Even before Viking 1 landed on Mars (20 July 1976), NASA and its contractors studied post-Viking robotic Mars missions. Prominent among these was Mars Sample Return (MSR), considered by many to be the most scientifically significant robotic Mars mission.
The Viking missions reinforced this view of MSR, and also revealed the perils of making assumptions when planning costly and complex Mars exploration missions. The centerpiece of the $1-billion Viking mission, a briefcase-sized package of three biology experiments, yielded more questions than answers. Most scientists interpreted their data as evidence of previously unsuspected reactive soil chemistry, not biology.
With that unsatisfying experience in mind, A. G. W. Cameron, chair of the National Academy of Science Space Science Board, wrote in a 23 November 1976 letter to NASA Administrator James Fletcher that
Soon after Cameron wrote his letter, NASA Headquarters asked the Jet Propulsion Laboratory (JPL) to study a 1984 MSR precursor mission. The JPL study, results of which were due by July 1977, was meant to prepare NASA to request "new start" funds for the 1984 mission in Fiscal Year 1979. NASA also created the Mars Science Working Group (MSWG) to advise JPL on the mission's science requirements. The MSWG, chaired by Brown University's Thomas Mutch, included planetary scientists from NASA, the U.S. Geological Survey (USGS), and Viking contractor TRW.
The MSWG's July 1977 report called the Mars 1984 mission the "next logical step" in "a continuing saga" of Mars exploration and a "required precursor" for an MSR mission, which it targeted for 1990. Mars 1984 would, it explained, provide new insights into the planet's internal structure and magnetic field, surface and sub-surface chemistry and mineralogy ("especially as related to the reactive surface chemistry observed by Viking"), atmosphere dynamics, water distribution and state, and geology of major landforms.
Mars 1984 would also seek answers to "The Biology Question." According to the MSWG's report,
Mars 1984 would begin in December 1983-January 1984 with two Space Shuttle launches. Each would place into low-Earth orbit a Mars 1984 spacecraft comprising one 3683-kilogram orbiter, three penetrators with a combined mass of 214 kilograms, and one 1210-kilogram lander/rover combination. The orbiter would serve as the spacecraft bus during interplanetary travel, providing propulsion, power, and communications to the lander/rover and penetrators. Together with an adapter linking it to a two-stage Intermediate Upper Stage (IUS), each Mars 1984 spacecraft would weigh 5195 kilograms.
The Shuttle orbiters would each deploy a spacecraft/IUS combination from its payload bay, then would maneuver away before IUS first-stage ignition. The MSWG calculated that the IUS would be capable of placing 5385 kilograms on course for Mars on 2 January 1984, near the middle of a launch window spanning 28 days.
The twin Mars 1984 spacecraft would reach Mars from 14 to 26 days apart between 25 September and 18 October 1984, after voyages lasting about nine months. Each would perform a final course-correction burn a few days before planned Mars Orbit Insertion (MOI). Their penetrators would separate two days before MOI and fire small solid-propellant rocket motors to steer toward their target landing sites. The rocket motors would then separate.
During MOI, each spacecraft would fire a solid-propellant braking rocket motor, then the orbiter's chemical-propellant engine would ignite to place it into a 500-by-112,000-kilometer "holding" orbit with a five-day period. Spacecraft #1's orbit would be near-polar, while spacecraft #2 would enter an orbit tilted from 30° to 50° relative to the martian equator. MOI completed, flight controllers would turn the orbiter's cameras toward Mars to assess weather conditions ahead of lander separation.
At about the time the twin spacecraft entered their respective holding orbits, the six penetrators would impact at widely scattered points. Each would split at impact into two parts linked by a cable. The aft body, which would include a weather station and an antenna for transmitting data to the orbiters, would remain on the martian surface after impact. The fore body would include a drill for sampling beneath Mars's surface and a seismometer. According to the MSWG, penetrators were "the only economic means" of establishing a Mars-wide sensor network.
After several months in holding orbit, spacecraft #2 would move to a 300-by-33,700-kilometer "magneto orbit," where it would explore Mars's magnetospheric bow wave and tail. It would then maneuver to a 500-by-33,500-kilometer "landing orbit" with a period of one martian day (24.6 hours). During a one-month landing site certification period, scientists and engineers would closely inspect orbiter images of the candidate landing site. Spacecraft #1, meanwhile, would proceed directly from holding orbit to landing orbit.
The primary purpose of the landers would be to deliver the Mars 1984 rovers to Mars's surface. Lander #2 would set down first at a high latitude, and lander #1 would land near Mars's equator at least 30 days later. JPL estimated that imaging data from the Viking orbiters would enable each Mars 1984 lander to set down within an "error ellipse" 40 kilometers wide by 65 kilometers long (for comparison, Viking's ellipse neasured 100 kilometers wide by 300 kilometers long). The Mars 1984 landers would each include a "terminal site selection system" that would steer them away from boulders and other hazards as they descended the final kilometer to the martian surface, but in other respects their de-orbit and landing systems would closely resemble those of the Vikings.
After lander separation, orbiter #1 would maneuver to a 500-kilometer circular orbit and orbiter #2 would move to a 1000-kilometer circular orbit. Orbiter #1's low near-polar orbit would permit global mapping at 10-meter resolution, while orbiter #2's higher near-equatorial orbit would enable it to map the equatorial region at 70-meter resolution. Orbiter #1 would serve as the radio relay for the six penetrators, while orbiter #2 would relay signals to and from the twin rovers.
The MSWG expected that most orbiter science operations would require minimal planning, since they would "be highly repetitive with most instruments acquiring data continuously and sending it to Earth in real time without tape recording." The exception would be imaging operations, since imaging data would be "acquired at a rate many times too great for real-time transmission." The MSWG proposed that the orbiters should relay to Earth about 80 images of Mars per day.
The MSWG envisioned that the Mars 1984 rovers would be "substantial vehicles" capable of traveling up to 150 kilometers in two years at a rate of 300 meters per day. Each would include four "loop-wheel" treads on articulated legs, a radioisotope thermal generator providing heat and electricity, laser range-finders for hazard avoidance, an "improved Viking-type manipulator" arm, twin cameras for stereo imaging, a microscope, a percussion drill for sampling rocks to a depth of 25 centimeters, and a sample processor for distributing martian materials to an on-board automated laboratory for analysis.
The MSWG acknowledged that a costly automated lab might be hard to justify on an MSR precursor mission, given that the MSR mission would be intended to return samples to Earth labs for analysis. The group argued, however, that clues to the nature of the reactive soil chemistry found by the Vikings might "reside in loosely bound complexes or interstitial gases" that "would be extraordinarily difficult to preserve in a returned sample." The rovers would also hold samples for later collection by the MSR mission and would test the effects of martian soil chemistry on MSR sample containers. The rovers would also each deploy three seismometer/weather stations to create a pair of 20-kilometer-wide regional sensor networks.
The rovers would employ three mission modes. The first, Site Investigation Mode, would permit "intensive investigation of a scientifically interesting site." The rover would be fully controlled from Earth.
In Survey Traverse Mode, the rover would operate nearly autonomously in a "halt-sense-think-travel-halt" cycle. Each cycle would last about 50 minutes and move the rover forward from 30 to 40 meters. Science operations would occur during the "halt" portion and while the rover was parked at night. Flight controllers would update rover commands once per day. The rover would cease autonomous operations and alert Earth when it encountered a hazard or a feature of scientific interest.
The third mode, Reconnaissance Traverse Mode, would occur when the terrain was sufficiently smooth (and scientifically dull) to allow the rover to move at its top speed of 93 meters per hour. The rover would make few science stops and would travel both by day and by night.
To conclude its report, the MSWG drew on USGS studies based on Mariner 9 and Viking orbiter data to offer two candidate landing sites for the Mars 1984 landers. Capri Chasma, at the eastern end of near-equatorial Valles Marineris, included heavily cratered (thus ancient) highlands terrain, lava flows of different ages, lava channels, and possible water-related channels and deposits. Candor Chasma, a north-central branch of Valles Marineris, included at least two rock types in its four-kilometer-high canyon walls. The group expected that a Mars 1984 rover might be able to sample ancient crystalline rocks on the canyon floor.
New Mars missions stood little chance in the late 1970s, when NASA's resources were devoted mainly to Space Shuttle development and public enthusiasm for the Red Planet was (thanks the equivocal Viking results) at a nadir. Though MSR remained a high scientific priority (as it does today), the planetary science community opted to seek support for missions to other destinations: for example, the Jupiter Orbiter and Probe mission, later renamed Galileo, got its start in NASA's Fiscal Year 1978 budget. NASA's next Mars spacecraft, Mars Observer, was approved in 1985 for a 1990 launch; launch was subsequently postponed until September 1992, then the spacecraft failed during Mars orbit insertion in August 1993. NASA would return successfully to Mars for the first time since Viking in July 1997, when the 264-kilogram Mars Pathfinder spacecraft landed in Ares Valles bearing the 10.6-kilogram rover Sojourner.
References:
A Mars 1984 Mission, NASA TM-78419, Mars Science Working Group, July 1977.
