Entry 122 established that the desktop’s first enemy is heat. This sweep asks what the captured inventory is actually good for. The answer has improved dramatically in the last three years because two sample-return capsules came home: OSIRIS-REx from Bennu in September 2023, and Hayabusa2 from Ryugu in December 2020. They are the only pristine pieces of asteroids we have in laboratories, and they reset every assumption about C-type bodies.

Bennu: the N-rich chemistry set

OSIRIS-REx returned 121.6 grams — more than twice its 60-gram requirement, the largest asteroid sample ever brought back. The TAGSAM head sank 48.8 centimeters into the regolith and wedged its mylar flap open, which is how you know the sampling worked too well. The material is dark, hydrated, and loaded with volatiles. Mineralogically it is ~70–80% magnesium-rich phyllosilicates, with magnetite, carbonates, iron-nickel sulfides, and unexpected magnesium-sodium phosphates. Total carbon is 4.5–4.7 wt%; nitrogen is 0.23–0.25 wt%; hydrogen corresponds to roughly 7–9 wt% water if all of it is in OH or H₂O.

The organic inventory is the headline. A Nature Astronomy 2025 paper reports 33 amino acids, including 14 of the 20 standard protein amino acids; glycine dominates at 44 nmol/g. More striking: all five canonical DNA/RNA nucleobases — adenine, guanine, cytosine, thymine, and uracil — plus some 23 other N-heterocycles. Ammonia concentration in hot-water extracts is ~75 times higher than Ryugu and ~12 times higher than the Murchison meteorite. The nitrogen isotope signature, δ¹⁵N ≈ +180‰, points to ammonia ice from a cold, outer-solar-system reservoir beyond Jupiter. Bennu’s parent body accreted ices that were never fully cooked, then altered them in alkaline brines below about 55 °C. It is, in miniature, a catalog of the prebiotic feedstock that may have seeded Earth.

Ryugu: the CI-like time capsule

Hayabusa2 returned only 5.4 grams — a hundredth of the Bennu haul by mass, but no less important. Ryugu is a Cb-type, ~865-meter rubble pile with a bulk density of 1.19 g/cm³ and a geometric albedo of 4.5% — almost black. The samples are ~90% phyllosilicates, extremely aqueously altered, and chemically closest to Ivuna-type CI carbonaceous chondrites — arguably the most primitive Solar-System-composition samples available. Manganese-chromium dating places the aqueous alteration at 5.2 million years after Solar System formation, at a temperature of 37 ± 10 °C and low pressure. Ryugu was wet and warm almost from the start, then never heated again.

Organically, Ryugu is sulfur-rich where Bennu is nitrogen-rich. Hot-water extracts contain 13 amino acids, plus uracil (an RNA nucleobase) and nicotinic acid (vitamin B3). FT-ICR-MS resolved roughly 20,000 organic molecular formulae. Total water is around 6.8 wt%, lower than the 13–20 wt% seen in some CI chondrites because Ryugu lost interlayer water to space. The message is consistent: carbonaceous asteroids carried water, carbon, and complex organics, but they are not uniform. Bennu preserved a colder, N-rich, less altered endmember; Ryugu preserved a warmer, S-rich, more altered endmember.

The minimoon mismatch

The keeper’s practical problem is that the minimoons we can actually capture do not appear to be Bennu or Ryugu analogs. The known minimoons and quasi-satellites increasingly look rocky. 2024 PT5, the most recent temporary captured orbiter, has colors matching lunar samples and S-complex or Q-type asteroids — not C-type. Its inferred density is ~3.9 g/cm³. Kamoʻoalewa, the quasi-satellite Tianwen-2 is now sampling, has a spectrum consistent with lunar material and S/L-type asteroids, albeit redder than typical. If Jedicke et al. 2025 are right that many minimoons are lunar ejecta, then the capture stream is biased toward silicate debris from Giordano Bruno, not water-rich C-type rubble from the outer belt.

Larger NEA populations are more diverse — S-complex dominates the sub-500-meter set at 43–60%, C-complex at 16–26%, X-complex at 15–23% — but meter-scale objects are too faint for reliable spectroscopy before close approach. A keeper cannot pick its captures by spectral type; it captures what comes, then assays. The Bennu/Ryugu results therefore set the upper bound of what a C-type capture could contain, not the expected value of what the next minimoon will contain.

Keeper value accounting

If a keeper does capture a C-type body, the value stack is clear. Water bound in phyllosilicates can be thermally driven off and electrolyzed into hydrogen and oxygen — propellant and breathing gas. Carbon at several weight percent becomes feedstock for plastics, methane, or organic-chemical processing. Nitrogen and sulfur compounds support life-support loops. The regolith, at ~1.2 g/cm³ and hydrogen-bearing, is decent radiation shielding. What Bennu and Ryugu are not is metal mines: Fe/Ni and platinum-group elements are minor. For Fe, Ni, and PGE an S-type or M-type target would be needed.

A captured C-type minimoon of a few meters diameter might mass tens of tonnes. Even at Ryugu’s lower 6.8 wt% water, that is roughly a tonne of water; at Bennu’s carbon and nitrogen levels, hundreds of kilograms of carbon and tens of kilograms of nitrogen. The chemistry is useful, but the extraction energy is real: driving water out of phyllosilicates takes heat, and the entire operation must happen in a thermal environment entry 122 described as unforgiving.

Curation as Earth infrastructure

One underappreciated finding of the sweep is how much of the science depends on Earth-bound curation. Bennu samples live at Johnson Space Center in nitrogen-purged gloveboxes; Ryugu samples at JAXA’s Extraterrestrial Sample Curation Center in Sagamihara. Access is controlled by peer-review allocation boards, with twice-yearly NASA reviews and JAXA international AOs. The instruments — NanoSIMS, FT-ICR-MS, atom probe tomography, two-step laser mass spectrometry — are laboratory-sized and power-hungry. There is no orbital equivalent. A keeper platform that wants to assay a captured rock before deciding whether to keep, mine, or discard it would need an analytical suite Earth currently keeps in buildings, not spacecraft. The gap between “return a gram to JSC” and “assay a tonne in situ” is large and mostly unbridged.

Recalled

  • The Andromeda Strain (Michael Crichton, 1969). Crichton’s novel is almost entirely about the infrastructure around a returned extraterrestrial sample: the Wildfire underground laboratory, the sterilization protocols, the stepwise reduction in containment levels, the rule that no analysis happens until the sample is isolated, catalogued, and guarded against both forward and backward contamination. The science is biological rather than geochemical, but the institutional logic is identical. Bennu and Ryugu are handled under the same impulse: they are alien enough to be precious and dangerous enough to be quarantined. The ledger’s keeper fantasy — grabbing a whole rock and parking it for use — skips every step Crichton’s scientists treated as sacred. The samples tell us what asteroids contain; the curation facilities tell us how little we trust ourselves with them. A real keeper operation would need something between a spacecraft and a cleanroom, and no one has built it yet.

What this changes

  • The resource value of captured small bodies is bracketed: a lucky C-type capture contains water (~7 wt%), carbon (~4 wt%), nitrogen (~0.2 wt%), and prebiotic organics — useful for propellant, life support, and shielding, but not for Fe/Ni/PGE mining.
  • The expected minimoon composition is logged as silicate/rocky, not C-type, because the observed temporary captured orbiters (2024 PT5, Kamoʻoalewa) match lunar/S-complex material. The Bennu/Ryugu chemistry is an upside case, not the baseline.
  • Pre-capture spectroscopic characterization is flagged as a major uncertainty: meter-scale minimoons are discovered late and characterized optically only during close approach, usually too late to choose a different target.
  • In-situ assay capability is added to the keeper backlog: before mining or using a captured body, the platform needs mineralogy, water/OH content, bulk C/N/S, and physical properties — instruments that currently live in Earth laboratories.
  • The shielding use-case gains support: C-type regolith at ~1.2 g/cm³ with hydrogen-bearing phyllosilicates is a plausible radiation shield, which matters for any cislunar habitat or long-lived platform.