The Importance of Sample Return Missions in the Solar System
Because humankind has never traveled into space beyond the Moon, most of the exploration of our solar system is done using robotic spacecraft. NASA has been conducting such exploration for over half a century and has explored planetary bodies from Mercury out to the Kuiper belt. But the solar system is an enormous place with a huge diversity of bodies, including giant, gaseous planets, terrestrial planets, dwarf planets, rocks and icy small objects, satellites of all sizes around these bodies, all the way down to dust. Despite 50 years of effort, NASA, now accompanied by other space-faring nations, is still only just beginning to fully explore the solar system. The OSIRIS-REx mission is a huge step in this massive endeavor.
Robotic exploration of any location in space is tailored to our science objectives and advances as we learn more and more about that target body. NASA has flown by objects like the ice giant planets Uranus and Neptune, and Kuiper belt objects like Pluto and Arrokoth; we’ve orbited Mercury, Venus, Jupiter, and Saturn, as well as the dwarf planets Vesta and Ceres, and asteroid Eros; we’ve landed on Mars, and our international partners have done the same on Venus, an asteroid, and a comet. But the climax of robotic exploration — sample return — has been much harder to achieve. NASA did it non-robotically on the Moon with the Apollo program, which was followed by several Soviet robotic missions. We did it for a comet in the Stardust mission, collecting greatly altered dust in a high-velocity flyby through the tail. We collected material from the Sun in the Genesis mission, not by touching the Sun, but by passively collecting solar wind in the vicinity of Earth. And Japan heroically survived adversity near an asteroid, as Hayabusa succeeded in collecting and returning microscopic dust particles. Other than a few dust grains embedded in orbiting spacecraft, this is the extent of human sample return missions to date. All other samples of extraterrestrial material that we have in scientific collections were delivered to Earth by natural processes, through its atmosphere, being compromised to varying extents in the process, and only in special cases do we know the point of origin.
Why is sample return so important? Although we can learn a huge amount about planetary bodies by remote sensing with cameras, spectrometers, and other instruments, and we can learn even more with instruments with in situ exploration, most sample science cannot be done this way. Exquisitely sensitive laboratory instruments on Earth are capable of determining the chemical, isotopic, mineralogical, structural, and physical properties of extraterrestrial samples from the macroscopic level down to the atomic scale, frequently all on the very same sample. This allows us to determine the origin and history of the material and answer questions far beyond the reach of current robotic technology. Moreover, sample return provides us with “ground truth” about the visited body, verifying and validating conclusions that can be drawn by remote sensing (both Earth-based and by spacecraft) and via landed instruments on other bodies. In addition, returned samples can be compared to astromaterials such as meteorites and cosmic dust, which give us clues about where those materials come from, potentially increasing their scientific value as natural space probes. And finally, returned samples can be preserved for decades and used by future generations to answer questions we haven’t even thought of yet using laboratory instruments that haven’t even been imagined. Thanks to the policies of NASA and other space agencies, returned samples are freely made available to qualified scientists around the world to study.
The 2020s promise a bounty of new sample returns, and some are even calling it the “Decade of Sample Return.” Leading off will be the return of samples from two small, carbonaceous asteroids, Ryugu, by JAXA’s Hayabusa2 mission in late 2020, and Bennu, by NASA’s OSIRIS-REx mission in 2023; these are in many ways sister missions, both in the kind of body being visited, and in the close cooperation of scientists and the sponsoring agencies. OSIRIS-REx promises to provide the largest sample returned since Apollo. The feature article in this issue explains some of the great science OSIRIS-REx hopes to achieve, including learning about organic material that may have played a role in the origin of life. NASA is aiming to return to the Moon in the mid-2020s and hopes to return samples from previously unexplored regions, including the lunar south pole, which may have deposits of water ice within the regolith of permanently shadowed regions. China is also planning to return samples from the Moon in the early part of the decade as part of the Chang’E 5 mission. JAXA’s Martian Moons eXploration (MMX) mission late in the decade, with NASA participation, hopes to return samples from Mars’ moon, Phobos. And the Mars 2020 mission, which launches in July 2020, begins a multi-mission, decade-long campaign to return samples from the surface of the Red Planet, which will help answer the question of whether Mars ever hosted life. It is an exciting time to be a planetary explorer!
— Lori S. Glaze, Director, NASA’s Planetary Science Division, January 2020