Euclid is a major, international, cross-disciplinary mission. On its face, it is an ESA space telescope to untangle the mysteries of dark matter and the history of the cosmos, by: 1) measuring the distortions of distant galaxies, galaxies which shine at us from behind dark matter and from earlier in cosmic history, and 2) catching supernovae and other standard candles… candles which may look different in early cosmic history. The difference is the expansion of space itself, which may have accelerated, decelerated, or both under the pulls of dark matter and dark energy.
Dig a little deeper though, and you’ll see: 1) Euclid is not ESA’s baby, strictly- the infrared detectors were contributed (at cost) by JPL, and the project’s staff span the world’s major astronomical institutions, and 2) a telescope like Euclid, good at catching supernovae from afar, must be pretty good at catching other things, too. Hence, posting here: in the process of surveying the sky for cosmic candles, we will also spot cosmic vandals like threatening asteroids.
Last week, the Euclid Consortium posted the first major results online: Euclid Quick Data Release 1, or Q1 for short. After launch in mid-2023, and inflight calibration, initial papers appeared in May 2024, detailing telescope and instrument checkouts. All seemed okay, except for a slight scare- the optics grew a fog. It seems water or similar volatiles were taken from Earth, then released once in vacuum. The volatiles re-condensed on the optics, clouding them. Fortunately, Euclid has thermal control via heaters, and the heaters could reach a temperature that drove off the fogging. After two ‘defrost’ cycles, all appears great- the end data is within spec. Now, after another cycle of roughly 6 months (plus writing time and redrafts), it’s time for the actual flight data of this first Euclid observing run. On its face, it seems the released papers include no asteroid results.
Dig a little deeper though, and you’ll see the data is very encouraging. Euclid is not just functioning, but if anything exceeding its stated mission requirements. And the results that did get released bode well for the asteroids:
Tarsitano, F. Fotopoulou, S. Banerji, M. et al. Euclid Quick Data Release (Q1): First study of red quasars selection arxiv.org 2503.15319 …also submitted to A & A
Bisigello, L. Rodighiero, G. Fotopoulou, S. et al. Euclid Quick Data Release (Q1): Extending the quest for little red dots to 2 < 4 arxiv.org 2503.15323 …submitted to A & A
Girardi, G. Rodighiero, G. Bisigello, L. et al. Euclid Quick Data Release (Q1): An investigation of optically faint, red objects in the Euclid Deep Fields arxiv.org 2503.15322 …submitted to A & A
Duffy, C. Cappellaro, E. Botticella, M. et al. Euclid Quick Data Release (Q1): Photometric studies of known transients arxiv.org 2503.15334 …submitted to A & A
To read these papers, go to the online preprint service (arXiv.org), and type in the paper’s code or title in the “search” box. And there’s also Ausset, H. et al. Euclid Quick Data Release (Q1): Data Release Overview, arxiv.org 2503.15302. But that’s largely redundant to the above, and to an extent the other Q1 papers I didn’t list above.
The papers I list above all cover the search for small, faint, elusive objects, many of which actually look brighter in the near-infrared (700-1100 nm). Gee, sounds like many asteroids! A significant fraction, maybe a majority, of asteroids are greyish, not reddish. But in near-Earth space, closer to the Sun, the carbonaceous (dark) asteroids are heated, and eventually split or disintegrate when their water and other volatiles boil. The population of hazardous asteroids is then overrepresented by bodies with ordinary-chondrite chemistries, and are mildly or significantly reddened.
Euclid, as a space telescope, was given imaging instruments not limited by the blurring effect of our atmosphere. Telescopes on the ground are often limited to no better than 1 arcsecond (1”) image sharpness in long exposures. Sometimes, particularly for short exposures, one may get lucky and see an image sharpness of 0.7”; the best sites, at the best times, have an atmospheric seeing limit reaching down to 0.56”. The pixels of Euclid’s VIS instrument are sized to give .101” resolutions, and without having to wait for good conditions and short shots. Even the NIR instrument, not meant to compete directly with VIS, has .3” pixels. The new data indicates that, while not giving “square stars” (one-pixel images of distant lights), the cameras are at their design resolutions. A star image of two pixels’ width then has an effective resolution of ~0.2”- handily beating ground scopes, and unquestionably so on faint objects which need long exposures. For asteroids, picking a target out from the background noise is then easier, just like other faint red objects.
If there’s an issue, it’s that the telescope’s search pattern (as of early 2025) didn’t revisit the same areas multiple times. Asteroid searches do this, to pick out moving objects. A full, multi-sweep dataset awaits Euclid’s full Data Release 1 …now planned for Oct 2026. By then, multiple “sweeps” will extend to sky fields observed jointly by Euclid and Vera Rubin. The survey telescopes complement each other, and the teams are coordinating. Watch this space in a year and a half!