for the 1974 launch; Comet Halley rendezvous would take place on 18 April 1985 or 21 June 1985, respectively.
In the second half of his paper, Michielsen gave close attention to the problem of precise prediction of Comet Halley's return, and it is in this context that his work is most often cited today. He noted that digital computers had enabled researchers to confirm that the gravity of the planets — in particular, Jupiter, Earth, and Venus — had caused Comet Halley's orbital period to vary by up to 1000 days over the centuries. In addition, a non-gravitational effect — the explanation of which he declared was beyond the scope of his paper — caused a shift in the perihelion date of about four days during each of the six apparitions spanning the period from 1456 to 1835.
The non-gravitational effect Michielsen was loath to explain had been attributed to jets of gas and dust that form when a comet nucleus is heated by the Sun. These jets would, it was believed, behave like natural rocket motors. This hypothesis would eventually be confirmed, but the Lockheed scientist was probably wise to treat the potentially controversial problem as an unnecessary distraction when he presented his study of Comet Halley rendezvous methods.
The shift in perihelion date meant that a Comet Halley probe launched in the late 1970s would need to perform additional propulsive maneuvers to ensure a close rendezvous. The magnitude of the maneuvers required would begin to become apparent, he predicted, in November 1983, when Earth's largest telescopes would begin to photograph Comet Halley between the orbits of Saturn and Jupiter at a distance of 8.5 AU from the Sun. Michielsen expected that, if reacquisition took place at that time, then a sufficient number of observations could occur to ensure that maneuvers requiring a total propulsive velocity change of just 1.2 kilometers per second would yield a "worthwhile rendezvous mission." Later reacquisition might demand a greater propulsive velocity change.
As it turned out, the advent of CCD technology enabled reacquisition of Comet Halley more than a year ahead of Michielsen's predicted date. On 16 October 1982, observers using the 200-inch Hale Telescope at Mount Palomar in California became the first humans to glimpse Comet Halley since 1911. The comet, which had yet to show a tail, lay beyond the orbit of Saturn when it was reacquired.
Advances in astronomy technology mean that Comet Halley has remained visible since its 16 October 1982 reacquisition. When it reaches perihelion in July 2061, it will have been visually tracked for 79 years.
This post is the first in a new series called "Preparing for Halley." It aims to describe U.S. efforts to launch a spacecraft to Comet Halley in 1985-1986. The series is timed to coincide with Comet Halley's aphelion passage late in 2023, after which it will be inbound for its 2061 apparition. Other posts on comet exploration relevant to Comet Halley missions in 1985-1986 can be found by following the "More Information" links below.
"A Rendezvous with Halley's Comet in 1985-1986," H. F. Michielsen, Journal of Spacecraft and Rockets, Volume 5, Number 3, March 1968, pp. 328-334; paper presented at the AIAA Guidance, Control, and Flight Dynamics Conference in Huntsville, Alabama, 14 August 1967.
|Scary ride: second stage and crew cabin of the Bellcomm minimum Mars Excursion Module (MEM) ascent vehicle. The unpressurized cabin would have included few displays and minimal communications. Image credit: Bellcomm/NASA.|
The piloted flyby spacecraft would have released the MSSR perhaps 10 days before Mars close encounter. A rocket motor on the MSSR would have boosted it toward Mars, enabling it to reach the planet and land at a preselected site several hours before the piloted flyby spacecraft arrived.
The crew on board the piloted flyby spacecraft would have used a variety of sample collectors on board the MSSR to gather about two pounds of Mars surface material and air by remote control as the planet grew ever larger in their viewports. These would have been packed into a three-stage ascent vehicle and launched to the piloted flyby spacecraft.
Phase 1 of the Planetary JAG piloted flyby study ended with distribution of an NASA report on 3 October 1966. The group then began work on Phase 2 of its piloted flyby study. Some members of the Planetary JAG foresaw a rosy future for the concept — they anticipated that MSSR study contracts might be awarded in Fiscal Year 1968 and the piloted flyby mission might become a NASA new start project in Fiscal Year 1969.
Even before the AS-204/Apollo 1 fire (27 January 1967), however, NASA planning for missions beyond Apollo was on shaky ground. Neither the Administration of President Lyndon Baines Johnson nor the Congress supported ambitious plans for post-Apollo spaceflight — for example, a long-term lunar base or humans on Mars. By the summer months of 1967, the fire, racial and anti-war tensions across the country, concerns about the Federal budget deficit, and military setbacks in Indochina had provided opponents of an expansive U.S. future off the Earth with ample justification for curtailing NASA efforts to define its future.
Not all advance planning halted, however. Bellcomm, NASA's Washington, DC-based planning contractor, continued its work as a matter of course. Most Bellcomm studies in the 1967-1969 period aimed to define the shape of the Apollo Program after the first successful piloted lunar landing, as well as that of Apollo's planned successor, the Earth-orbital and lunar Apollo Applications Program (AAP). A modicum of work toward more ambitious goals beyond Earth orbit and the Moon also continued.
In July 1967, for example, Bellcomm planners D. Cassidy and H. London completed a short technical memorandum in which they explored how the MSSR probe might form the basis for a piloted Mars Excursion Module (MEM) lander. Their study kicked off a series of related studies at Bellcomm in the year that followed.
Cassidy and London assumed a 15,000-pound MSSR with a two-stage ascent vehicle capable of launching 80 pounds to a passing piloted flyby spacecraft launched in 1975, 1977, or 1979 on a Mars Twilight flyby path. The "Twilight" mission owed its name to the geometry of its Mars flyby — closest approach to Mars took place over the planet's night hemisphere near the dawn terminator, the line dividing pre-dawn darkness from daylight.
The Bellcomm engineers calculated that MSSR Mars atmosphere entry velocity would reach 32,500 feet per second (fps) in 1975, 34,500 fps in 1977, and 39,000 fps in 1979. As it passed at a shallow angle through the thin martian atmosphere, the automated MSSR would undergo deceleration equal to up to 40 times the pull of gravity on Earth's surface (that is, 40 Gs). To accomplish rendezvous with the passing piloted Mars flyby spacecraft, the MSSR ascent vehicle would have to boost an 80-pound third stage and sample container to 36,000 fps in 1975, 38,000 fps in 1977, and 42,500 fps in 1979.
An MSSR-derived MEM released during piloted Mars orbiter approach to Mars — that is, before the orbiter fired its rocket motors to slow down so that Mars's gravity could capture it into an elliptical orbit with a one-day period — would, on the other hand, in 1978, 1982, 1984, 1985, and 1986 enter the martian atmosphere moving at between 20,000 fps and 25,000 fps. It would decelerate at about 10 Gs, which Cassidy and London judged to be acceptable for an astronaut.
|MSSR-derived minimum MEM. Image credit: Bellcomm/NASA.|
Cassidy and London explained that a separate Bellcomm study, not yet completed when they finished their July 1967 memorandum, had determined that a minimum one-person MEM with a crew capsule weighing as little as 600 pounds without an astronaut on board might be possible. Such a MEM would carry only enough life support consumables to remain on Mars for a short time and no scientific exploration equipment.
Anticipating that a mission so limited might not enjoy much support, they briefly examined one that employed three MSSR-derived landers: a pair of minimum MEMs carrying one astronaut each and an automated one-way cargo lander for delivering 5,500 pounds of life support supplies and science gear. The three landers would touch down near each other so that the two astronauts could meet up and make use of the cargo.
Skeer eliminated 47% of Mercury's weight immediately by deleting the 1119-pound launch escape tower, 315-pound heat shield, 317-pound retrograde propulsion system (used to deorbit the Mercury capsule), landing systems and recovery gear together weighing 343 pounds, and 51 pounds of experiments. He then treated remaining Mercury systems in detail. He wrote, for example, that McDonnell engineers had told him that the 75-pound Mercury couch could be trimmed to 15 pounds. Skeer arrived at a total MEM ascent stage weight of 738 pounds including a 170-pound astronaut.
|Minimum MEM entry and landing. Image credit: Bellcomm/NASA.|
|Useful payload: two-stage minimum MEM ascent vehicle. Image credit: Bellcomm/NASA.|
The astronaut packed into the coffin-like MEM cabin would rely for life support on a 40-pound space suit with a 100-pound life support backpack containing sufficient life support consumables for 12 hours of operations. This combination would replace a Mercury cabin life support system weighing 248 pounds.
Skeer briefly examined a two-person minimum MEM with a descent stage not based directly on the piloted flyby MSSR. This would, he wrote, have a total weight of less than 35,000 pounds. Of this, 1360 pounds would comprise the MEM ascent stage and crew. He clearly favored the single-person minimum MEM, however.
Skeer subsequently conducted a pair of follow-on studies of MSSR-derived piloted spacecraft. The first, completed on 8 May 1968, looked at a 4064-pound MSSR-derived two-person surface shelter that would enable a two-week Mars surface stay by astronauts landed separately in a pair of MSSR-derived minimum MEMs. Expendables supporting the two-week stay — mostly for life support and power generation — would account for 1053 pounds of the shelter's weight.
|Cutaway view of MSSR-derived Mars surface shelter. Image credit: Bellcomm/NASA.|
|Plan view of MSSR-derived Mars surface shelter. Image credit: Bellcomm/NASA.|
Scientific exploration equipment accounted for 1460 pounds of the shelter's weight. This included a single 367-pound one-person surface rover or flyer with a total range of 420 kilometers, 77 pounds of multiband photography/radiometry equipment, a 107-pound shelter-mounted drill capable of reaching a depth of 30 meters, 100 pounds of "surveying tools," a 250-pound "Emplaced Science Station" and three "satellite science stations" (total weight 140 pounds) meant to be left behind on the martian surface, and "local sampling and environmental equipment" weighing a total of 395 pounds. The shelter's pressurized cabin would contain 50 pounds of equipment for geologic analysis. (Skeer made no reference to return of samples in the two minimum MEM vehicles; presumably sample analysis on Mars was meant to replace return of samples to Earth.)
Skeer's second follow-on study, a more detailed examination of the minimum MEM ascent stage dated 8 July 1968, sought to identify "fruitful areas of technological research and development needed for evaluation and future program planning options." He argued for development of new propulsion systems capable of burning new high-energy propellants (for example, fluorine-LOX/methane), compact and lightweight refrigeration systems for long-term storage of such propellants, and development of new lightweight materials to permit further minimum MEM weight reduction.
He also noted that flights of MSSR probes during piloted flyby missions could be seen as test flights of minimum MSSR technology. Unfortunately, by the time Skeer completed his second follow-on study, work within the NASA Planetary JAG toward a piloted flyby with MSSR probe had been largely abandoned for nearly a year.
"MSSR/MEM Commonality - Case 233," D. E. Cassidy and H. S. London, Bellcomm, Inc., 19 July 1967.
"Preliminary Sizing of a Mars Excursion Module Ascent Capsule Based on Mercury Spacecraft Design - Case 233," M. H. Skeer, Bellcomm, Inc., 25 September 1967.
"Preliminary Mars Excursion Module Shelter Design - Case 730," M. H. Skeer, Bellcomm, Inc., 8 May 1968.
"Mars Excursion Module Ascent Propulsion Stage Design," M. H. Skeer, Bellcomm, Inc., 8 July 1968.
|The Tharsis hemisphere of Mars. The four large volcanoes are marked by clouds and the western half of Valles Marineris is visible at right. Image credit: NASA.|
The MSWG scientists divided into Site Selection and Sample Acquisition teams. The teams held two joint workshops and produced 10 detailed reports before the middle of 1979. Edited by JPL's Neil Nickle, they did not see print until November 1980.
Publication was delayed in part because Mars planning at JPL slowed markedly in early 1979. It would not begin to emerge from the doldrums again until the following year, after President Jimmy Carter's NASA Administrator, theoretical physicist Robert Frosch, created the Solar System Exploration Committee in an (ultimately successful) effort to revitalize the space agency's flagging robotic exploration program.
Because the MSWG reports were based on limited data, they may appear archaic to some readers. Nevertheless, they remain important, for they capture snapshots of the state of Mars science as the busy first era of robotic Mars exploration ended and the long gap began between the Viking missions, which reached Mars in 1976, and Mars Pathfinder and Mars Global Surveyor, which arrived at the planet in 1997.
The first MSWG report, which looked at polar landing sites for the minimum MSR mission, was authored by J. Cutts, K. Blasius, W. Roberts, and K. Pang of the Planetary Science Institute (PSI) of Science Applications, Inc., and A. Howard of the University of Virginia (UV). They submitted their report to JPL on 30 April 1979.
The PSI/UV team began by pointing out that humans had already explored Mars's poles for more than a decade. Mariner 7 had begun close-up martian polar exploration by imaging the entire southern ice cap at low resolution during its August 1967 flyby. Mariner 9 imaged both caps from Mars orbit during 1971-1972, and the Viking 2 orbiter began high-resolution polar imaging in 1976.
In many respects, polar MSR sites constituted a special case, the PSI/UV team wrote. Whereas missions to the other MSR sites would focus mainly on rock samples, the polar MSR mission would acquire meter-long core samples of ice or dust and ice. Rock samples would be "an unplanned bonus."
|The north pole of Mars. Image credit: NASA.|
They assumed that a landing might safely occur anywhere within a target ellipse 25 kilometers wide by 40 kilometers long, and calculated that a lander that set down in the ellipse would stand at least a 99% chance of landing on perennial ice. For this reason, no mobility (that is, no rover) would be required at Site A.
The second polar site, Site B (84.5° N, 105° W), included perennial ice and "partially defrosted" terraced troughs. The latter, the PSI/UV scientists explained, would "form windows through the layered deposits and cross-sections through martian history." The 25-kilometer-by-40-kilometer Site B target ellipse would also overlap the edge of the permanent ice cap.
Selecting such a varied area would, they warned, reduce the probability of landing on perennial ice to between 60% and 90%. If, however, the Site B mission included a short-range (about 10 kilometers) rover, then the probability of sampling more than one terrain and of sampling perennial ice would increase to greater than 90%.
In discussing the engineering problems of a polar MSR mission, the PSI/UV team cited Purdue University's 1976-1977 Mars Polar Ice Sample Return study (see "For More Information," below), but otherwise left engineering to the engineers. Potential problems identified included acquisition and preservation of ice and permafrost cores, mechanical operations at extremely low temperatures, and water and carbon dioxide frost accumulation and evaporation that might impede a rover.
As a "next logical step" toward a polar MSR mission, the PSI/UV scientists recommended establishment of a science working group with "substantial participation by earth scientists involved with studies of terrestrial sedimentary records[,] particularly those pertaining to climate change." They did not recommend an MSR precursor mission; that is, they judged that the Viking missions had provided data adequate for planning a minimum MSR mission to Mars's north pole.
Arizona State University (ASU) geologists R. Greeley, A. Ward, A. Peterfreund, D. Snyder, and M. Womer submitted the second of the 10 MSWG reports to JPL in March 1979. Their quest for a young volcanic MSR site was hampered, they explained, by a dearth of high-resolution (better than 50 meters per pixel) orbital images. Nevertheless, they located six candidate sites that looked to be volcanic and had few craters, signifying youth. (Planetary scientists count craters to estimate terrain age; the more densely craters pock a landscape, the older it is likely to be.)
|Arsia Mons (right of center) is southernmost of the four great Tharsis volcanoes. ASU's "Arsia Mons West" MSR site is located near the center of the left (west) half of the image. Image credit: NASA.|
The Arsia Mons West site, which had been imaged by the Viking orbiters at 34-meters-per-pixel resolution, included eight overlapping lava flows. The flows measured from eight to 35 kilometers wide and averaged 51 meters thick.
The ASU team found room for two target ellipses 80 kilometers long by 50 kilometers wide on either side of a five-kilometer crater at their site's center. They calculated that a rover with a 14-kilometer range would have a "complete guarantee" of reaching an outcrop of young volcanic rock.
At JPL's request, the ASU geologists also assessed Viking 1's Chryse Planitia landing site as a potential MSR landing site. The volcanic rocks were old at Chryse, a smooth-floored basin at the confluence of several large flood-carved channels. Based on the in-situ evidence provided by Viking 1 lander images, it was clear that no mobility would be needed to acquire a rock sample.
The ASU team noted, however, that the "value of a returned sample [would be] severely diminished because it may be impossible to determine if the material represents local [lava] flows. . .[or] if it has been deposited from the floods that eroded the channels." The ASU team added that "[w]ithout mobility of at least 200 to 300 kilometers, the [Chryse Planitia] site [would be a] a poor choice to answer basic scientific questions about Mars." For neither site did they recommend an MSR precursor mission.
A Young-Lavas Landing Site Northwest of the Volcano Apollinaris Patera and a Landing Site on the Ancient Terrain Southeast of the Schiaparelli Basin, had a single author: Brown University geologist P. Mouginis-Mark. He argued for mobility at his young Elysium Lavas (5° S, 190° W) and Ancient Terrain (8° S, 336° W) minimum MSR sites. The former, 150 kilometers from the Apollinaris Patera volcano, comprised rolling plains with scattered volcanic domes and shields, stratovolcanoes, and fresh impact craters. He identified a ridge running through the center of the 80-by-50-kilometer target ellipse as the feature most likely to yield a "good sample" (that is, a well-preserved volcanic rock representative of the site).Mouginis-Mark calculated that without mobility the probability of obtaining a good sample would be nil, while the probability of landing on a sand dune and obtaining no sample at all would be as high as 22%. The probability of obtaining a good sample would increase to 91%, however, if the mission included a rover with a round-trip range of 20 kilometers.
|The smooth-floored crater Schiaparelli (top of image, just right of center). Mouginis-Mark's "Ancient Terrain" MSR site is located near the center of the image. Image credit: NASA.|
Mouginis-Mark expected that a good sample might be found on the rim of a fresh crater more than two kilometers across, five of which occurred in Ancient Terrain target ellipse. He calculated that a rover round-trip range of 50 kilometers would be needed to achieve a 90% probability of acquiring a good sample.
For their contribution, USGS geologists H. Masursky, A. Dial, M. Strobell, G. Schaber, and M. Carr recycled four sites that they had studied in 1977-1978 for a proposed Viking follow-on long-range rover mission. Masursky and Dial were co-authors of the Viking '79 traverse study in 1974, while Carr led the Viking orbiter imaging team (and thus was involved in capturing the high-resolution images the minimum MSR Site Selection Team used in preparing its reports).
The USGS sites represented two martian terrain types. Tyrrhena Terra and Iapgyia Terra included ancient cratered terrain similar to that at Mouginis-Mark's Schiaparelli site, which is perhaps unsurprising given that such terrain covers more than 60% of Mars. The sites contained a jumble of overlapping craters and an intercrater mantle of old lava flows.
Samples collected in Tyrrhena and Iapgyia would permit age-dating of the oldest martian crustal material, the USGS geologists wrote. This would enable calibration of the crater counts used for dating martian terrains. In addition, data from the samples could "be compared to comparable analyses made of ancient lunar crustal materials returned by Apollo 16 and [to] ancient terrestrial rocks in order to make interplanetary comparisons of [how rocks are formed], physical and chemical properties, and age."
Of the two sites, Tyrrhena was "superior as a potential sample site in all respects," the USGS team wrote. They proposed that the minimum MSR lander set down where the old lava flows appeared to be thin, near a six-kilometer-diameter crater - one large enough, they judged, to have excavated ancient crust buried beneath the flows. They calculated that a landing ellipse 30 kilometers long and a rover with a 10-kilometer round-trip range would reach only old lava samples.
Obtaining an ancient crustal rock sample ("the primary science objective"), on the other hand, would demand a five-kilometer landing ellipse and a 14-kilometer round-trip rover. Achieving such landing accuracy implied that the minimum MSR lander would be capable of automated guidance and precision maneuvers during descent.
The other two USGS sites, Candor Chasma and Hebes Chasma, were both part of Valles Marineris, Mars's great equatorial canyon system. "These sites," the USGS team wrote, would "offer a unique opportunity to sample rock layers and their interbedded soils that would reveal the petrochemical history, age dates[,] and the history of environmental changes that may correlate with episodes of channel formation" on Mars. They might also yield organic material ("if the present red anorganic climate did not exist at times in the past") and a record of "the history of solar variations."
|Martian Canyonlands: Candor Chasma. Image credit: NASA.|
The MSWG's fifth report, the first of the six prepared by members of the MSWG Sample Acquisition Team, looked at the availability of rocks on Mars with emphasis on the equatorial Central Latitude Belt, which spanned between 30° N and 30° S. The report's author, University of Houston geologist E. King, explained that celestial mechanics and MSR lander engineering constraints would probably dictate that the Belt contain the first MSR landing site.
The twin Viking landers had had trouble collecting small rocks on Mars, King noted. This had led some to suggest that what looked like rocks at the Viking sites were in fact soft "clods" of martian dirt. If correct, then this hypothesis would mean that rocks were rare on Mars, which would in turn eliminate the primary motivation for an MSR mission; that is, to collect rocks.
King reported that his "evaluation of all of the presently available relevant data" had eliminated this concern "completely" for large parts of Mars, including for the Central Latitude Belt. Especially encouraging were data from the Viking orbiter Infrared Thermal Mapping (IRTM) experiment, which mapped thermal inertia (that is, how long it takes a given surface to become cool at night). Rocky surfaces need longer to cool down than do dusty surfaces.
Viking IRTM data indicated that much of the Central Latitude Belt has thermal inertias as high as 12. "It is very difficult to construct a reasonable model of the martian surface that has a thermal inertia of more than about 3 that does not have a substantial percentage of the surface area covered with rocks," King wrote.
He attributed the Vikings' inability to collect small rocks to inadequacies in the Viking sampler design. After it scooped a sample containing small rocks, controllers on Earth commanded the sampler to turn upside-down and shake for up to two minutes to sieve out dust. King noted that shaking the sampler caused its lid to flap open as much as an inch. This would allow any pebbles it contained to escape.
He advocated collecting rock samples in the form of drilled cores, since drilling could penetrate past any weathered rock rinds. Drilling could also collect uniform cylindrical samples that could be handled easily and stored efficiently in the MSR spacecraft.
King was ambivalent about the need for mobility in an MSR mission; he wrote that, if the objective of the mission were to collect fresh igneous rocks, and if the MSR landing site were similar to the Viking landing sites, then little mobility would be necessary. He added that, while it might be prudent to "build in some additional mobility as a margin of safety and to afford additional possibilities for sample collection. . .such provisions [had to be] traded off against lander science and returned sample weight."
USGS geologist H. Moore wrote the sixth MSWG report, which constituted a tour of the landscape within view of the Viking 1 and Viking 2 lander cameras. Viking 2 landed in Utopia Planitia, near the large impact crater Mie, a region more northerly than Viking 1's site in Chryse Planitia. Like King, Moore wrote that Viking 1 rocks were varied (there were 30 types) and tended to be smaller than Viking 2 rocks. The Viking 2 rock population, for its part, appeared to be dominated by ejecta from Mie.
Moore then described hypothetical rover traverses at the two sites. In each, the rover would visit 17 sampling stations, traverse about 100 meters, and range up to 20 meters from its lander.
|The boulder named "Big Joe" at the Viking 1 landing site in Chryse Planitia. Image credit: NASA.|
The seventh MSWG report sought to estimate the number of crystalline rocks — that is, volcanic rocks such as basalt — at the Viking landing sites and to plan traverses that would adequately sample them. Its authors, R. Arvidson, E. Guinness, S. Lee, and E. Strickland, geologists in the Department of Earth and Planetary Sciences at Washington University in St. Louis, Missouri, argued that any rock larger than about 10 centimeters in diameter at the Viking sites was a good candidate for being crystalline.
Such rocks, they added, cover 9% of the Viking 1 site and 17% of the Viking 2 site. The former, they wrote, included bedrock exposures and at least four soil types, while the latter included two soil types and no bedrock. They pointed out that, while a sampler arm could probably reach a crystalline rock at either site, it would not be able to sample all of the available materials. For that reason, they proposed that MSR landers at the Viking sites should each deploy a "mini-rover."
The Viking 1 site was "such an interesting place," the Washington University team wrote, that they had planned for it a 40-meter traverse with seven sampling stations (with an option to extend to 50 meters and 10 stations). The basic traverse would collect 10-centimeter core samples from three rocks and four soil samples. The extended traverse would sample two more rocks, including Big Joe, and would gather a total of five soil samples, including very red soil from atop Big Joe.
The Viking 2 site, by contrast, featured minimal variety, so the Washington University team's traverse there would cover only 25 meters and seven stations. The mini-rover would collect four soil samples and core samples from three rocks.
N. Nickle of JPL's Flight Projects Planning Office authored the eighth MSWG report, which was titled Requirements for Monitoring Samples. The report was published originally as a JPL Interoffice Memorandum dated 20 October 1978. Nickle wrote that the "scientific integrity of the returned Martian samples is of prime importance." "Scientific integrity," he explained, meant "the preservation of the physical and chemical state of the acquired samples."
To maintain the scientific integrity of the samples collected during the minimum MSR mission, Nickle recommended that they be kept 20° C cooler than the estimated minimum temperature they had experienced on Mars, and that they be sealed within a container with martian air at typical martian surface pressure. In addition, he recommended that the samples be exposed to no more galactic cosmic and solar radiation than they had been on Mars, and to no magnetic field stronger than Earth's natural field.
The minimum MSR mission sought to control cost in part by avoiding science instrumentation not required for sample collection. In the MSWG's ninth report, J. Warner of NASA's Johnson Space Center (JSC) in Houston, Texas, looked at low-mass, low-power MSR science instruments designed to "provide adequate information to select samples."
His candidate instrument suite included a steerable imager, a reflectance spectrometer, a chemical analyzer on a boom, a boom-mounted densitometer, and a tool for measuring hardness (this might, Warner suggested, be made a function of the sample scoop; the Viking arm and claw had been used to scratch and chip at rocks to judge their hardness).
Warner also prepared the tenth and last report of the Site Selection and Sample Acquisition Study, which he titled A Returned Martian Sample. In it, he looked at the form the minimum MSR sample should take. He looked at two different landing site types: a Viking-like site "laden with a variety of rocks and soils" and a hypothetical "smooth plains site."
The JSC geologist cited Moore's report when he wrote that, at a Viking-like site, an adequate sample could be "obtained on a traverse of a few hundred meters that never leaves the field of view of the lander." He estimated that an atmosphere sample, a soil core, nine rock cores, four small rock fragments, two duricrust samples, and six scoops of soil would adequately represent a Viking-like site. Together these samples would have a mass of 4.1 kilograms.
An eight-month, 15-station traverse could adequately sample a rock-poor smooth plains site, Warner wrote. The rover would range widely over the smooth terrain. Sampling stations would occur at "obstructions" (for example, craters). The rover would drill two or three rock cores and collect one rock fragment at each station, scoop soil at every other station, and collect duricrust at every fifth station. Adding a soil core and an atmosphere sample would bring the total sample mass to 5.7 kilograms if two rock cores were collected and 6.9 kilograms if three cores were collected.
Mars Sample Return: Site Selection and Sample Acquisition Study, JPL Publication 80-59, Neil Nickle, editor, NASA Jet Propulsion Laboratory, 1 November 1980.
Detailed Reports of the Mars Sample Return Site Selection and Sample Acquisition Study, JPL 715-23, Volumes I-X, Mars Science Working Group Mars Sample Return Study Effort, NASA Jet Propulsion Laboratory, November 1980.