Sample return missions
DraftSample return is the most expensive way to do planetary science and often the only way to answer the question that was asked. The reasoning is worth setting out, because it explains why agencies keep attempting it despite the cost.
Why return material
Laboratory instruments vastly exceed flight instruments. A mass spectrometer on a spacecraft must fit a few kilograms, run on a few watts, survive launch and operate unattended for years. A terrestrial laboratory instrument has none of those constraints and is orders of magnitude more sensitive and more precise. Isotope ratios that resolve a sample’s age and origin are routinely measured on Earth and are beyond any flight instrument.
Samples can be re-analysed indefinitely. A flight instrument makes the measurement it was designed for and no other. A returned sample can be measured again with techniques that did not exist when it was collected. Apollo sample 73001 was deliberately kept sealed for fifty years and opened in 2022 using methods unavailable in 1972.
Context can be recovered later. A returned sample can be sectioned, photographed, and examined at scales from centimetres to nanometres, with the analysis directed by what earlier analysis found. That iterative process is impossible with a spacecraft.
Ambiguity can be resolved by consensus. Multiple independent laboratories can examine the same material. The Venus phosphine dispute, discussed in Venus, turned on data reduction choices in a single remote measurement. Material in hand is a different epistemic situation.
What has been returned
| Mission | Target | Mass returned | Year |
|---|---|---|---|
| Apollo 11 to 17 | Moon | 382 kg | 1969 to 1972 |
| Luna 16, 20, 24 | Moon | ~301 g total | 1970 to 1976 |
| Genesis | Solar wind ions | Trace | 2004 |
| Stardust | Comet Wild 2 dust | Trace, in aerogel | 2006 |
| Hayabusa | Asteroid Itokawa | ~1500 grains, under 1 mg | 2010 |
| Chang’e 5 | Moon, near side | 1731 g | 2020 |
| Hayabusa2 | Asteroid Ryugu | 5.4 g | 2020 |
| OSIRIS-REx | Asteroid Bennu | 121.6 g | 2023 |
| Chang’e 6 | Moon, far side | 1935.3 g | 2024 |
The masses are strikingly small outside Apollo. Hayabusa returned under a milligram and still produced a substantial body of literature, because the question it answered, whether S-type asteroids are the parent bodies of ordinary chondrite meteorites, needed mineralogy rather than bulk material.
Two of these missions succeeded despite failing. Genesis crashed on return when its parachute did not deploy because an accelerometer had been installed inverted, and much of the science was still recovered from the contaminated fragments. Hayabusa suffered engine failures, a lost lander, a fuel leak and a three-year delay, and returned anyway. Both are arguments for building the capsule robustly and for not abandoning a mission that has degraded.
Mars Sample Return
Mars is the most scientifically compelling target and the hardest. The architecture requires several elements that have never been done at Mars.
Perseverance has already done its part, collecting and sealing sample tubes. Every subsequent box is unprecedented.
| Step | Why it is hard |
|---|---|
| Landing near the cache | Requires precision landing better than any Mars mission has achieved |
| Mars Ascent Vehicle | Nothing has ever launched from another planet’s surface. It must survive years on Mars, then work once |
| Orbital rendezvous at Mars | Autonomous rendezvous with a small passive container, with no ground intervention at 4 to 21 minutes light time |
| Break the chain | The returning container’s exterior must be certified free of Martian material |
| Receiving facility | A containment laboratory that does not exist and would take years to build and certify |
The programme’s difficulty has been programmatic rather than technical. Independent review found cost growth toward the $8 to 11 billion range and a return date slipping into the 2040s, which is unaffordable within a planetary science budget that must also fund everything else. NASA has since sought lower-cost architectures, including commercial proposals.
The episode illustrates the flagship risk described in Mission design and operations: a single directed mission whose overrun consumes the budget of the programme around it.
Planetary protection
Sample return from a body that might harbour life falls under Category V restricted Earth return in the COSPAR framework, which is the most stringent classification that exists.
The requirement is that Martian material must not be released into Earth’s biosphere. In practice this means:
- Break the chain
- Nothing that touched Mars may touch the outside of the returning container. The sample container is sealed and its exterior sterilised in Mars orbit before it enters the Earth-return vehicle.
- Containment on entry
- The entry capsule must survive landing without breaching, including credible off-nominal cases. Parachute failure is assumed rather than excluded, which drives a capsule that survives hard impact.
- Receiving facility
- A laboratory combining biosafety containment, to keep Mars in, with cleanroom conditions, to keep Earth out. These two requirements pull in opposite directions and no facility currently does both.
- Release only after testing
- Samples remain contained until an agreed protocol establishes they are safe, or they are sterilised before release.
The dual requirement in the third row is the genuinely novel engineering problem. A biosafety laboratory maintains negative pressure so nothing escapes; a cleanroom maintains positive pressure so nothing enters. A Mars receiving facility must do both simultaneously, and the sample is scientifically worthless if terrestrial contamination reaches it.
Lunar and asteroid returns are unrestricted, because those bodies are judged to have no potential for life. Apollo nonetheless quarantined its early crews for three weeks, a precaution abandoned by Apollo 15 once it was clear the Moon was sterile.
Curation
Curation is a permanent obligation rather than a project task, and it is routinely under-resourced.
A returned sample must be stored without contamination or alteration for decades, in an inert atmosphere or vacuum, at controlled temperature, with complete documentation of every handling step. It must be catalogued and allocated to researchers worldwide through a process that balances access against preservation.
The Apollo collection remains the model. More than fifty years on, it is still being allocated, still producing results, and portions remain deliberately unopened for future techniques. That policy of holding material in reserve is the clearest institutional recognition that the analysis capability of 2075 will exceed that of today by as much as today’s exceeds 1972’s.
The same discipline now applies to Bennu and Ryugu material, which is distributed internationally with a fraction archived untouched.
References
- National Academies, Assessment of Planetary Protection Requirements for Mars Sample Return Missions, 2009, and subsequent reviews.
- Lauretta, D. S. et al. “OSIRIS-REx: Sample Return from Asteroid (101955) Bennu”, Space Science Reviews, 212:925, 2017.
- Tachibana, S. et al. “Pebbles and sand on asteroid (162173) Ryugu”, Science, 375:1011, 2022.
- Muirhead, B. K. et al. “Mars Sample Return Campaign Concept Status”, Acta Astronautica, 176:131, 2020.
- Mars Sample Return Independent Review Board-2 Final Report, NASA, 2023.
- Shearer, C. K. and Borg, L. E. “Big returns on small samples: Lessons learned from the analysis of small lunar samples”, Chemie der Erde, 66:163, 2006.