Throughout the 1980s, engineers at the Jet Propulsion Laboratory in Pasadena, California, and at NASA's Johnson Space Center in Houston, Texas, worked with planetary scientists and contractor engineers to develop what came to be called a Mars Rover Sample Return (MRSR) mission for the 1990s. The MRSR mission would have seen a sophisticated large rover land on Mars and roll over the surface for tens or even hundreds of kilometers. A Mars-orbiting satellite with a massive telescopic camera for traverse route imaging would have helped engineers and scientists select the safest, most scientifically productive path across Mars's surface, and a powerful communications relay orbiter would have kept controllers on Earth in constant touch with the rover.
The rover, which might have weighed several tons, would have carried a complex suite of sensors and tools to enable collection of a suite of geologic samples representative of a large area of Mars. The samples would have been sealed in a container, transferred to an ascent vehicle, and launched into Mars orbit, where they would have been handed off to an orbiting Earth Return Vehicle (ERV). The ERV would have installed the sample container in an aeroshell and launched it to Earth, where it would have aerobraked into orbit for Space Shuttle or Space Tug recovery. Some plans called for a separate quarantine space station for preliminary sample analysis.
The mission was very complex, with many opportunities for malfunctions, so to help to ensure its success all MRSR vehicles would have been redundant. This would have required multiple Space Shuttle or expendable rocket launches and possibly assembly at NASA's Earth-orbiting Space Station. It is not to surprising, then, that a 1988 independent cost estimate placed the MRSR mission's cost at $13 billion. Including the precursor orbiter-rover-penetrator mission some declared to be necessary would have driven the cost of returning a kilogram or two of Mars to Earth even higher.
The 1980s MRSR debacle impressed on many the idea that automated return of Mars samples must be very costly. Before the 1980s were out, however, groups within JPL and JSC and their contractors, as well as independent scientists and engineers, sought less costly methods of sampling Mars. Most sought to eliminate the large rover in favor of a lander that would collect samples only within reach of its robot arm. At least one sought to eliminate even the lander.
In a brief paper in the April 1989 issue of Journal of Spacecraft and Rockets, Alan Stern, a researcher at the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder, noted that scientists using data from the Mariner 9 orbiter - which arrived at Mars on 14 November 1971 during a dense, long-lasting, global martian dust storm - had observed that seasonal dust storms loft fine-grained material from Mars's surface up to 60 kilometers into its thin atmosphere. The twin Viking Orbiters also observed high-altitude dust. Stern then proposed a novel, bare-bones approach to Mars sample collection: that a Mars-orbiting spacecraft lower a "collection platform" on a sturdy tether to an altitude of 50 kilometers above the surface during a seasonal dust storm.
Stern estimated that his Mars Tethered Sample Return scheme could gather a 100-gram sample of airborne Mars dust in 55 hours. He acknowledged that atmospheric drag on the tether and collection platform would slow the Mars orbiter, causing it to lose orbital altitude. He calculated, however, that its altitude would decrease at a rate of only five kilometers per kilogram of dust collected. Tether and platform erosion by high-velocity dust impacts might be of greater consequence, he wrote.
With sample collection complete, the orbiter would reel in the platform and dust sample and load the latter into a reentry capsule. An ERV would then launch the capsule out of Mars orbit to waiting scientists on Earth.
Stern's Mars Tethered Sample Return proposal did not influence NASA Mars Sample Return planning. Partly this was because its "random sample" approach could not permit material to be collected from specific known sites on Mars. Instead, it would collect dust grains that had potentially blown from sites all over the planet. Without knowing where the samples originated, researchers could not use them to characterize specific geologic units on Mars.
In the nearly quarter-century since 1989, however, the science and technology of small-particle sample collection and analysis have made great strides. Particles the Stardust comet sample-returner captured intact from Comet Wild 2 in January 2004 and returned to Earth in January 2006, for example, have yielded invaluable data about the nature of comets and the regions of space through which they travel. Given the large quantity of geologic* *data Mars orbiter and lander spacecraft have collected since Stern wrote his paper - data that could provide at least a general context for very small, randomly collected samples - it seems possible that, were it to be carried out now, his proposed Mars Tethered Sample Return mission could yield a Mars sample with a value at least commensurate with its likely low cost.
Reference:
"Mars Tethered Sample Return, S. Alan Stern, Journal of Spacecraft and Rockets, Vol. 26, No. 4, April 1989, pp. 294-296.
