“This was a very challenging observation for the instrument. The comet is around 10 000 to 100 000 times fainter than our usual target.”This kind of description by Nick Thomas of a camera being pushed to the outside of its comfort zone has become a common theme in the modern world of engineering: the best data is often obtained when one has hardware that is forced to perform something that it was never designed to perform.
That is precisely the type of stress test that Comet 3I/ATLAS provided. Through Earth measurements and other astrometry measurements by the ESA Trace Gas Orbiter around Mars, the path of the comet was reduced by ten times. The geometry was more than the glamour: a second observing baseline, tens of millions of kilometres distant, had made a faint blur into an object located precisely. That accuracy is non-academic. At velocities of up to 250,000 km/h, uncertainty grows rapidly and the estimates of the trajectory have to incorporate also non-gravitational nudges due to asymmetric outgassing.
The Mars observation point also highlighted the embarrassing aspect of seeing visiting fast food. CaSSIS camera on ExoMars TGO was unable to distinguish between the nucleus and the coma with a distance approximate of 30 million km; ESA likened this to the view of trying to identify a mobile phone on the Moon with the help of the earth. What it was able to see was the coma, the halo a few thousand kilometres in diameter formed because the sunlight makes the comet live by pushing away its gas and dust. Even the summary by ESA observes that the coma can easily be seen when the tail is too faint to be isolated on the noise.
It is composition, not position, that makes 3I/ATLAS really alien. A ratio of CO 2/H 2 O has been determined by spectroscopy at 7.6 (0.3) which is much higher than the expected ratio in Solar System comets at similar distances. A suggestion is the long-period exposure to galactic cosmic rays which in the laboratory have been suggested to convert CO to CO 2 and a crust-like layer rich in organic matter. The engineering impact is subtle yet significant: when outgassing is dominated by sampling an irradiated outer shell of about 1520 metres in depth, then instruments can give readings exquisitely precise, although they still cannot give a single read out of the most pristine material in the inside. The comet is a natural thing, as NASA scientist Tom Statler said: It looks like a comet. It is comet things… the testimony is too much against this body being a natural one.
It is that of measurement and misdirection–of seeing clearly but not always seeing what matters most–that pervades also a very divergent direction of enquiry: Neanderthal anatomy. A 3D reconstruction of the skull of an Altamura Man in Italy rekindled an old popular theory that the giant Neanderthal noses were heat exchangers in ice age air. The fine internal architecture which would bear such a statement was not discovered, and Todd Rae summed up the change: two out of three of the earlier-suggested distinct characteristics of the Neanderthal nasal cavity do not seem to exist in this specimen. The more lasting educational point is methodological: the existence of a dramatic survival of a weakly preserved structure, one that is generally not fossilised, can topple decades of theory about the ways that adaptation might have happened.
Fragility is not uncommon at all in orbit, it is the default position. The Tiangong station incident, where it was found that parts of a returning capsule window were craked in a pattern suggesting impact by debris, was a reminder of the amount of material that is required to pose a risk to a crewed system in cases where relative velocities are over 7 km/s. According to one of the television reports, Shenzhou designer Jia Shijin said, we initially assumed it was something less than 1 millimetre in size, but quite fast moving. The crack that results goes more than a centimetre long.
The longer-term trends of debris increase that vulnerability to become an outlier harder to treat. ESA approximates a figure of approximately 40,000 objects in orbit with an estimated figure of more than 1.2 million objects that are bigger than 1 cm. It also reports that intact satellites or rocket bodies are now regularly re-entering the atmosphere of Earth more than three times on average daily, an indication that it is indeed being heavily used in orbit as well as of better compliance with disposal. However, net growth is still indicated in the modelling made by ESA: even without new launches, the fragmentation can even add debris at the same rate as the drag is removing it, which confirms the argument advocating active debris removal and stricter end-of-life regulations.
Through these stories, spectacle is not the connecting thread. It is the trained increase in prowess it will be the orbiter of Mars as a tracking beam, it will be the image of medicine interrogating ancient bone, it will be the restructuring of working margins around debris that cannot be entirely observed or consistently evaded.
