Artist's conception of the MoonRise lunar return mission that is a finalist for the current New Frontiers selection.
If you look at the list of missions being considered by the Decadal Survey, a few themes stand out. One is the exploration of the ice and ocean moons of Jupiter and Saturn (Europa, Ganymede, Titan, and Enceladus), another is the exploration of Venus, and a third are sample return missions. (A sprinkling of individual missions to other worlds such as Mercury, Chiron, and Neptune-Triton round out the list.) [Tidbit: One recent article listed the number of missions being considered by the Survey as 28, up from the list of 25 posted on its website. The potential missions to select from may be even more interesting than the current list -- which is already quite an interesting list.]
A number of my blog entries have summarized ideas for all of these themes, but I have given more space to discussing campaigns of missions to ice-ocean moons and Venus than I have a campaign of sample return missions. This blog entry will try to provide balance by considering sample return missions.
The return of samples from solar system bodies has been a focus on planetary mission planning for decades. While the instruments carried by spacecraft are marvels of engineering, they must be built to meet severe constraints for mass, size, and power. Many of the instruments routinely used on Earth exceed the size and mass of the entire spacecraft; in the case of the synchrotrons used to study the Stardust comet grains, the instruments exceeded the size of the launch pad used for the mission. The instruments in laboratories can probe individual grains in exquisite detail and compositional resolution that no spacecraft can. Unlocking many of the secrets of the solar system and its worlds will require bringing back samples to Earth.
Unfortunately, sample return missions are inherently costly. As with a non-sample return mission, a spacecraft must be delivered to its target world. Most mission concepts include a fairly robust suite of instruments to survey the target body and to select optimal sampling sites. (The MoonRise lunar sample return mission is an exception; the fleet of recent missions to the moon already have provided the remote sensing context.) Just to get there and survey the locale requires a fairly capable mission. Then the sample must be collected, which can require complex sample acquisition, handling, and storage devices. The spacecraft, or a portion of it, must maneuver back to Earth and successfully deploy a return capsule with the sample through the atmosphere.
There are a range of mission complexities and corresponding costs. At the low end, the Genesis solar wind and the Stardust comet missions were able to capture their samples in flight. The next step up would be to sample a near Earth asteroid or a comet. Such missions do not require sophisticated ascent vehicles, but the ultra-low gravity and poorly understood surfaces create their own engineering problems. A sample return from our moon would require a capable lander and ascent vehicle. And at the far end of the scale would be a Mars sample return mission that would require multiple launches and a small flotilla of craft to gather and return the samples.
Five possible targets for sample return missions are being studied by the Decadal survey: the moon, a near Earth asteroid, a comet, Enceladus, Mars. The first three targets possibly could be accomplished within the budget of a New Frontiers mission (~$650M). Comet sampling missions could range across a wide range of prices from mission that collects dust during repeated low-speed passes above the nucleus (Discovery class?), to a mission that returns a warm sample with volatiles in liquid form (New Frontiers?), to a mission that returns frozen volatiles (perhaps 2X New Frontiers cost?). At the high end of the missions would be a Mars sample return mission at a possible cost of $6-7B. (I have yet to see a cost estimate for an Enceladus sample return that would collect ice particles while passing through that moon's geysers.)
The Decadal Survey could decide that further missions to flyby, orbit, and land on these worlds for remote sensing and in-situ studies are unlikely to provide more than incremental increases in our knowledge of these worlds. By the time a sample return mission could be flown to each target, we will have orbited or rendezvoused with three asteroids and flown by several more, have rendezvoused with and landed on a comet and flown by several more, have studied the moon in depth from orbit and sampled the near side, and will have orbited and landed on Mars many times. A revolutionary increase in our knowledge may require bringing home samples.
What might a sample-return focused set of missions look like? At 2011 spending levels (adjusted for inflation), NASA will have ~$13B to spend on planetary missions in the coming decade. Here is a possible breakdown of missions and possible costs (costs are best guesses from estimates published in various sources; many are probably wrong):
Near Earth asteroid sample return $1.2B*
Warm comet sample return $1.2B*
Lunar sample return $1.2B*
Mars Trace Gas Orbiter** $0.5B
Mars sample cache rover $2.5B
Technology development for
subsequent Mars sample return
$8.2B from a projected decade budget of ~$13B
*Assuming the mission could be done for the fully burdened cost of a New Frontiers mission
**Needed to image landing sites and serve as data relay for the Mars sample return missions; cost is a guess and probably doesn't reflect NASA's planned costs
****A guess and possibly low
Editorial thoughts: A program focused on ice-ocean moons, Mars, or sample returns would be intellectually sound. Where the Decadal Survey ultimately decides to place its focus will be known in a few months. I've discussed this with a few members of the planetary science community. In many respects, the mission mix reflects which scientific specialties receive favor. Missions to ice-ocean moons, for example, favor scientists with expertise in remote sensing and in-situ instruments. Sample return missions favor scientists with expertise in laboratory analysis and laboratories with the right instruments. The Survey may ultimately decide on a program that is a mixture of elements to serve the needs of all groups.
Long term readers of this blog know that I am skeptical that a Mars sample return mission will every fly. That's not because I don't believe that a sample return would return scientific knowledge equal to the price tag. Rather, I doubt that the political systems of space faring nations will foot the cost short of a previous mission finding clear signs of possible life, past or present. (Note to my Congressman and Senators: Feel free to prove me wrong.)
Whatever goals the Decadal Survey sets for NASA, the Japanese space agency is planning a second near Earth asteroid mission and the Russian space agency is planning a sample return from the Martian moon Phobos. The ESA is planning to issue a call for another round of proposals for medium sized science missions, and a near Earth asteroid mission is likely to be proposed again. Two of the finalists for the current NASA New Frontiers mission are sample returns (from the moon and a near Earth asteroid). It would seem that sample return missions will be playing an increasingly large role in programs of planetary exploration.