 |
| This oblique view of the southern polar ice cap of Mars (bottom) includes the entire permanent cap and a portion of the adjoining seasonal temporary cap. Many features of the southern hemisphere are in view; for example, the large Hellas impact basin is visible at center left. At its fullest extent in southern hemisphere midwinter, the seasonal cap expands to touch the southern rim of Hellas. Image credit: NASA. |
The three-kilometer-thick permanent caps cover a little more than 1% of the martian surface. In northern hemisphere midwinter, the seasonal carbon dioxide cap expands to about 60° north latitude. Roughly 13 Earth months later, in southern midwinter, carbon dioxide ice covers the cratered landscape to about 60° south latitude.
Confirmation that the permanent polar caps are made up mainly of water ice did not come easily. The polar caps were first glimpsed using crude telescopes during the 17th century, and were widely believed to be made of water ice by the end of the 18th. In 1965, however, data from Mariner IV, the first spacecraft to fly past Mars, indicated that the permanent caps were made of frozen carbon dioxide, an interpretation the Mariner 6 and 7 flybys (1969) and the Mariner 9 orbiter (1971-1972) did little to contradict.
In the late 1970s, however, the twin Viking Orbiters revealed that the northern permanent cap is made of water ice. Confirmation that the southern permanent cap is also made of frozen water had to wait, however, until 2003, when data from the Mars Global Surveyor and Mars Odyssey orbiters had become available.
 |
| Close-up of the southern permanent ice cap of Mars in southern hemisphere summer. In winter, the entire image would be cloaked in red dust and carbon dioxide frost and ice. Image credit: NASA. |
The Purdue students assumed that the permanent ice caps of Mars are, as on Earth, built up of layers of snow or frost deposited annually. They anticipated that each layer would contain a sample of the dust and gases in the atmosphere at the time it was laid down, making it a record of atmospheric particulates and climate conditions. The range of materials captured in the core would enable multiple methods of age determination.
On Earth, ice cores from Greenland record lead smelting in the Roman Empire and vegetation changes in Ice Age Europe. A martian polar ice core, the students believed, might yield a planet-wide record of dust storms, asteroid impacts, volcanic eruptions, flowing surface water, and, quite possibly, the existence of microbial life.
 |
| Section of an ice core collected from deep beneath the Greenland ice cap. Image credit: Greenland Ice Sheet Project. |
Viking 1 fired its Orbiter-mounted rocket motor on 19 June 1976 to slow down so that the red planet's gravity could capture it into orbit. The Viking 1 Lander was scheduled to land on the American Bicentennial (4 July), but the landing was postponed after Viking Orbiter images of its prime and backup landing sites, which had been selected using Mariner 9 data, showed them to be too rough. The Viking 1 Lander separated from its Orbiter and performed the first successful Mars landing in Chryse Planitia on 20 July 1976.
Viking 2 reached Mars orbit on 7 August. Its pre-selected landing sites were also found to be too rugged, so touchdown in Utopia Planitia did not take place until 3 September 1976.
Viking development cost close to a billion U.S. dollars, making it the most expensive automated exploration program of its time. For some planners — possibly unacquainted with the untimely end of the Apollo program — it seemed reasonable to assume that NASA would exploit Viking hardware to the fullest to cash in on its investment. JPL planners, for example, widely expected that a third Viking spacecraft — probably including a rover — would depart for Mars in 1979. For this reason, the Purdue students assumed that Viking hardware would continue to be manufactured into the 1980s and that their MPISR spacecraft could be derived from it.
 |
| Schematic of the Viking Orbiter with attached Viking Lander aeroshell capsule. Image credit: NASA. |
 |
| Schematic of Viking lander deployed on Mars. Image credit: NASA. |
The need for a short-duration flight from Mars to Earth and for south pole conditions safe for landing dictated the MPISR mission's Earth departure date. A long flight back to Earth would place great demands on sample refrigeration equipment, so the Purdue students sought the shortest return opportunity they could find.
Data from the Viking Orbiters had shown the south pole ice cap to be too unstable for landing and sample collection in the spring and summer, when the temperature climbs too high for carbon dioxide to remain solid. At mid-winter, on the other hand, snow and frost accumulation might bury the MPISR Lander. The team proposed, therefore, that the Lander should set down in late summer, about 75 days before southern hemisphere autumnal equinox.
Data from the Viking Orbiters had shown the south pole ice cap to be too unstable for landing and sample collection in the spring and summer, when the temperature climbs too high for carbon dioxide to remain solid. At mid-winter, on the other hand, snow and frost accumulation might bury the MPISR Lander. The team proposed, therefore, that the Lander should set down in late summer, about 75 days before southern hemisphere autumnal equinox.
 |
| Schematic of MPISR spacecraft after lander separation but before AV third stage arrival. The Earth Return Vehicle near the bottom of the image — which would carry within it the Earth Orbit Vehicle — would be based on the Pioneer Jupiter/Saturn spacecraft bus design. Image credit: Purdue University. |
On 16 November 1986, after a flight lasting nearly seven months, the MPISR Orbiter propulsion system would slow the spacecraft so that martian gravity could capture it into a polar orbit. It would then begin a 14-month orbital survey of the martian poles.
The MPISR Orbiter would map the poles using Viking-type cameras, a Viking-type thermal mapper, and a new-design Radar Ice Sounder for determining ice depth. The sounder, which is not depicted in the MPISR Orbiter image above, would employ an 11.47-meter-diameter dish antenna that would unfold from the Orbiter soon after Mars orbit arrival. Scientists would use data from the Orbiter's instruments to select a safe and scientifically interesting south pole landing site for the MPISR Lander.
On 3 February 1988, the Lander would separate from the Orbiter, ignite solid-propellant rockets to slow down and drop from Mars orbit, then descend through the planet's thin atmosphere to the selected landing site. Because it would have nearly twice the mass of the Viking Lander from which it was derived, the MPISR Lander would lower on six parachutes and six terminal descent rocket engines (in each case, twice as many as Viking). The engines would be arranged in three clusters of two engines each.
Extra engines would complicate deployment of the Lander's most important science system, the 16.3-kilogram Ice Core Drill (ICD). Soon after touchdown, the MPISR Lander would reach out with its modified Viking sampler arm to detach one of the three descent engine clusters to clear the way for ICD deployment.
Sixty-seven times over the next 90 days, the ICD would collect a 75-centimeter-long ice core, raise it to the surface, and deposit it in an insulated 12-kilogram sample container. The final core would sample ice and dust layers hidden 50 meters below the surface.
The students did not describe ICD operation in detail. No doubt the drilling and core acquisition process would face many challenges. The slender drill might, for example, encounter a patch of compressed crystallized ice and dust at depth and need to start again at a new place within its drill site, which would measure at most two or three square meters in area.
The MPISR Lander's south pole landing site would mean that it could not transmit radio signals directly to Earth. The MPISR Orbiter, for its part, would be able to keep Lander and Earth in view simultaneously for at most 25 minutes per day, sharply restricting radio relay time. This meant that the continuous drilling operation would need to take place autonomously.
Communication limitations, combined with slowly changing, generally unfavorable polar lighting conditions, led the Purdue students to replace the twin Viking scanning facsimile cameras with a simpler vidicon camera akin to that carried on the 1960s Surveyor lunar landers. The camera package would include three strobe lights. This would, they explained, permit the MPISR Lander to capture "snapshots" of the drill site and its novel frosty surroundings for transmission to the Orbiter.
Radioisotope Thermal Generators (RTGs) would power and warm MPISR Lander systems. The Lander's three footpads and underside would be insulated to prevent heat transmitted through its structure from melting the ice, helping to ensure that it would not sink from sight during the three-month sample-collection period.
The Mars southern hemisphere autumnal equinox would occur on 17 April 1988. On 2 May 1988, with winter gradually settling in at the martian south pole, the first of the AV's three rocket stages would ignite to blast the ice core samples to Mars orbit. The AV third stage would provide refrigeration in the sample container to keep the ice core sections pristine.
The AV first stage and second stage would burn solid propellants. The liquid-propellant third stage would boost the sample container into a 2200-kilometer circular orbit about Mars, then would commence active maneuvers to perform a rendezvous with the MPISR Orbiter.
On 17 May 1988, the MPISR Orbiter would maneuver to a docking with the AV third stage. A docking collar on the ERV/EOV would dock with the third stage, then the sample container would automatically transfer to the ERV/EOV and the third stage would be cast off.
On 27 July 1988, the ERV/EOV would separate from the MPISR Orbiter and fire its engine to leave Mars orbit for Earth. To reduce the period of time during which the ice core would need refrigeration, the ERV/EOV would expend propellants to speed Earth return. A minimum-energy transfer in the 1988 Mars-Earth opportunity would last 122 days; the ERV/EOV's energetic Mars-departure burn would slash this to as little as 98 days. Arrival in Earth orbit would take place between 2 November and 14 November 1988.
Nearing Earth, the cylindrical 1.5-meter-long EOV would separate from the ERV and fire a solid-propellant rocket motor to slow down so that Earth's gravity could capture it into a 42,200-kilometer circular orbit. The ERV, meanwhile, would speed past Earth into solar orbit.
Discarding the ERV ahead of Earth-orbit capture would slash Earth-orbit insertion mass, dramatically reducing the quantity of propellants needed to place the Mars ice sample into Earth orbit. The Purdue team found that this approach would have mass-saving knock-on effects throughout the MPISR mission design, yielding a 6% reduction in total spacecraft mass at Earth launch.
The Purdue students described an EOV with enough refrigerant on board to passively cool the ice sample for up 28 days in Earth orbit, meaning that it would need to be retrieved no later than 30 November 1988 if Earth-orbit capture took place on 2 November and no later than 12 December if it occurred on 14 November. During the 28-day period, an automated Tug would climb from low-Earth orbit to retrieve the EOV.
Following retrieval, the Tug would convey the sample container to a waiting Shuttle Orbiter or to a laboratory on board an Earth-orbiting space station. Concerns about planetary protection would drive the selection. If risk of terrestrial contamination were judged to be acceptable, then the Shuttle Orbiter would deorbit and transport the sample container to Earth's surface. If, on the other hand, a more conservative approach seemed warranted, then the Mars sample would be subjected to initial examination on the Station.
Purdue's MPISR concept generated considerable interest and demonstrated surprising longevity for a student project. After a summary of the study appeared in the pages of the British Interplanetary Society publication Spaceflight, two of its authors (Staehle and Skinner) briefed JPL engineers on the concept. They discussed the possibilities of "life indigenous to polar ice" on Mars and the significance of detection of "alternate chemistries" of life.
They also adjusted some of the dates of critical MPISR mission events. Departure from Earth orbit, arrival in Mars orbit, and Mars landing would take place as in the original study, but Mars surface liftoff would take place about two weeks early (24 April 1988). Subsequent dates were adjusted accordingly: the MPISR Orbiter/ERV/EOV combination would dock with the third stage of the AV on 9 May 1988; the ERV/EOV would depart Mars orbit on 20 July 1988; the EOV would capture into Earth orbit on 8 November 1988; and 6 December 1988 was the latest sample retrieval date.
In January 1978, JPL new-hire Staehle pitched the scientific benefits of the MPISR plan at the Lunar and Planetary Institute in Houston, Texas. In his presentation, he called "reasonable" the automated acquisition of an ice core up to 200 meters long made up of segments up to 15 millimeters in diameter.
Sources
"Mars Polar Ice Sample Return Mission - 1," Robert L. Staehle, Spaceflight, November 1976, pp. 383-390
"Mars Polar Ice Sample Return Mission, Part 2," Robert L. Staehle, Sheryl A. Fine, Andrew Roberts, Carl R. Schulenburg, and David L. Skinner, Spaceflight, November 1977, pp. 399-409
"Mars Polar Ice Sample Return Mission, Part 3," Robert L. Staehle, Sheryl A. Fine, Andrew Roberts, Carl R. Schulenburg, and David L. Skinner, Spaceflight, December 1977, pp. 441-445
Mars Polar Ice Sample Return Mission, R. Staehle and D. Skinner, Jet Propulsion Laboratory, September-October 1977
Mars Polar Ice Sample Return Mission - Overview, R, Staehle, Jet Propulsion Laboratory, January 1978
More Information
Bridging the 1970s: Lunar Viking (1970)
Prelude to Mars Sample Return: the Mars 1984 Mission (1977)
Mars Airplane (1978)
Making Propellants from Martian Air (1978)
Catching Some Comet Dust: Giotto II (1985)
Safeguarding the Earth from Martians: The Antaeus Report (1978-1981)
I remember those Viking landings and the first pictures well. A very special moment indeed.
ReplyDelete