English (UK)
In the framework of the Space Safety program of the European Space Agency (ESA), Rainer Kresken, an ESA engineer observing remotely with the Calar Alto Schmidt telescope, reported in December 2025 the discovery of a new comet, P/2025 W3 (Kresken). This is the second comet “Made in CAHA”, after comet Thiele (C/1985 T1) discovered forty years ago con el mismo telescopio.
On the night of November 28th, 2025, Rainer Kresken, an aerospace engineer at the European Space Agency (ESA) was testing a new camera recently installed on the Calar Alto Schmidt telescope. Kresken, who works for the ESA Near-Earth Object Coordination Centre (NEOCC), was observing remotely from Darmstadt (Germany). He pointed the telescope to a field in the constellation of Gemini, close to the bright stars Castor and Pollux.
After inspecting carefully the new, wide-field images taken, he detected a faint object, showing a thin, elongated tail. On the images of the field taken in the following nights, the object had moved slightly on the sky, with its tail remaining nicely elongated: it was definitely a new comet-like object! As an experienced observer, Rainer Kresken has already co-discovered over a hundred of asteroids, but this finding had something special: "The discovery was made during the very night of tests with a very sensitive new CMOS camera. That camera, together with the excellent telescope, allows astronomers to detect and observe very faint objects like that comet. "
On Dec. 6th, 2025, Kresken’s discovery was officially recognized by the Minor Planet Center of the International Astronomical Union, with the name P/2025 W3 (Kresken). Further observations suggest that it may be an active asteroid form the main belt, a torus-shaped region located between Mars and Jupiter, rich in asteroids (or minor planets) with 1.3 million reported to date. P/2025 W3 (Kresken) revolves around the Sun in four years in a pretty ellipsoidal orbit at least 300 million km away from our star.
Main-belt comets (MBCs) are active asteroids, that is, asteroids that show a coma or a tail. The activity of some of them is driven by ice sublimation, like in all the comets. However, other active asteroids may be releasing dust because of an impact with another object or by rotational mass loss. It will require some time to know which mechanism is the one responsible for the activation of comet Kresken.
This is the third discovery of a comet by the Schmidt telescope. The first one was comet Kohoutek (C/1973 E1), back in 1973, when the telescope was still at Hamburg (Bergedorf) observatory. A few years after moving the Schmidt optical tube to Calar Alto in 1980, comet Thiele (C/1985 T1) was discovered on a photographic plate taken on October 9th, 1985, by the astronomer Ullrich (Ulli) Thiele. This was by then the first comet found from Spain since 1932...
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| Discovery image of comet P/2025 W3 (Kresken), which appears as a thin horizontal trace at the center (the stars in the field appear like diagonal trails). In this image, stack of 64 exposures of 30 s each, the faint but elongated comet tail is clearly visible (see zoom in), aligning with the Sun (arrow in the east-west direction). Credits: R. Kresken / ESA / CAHA |
Forty years later, another German observer would finally discover a second comet from Calar Alto, P/2025 W3 (Kresken)! This is a timely scientific result to celebrate the ESA 50th anniversary and the ten years of the ESA-CAHA agreement for the exclusively use of the robotized Schmidt telescope, in particular to search for Near Earth Objects (NEOs), potentially hazardous for our planet.
Luca Conversi, NEOCC manager at ESA, concludes: “The upgrade to a new camera is an essential step for the continuation of this successful collaboration. CAHA Schmidt camera has become one of our observational workhorses, being in the top-5 facilities worldwide for tracking NEOs. The new camera improves its performance dramatically for the follow-up and search of NEOs: better sensitivity, wider field of view, better resolution and faster read-out time. “
The Calar Alto Observatory is one of the infrastructures that belong to the national map of Unique Scientific and Technical Infrastructures (Spanish acronym: ICTS), approved on March 11th, 2022, by the Science, Technology and Innovation Policy Council (CPCTI).
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Calar Alto (CAHA) Observatory - Gilles Bergond
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