|The Viking 2 rover on the frosty plain at Utopia. Image credit: Pat Rawlings/NASA|
The Viking missions reinforced this view of MSR, and also revealed the perils of making too many 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. The truth, however, was that no one could be certain what the Viking biology experiment results meant.
With that unsatisfying experience in mind, A. G. W. Cameron, chair of the National Academy of Sciences Space Science Board, wrote in a 23 November 1976 letter to NASA Administrator James Fletcher that
[to] better define the nature and state of Martian materials for intelligent selection for sample return, it is essential that precursor investigations explore the diversity of Martian terrains that are apparent on both global and local scales. To this end, measurements at single points. . .should be carried out as well as intensive local investigations of areas 10-100 [kilometers] in extent.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 several NASA centers, the U.S. Geological Survey (USGS) Astrogeology Branch, 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.
The Mars 1984 mission would also seek answers to "The Biology Question." The MSWG declared that
on-going exploration of Mars must address the issue of biology. Although there does not appear to be active biology at the two Viking landing sites, there may be other localities with special environments conducive to life. Life-supportive aspects of the Martian environment must be defined in greater detail. The characterization of former environments [and] a search for fossil life. . .should be conducted.Mars 1984 would begin in December 1983-January 1984 with two Space Shuttle launches no less than seven days apart. The piloted, reusable Space Shuttle Orbiters would each place into low-Earth orbit a Mars 1984 spacecraft comprising one 3683-kilogram orbiter based on the Viking Orbiter design, three penetrators with a combined mass of 214 kilograms, and one 1210-kilogram lander/rover combination housed in an extended Viking bioshield/aeroshell. Together with an adapter linking it to a two-stage Intermediate Upper Stage (IUS), each Mars 1984 spacecraft would weigh a total of 5195 kilograms.
|A Viking orbiter releases an aeroshell containing a Viking Mars lander. The Mars 1984 orbiter would have a similar design; the aeroshell, however, would stand taller to provide sufficient room for the lander/rover combination within it.|
|Viking aeroshell (left) and Mars 1984 aeroshell. Image credit: Martin Marietta|
The twin Mars 1984 spacecraft would reach Mars from 14 to 26 days apart between 25 September and 18 October 1984, after voyages lasting a little more than nine months. Each would perform a final course-correction rocket burn using attitude control thrusters 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 impact sites on Mars. The motors would then separate from the penetrators.
During MOI, each spacecraft would fire a solid-propellant braking rocket motor, then the orbiter's liquid-propellant maneuvering 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.
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 Mars 1984 landers, based on a Martin Marietta design, would each include a "terminal site selection system." This would steer them away from boulders and other hazards as they descended the final kilometer to the martian surface. In other respects, their deorbit and landing systems would closely resemble those of the Vikings.
After lander separation, orbiter #1 would maneuver to a 500-kilometer near-polar circular orbit and orbiter #2 would move to a 1000-kilometer near-equatorial circular orbit. Orbiter #1's low near-polar orbit would permit global mapping at 10-meter resolution, while orbiter #2's more lofty near-equatorial orbit would enable it to map the equatorial region at 70-meter resolution. Low-flying Orbiter #1 would serve as the radio relay for the six penetrators, which would transmit relatively weak signals, 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 suggested that the orbiters relay to Earth about 80 images of Mars per day.
The MSWG acknowledged that a costly automated lab on an MSR precursor mission might be hard to justify, given that the MSR mission meant to follow it was intended to return samples to well-equipped labs on Earth for detailed 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 scientists might also have worried that the planned MSR mission would be postponed or cancelled, leading them to attempt to exploit every opportunity to acquire new data.
The rovers would store particularly interesting samples for collection during the MSR mission and test the effects of Mars's reactive soil chemistry on MSR sample container materials. They would also each drop off three seismometer/weather stations as they moved over the surface to create a pair of 20-kilometer-wide regional sensor networks.
The rovers would employ three Mars surface operation modes. The first, Site Investigation Mode, would enable "intensive investigation of a scientifically interesting site." The rover would be fully controlled from Earth.
In Survey Traverse Mode, the second mode, the rover would operate nearly autonomously in a "halt-sense-think-travel-halt" cycle. Each survey/traverse 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.
|Valles Marineris with Mars 1984 landing ellipses marked in red and labeled. Image credit: NASA|
New Mars missions stood little chance of acceptance in the late 1970s, when NASA's limited resources were largely devoted to Space Shuttle development and public enthusiasm for the Red Planet was (thanks the equivocal Viking biology 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, the Mars Observer orbiter, 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.
Post-Viking Biological Investigations of Mars, Committee on Planetary Biology and Chemical Evolution, Space Science Board, National Academy of Sciences, 1977
Mars '84 Landing System Definition: Final Report, "Technical Report," Martin Marietta, April 1977
A Mars 1984 Mission, NASA TM-78419, "Report of the Mars Science Working Group," July 1977
Robot Rendezvous at Hadley Rille (1968)
The Russians are Roving! The Russian are Roving! A 1970 JPL Plan for the 1979 Mars Rover
Safeguarding the Earth from Martians: The Antaeus Report (1978-1981)