editorial Small in size only Planets and their systems have long held the spotlight, but researchers, space agencies and even the private sector and the public have turned their attention to small bodies.

stronomy in 2019 started with a collaboration), a spacecraft that will lurk at System, compositionally, structurally bang: the flyby of the farthest body a Lagrange point until a special target — a and dynamically. Aever visited by a human artefact. The pristine comet, or maybe an interstellar Beyond science, the community is also close approach of the New Horizons probe object — will pass by. The probe will then motivated by planetary defence, for which to the recently named Arrokoth, a contact be ready to approach it and perform close we need to characterize near-Earth asteroids binary Kuiper belt object, allowed scientists observations. Other agencies are also active: in order to understand what can be done to to look for the first time at a pristine body, JAXA has laid down a full small-bodies avert a catastrophic impact on our planet. probably undisturbed for the past 4 billion programme (see the Comment by Masaki Both NASA and ESA are stepping up their years or so. Fujimoto and Elizabeth Tasker) and China efforts, which translates into the approval We are living in a golden age for small announced plans to launch in 2022 an of new missions. Two spacecraft that had bodies, as various space exploration events ambitious mission involving flybys of an been previously rejected got approved this of 2019 clearly show. A major theme of the asteroid and a comet, with sample return year. ESA’s Hera, a probe that will look year was sample return. As discussed in a from the asteroid. at the aftermath of the impact created previous Editorial, sample return is more One of the main reasons that small by the NASA mission DART on asteroid than a curiosity or a technical achievement. bodies have become so popular is their Didymos’s moon, got funded in the recent Simple remote sensing or in situ analysis are extreme versatility in terms of science — November 2019 ministerial conference, not able to provide the same level of exemplified by the content in this issue Space19+. The combination of DART and scientific information. NASA’s OSIRIS-REx of Nature Astronomy. In a pair of Letters, Hera, a project called AIDA, will assess our team published their first results from the Johnson et al. and Zhu et al. study specific capability to deflect asteroids in case we monitoring of asteroid Bennu and JAXA’s phenomena that affected the formation need to. From NASA’s side, the Near-Earth Hayabusa2 performed a series of amazing and evolution of asteroids. In particular, Object Surveillance Mission (formerly called manoeuvres throughout the year, which Johnson et al. highlight the possible NEOCam) was finally greenlighted, aimed included retrieving two samples from existence of a new kind of volcanism, called at monitoring the skies to detect NEOs asteroid Ryugu’s surface, creating an artificial ferrovolcanism, that could have created (usually fast and dark, thus difficult to find crater on it, and deploying a second rover, eruptions of metal-rich material from the with telescopes). MINERVA-II-2 (which failed before deploy­ core to the surface of metallic asteroids like Finally, small bodies are considered as ment but was released anyway in orbit to Psyche. Zhu et al.’s laboratory experiments enablers for human exploration. Resource perform some gravitational measurements). explain instead how water ice can be utilization and asteroid mining have started Among the other exciting small-bodies continually generated by space weathering to leave the realms of pure science fiction, events of the year, the discovery of the on airless bodies. An important component being increasingly discussed by scientists, second interstellar object, in September, of small-bodies studies is looking at families agencies and even the private sector in deserves mention. 2I/Borisov, contrary — groups of dynamically related objects practical terms. The recent announcement to 1I/’Oumuamua, has a clear cometary created by some catastrophic impact — as of a Luxembourg Space Resources character, highlighting the fact that the they hold crucial clues on the evolution and Innovation Centre demonstrates that the diversity of small bodies in our Solar System history of the asteroid belt. In an Article, topic is here to stay. Before it can happen, is mirrored in interstellar interlopers. And Carruba et al. study the very young families, a serious discussion concerning resource we were able to detect 2I/Borisov on its way the only ones for which sub-families (groups exploitation, sustainability and ethical issues in, before it reached perihelion, so we will be of bodies generated by subsequent impacts) will need to be established. able to acquire data for much longer than we can be discerned. In their Article, Pike et al. 2020 will also be rich in milestones in could for ‘Oumuamua. also studied a family, but a specific one: small-bodies exploration. In the first half 2019 was also a good year for the future the Haumea family, the only one identified of the year OSIRIS-REx will try to acquire of small-bodies exploration. Both Lucy and so far in the Kuiper belt. The last paper, a sample from the recently chosen landing Psyche, the latest NASA Discovery missions a Letter by Guzik et al., analyses the first site, a place in the northern hemisphere to be selected, passed critical technical detailed observations of 2I/Borisov. Finally, covered in dark regolith called Nightingale. reviews and are on course for their expected the Review Article by Kruijer et al. shows At the very end of the year, Hayabusa2 will launch dates in 2021 and 2022, respectively. how the isotopic composition of meteorites arrive back at Earth with its container of Both missions target small bodies (a tour holds clues on the very first years of the fresh asteroidal material. Interstellar comet of some Jupiter Trojans and the possibly Solar System, with the existence of two non- Borisov will remain visible for most of the metallic Psyche asteroid, respectively), and mixing reservoirs of material (carbonaceous year, probably until September. It’s a safe bet in general NASA likes to send their small- and non-carbonaceous, respectively) that that these small bodies will be big on impact budget missions towards such targets, as generated the diversity of small bodies we in 2020 and beyond. ❐ discussed by Mike A’Hearn in a Comment. observe today. These papers demonstrate There is good news from the ESA side the main role of small bodies in research: too, as it approved the innovative Comet they hold unique clues to the understanding Published online: 14 January 2020 Interceptor mission (actually an ESA/JAXA of the formation and evolution of the Solar https://doi.org/10.1038/s41550-019-1006-6

Nature Astronomy | VOL 4 | January 2020 | 1 | www.nature.com/natureastronomy 1