The moving, point-like image of the newly discovered Jupiter moon, S/1999 J 1, as observed with VLT ANTU and the FORS-1 instrument, low in the sky and just before sunrise on July 28, 2000. The photo is a composite of three CCD frames, obtained through a R(ed) optical filter. Because of the motion between the exposures, there are three images of the object near each other. Photo: ESO

Two astronomers, both specialists in minor bodies in the solar system, Have performed observations with ESO telescopes that provide important information about a small moon, recently discovered in orbit around the solar system's largest planet, Jupiter.

Brett Gladman (of the Centre National de la Recherche Scientifique (CNRS) and working at Observatoire de la Cote d'Azur, France) and Hermann Boehnhardt (ESO-Paranal) obtained detailed data on the object S/1999 J 1, definitively confirming it as a natural satellite of Jupiter. Seventeen Jovian moons are now known.

The S/1999 J 1 object
On July 20, 2000, the Minor Planet Center (MPC) of the International Astronomical Union (IAU) announced on IAU Circular 7460 that orbital computations had shown a small moving object, first seen in the sky in 1999, to be a new candidate satellite of Jupiter.

The conclusion was based on several positional observations of that object made in October and November 1999 with the Spacewatch Telescope of the University of Arizona (USA). In particular, the object's motion in the sky was compatible with that of an object in orbit around Jupiter. Following the official IAU procedure, the IAU Central Bureau for Astronomical Telegrams designated the new object as S/1999 J 1 (the 1st candidate Satellite of Jupiter to be discovered in 1999).

Unfortunately, Jupiter and S/1999 J 1 were on the opposite side of the Sun as seen from the Earth during the spring of 2000. The faint object remained lost in the glare of the Sun in this period and, as expected, a search in July 2000 through all available astronomical data archives confirmed that it had not been seen since November 1999, nor before that time.

With time, the extrapolated sky position of S/1999 J 1 was getting progressively less accurate. New observations were thus urgently needed to "recover" this object and "secure" its orbit.

Recovery of S/1999 J 1 at La Silla
Jupiter and its moons would again become visible in the early morning hours in late July with telescopes in the southern hemisphere. By a fortunate coincidence, observing time for observations of comets and asteroids had been allocated to Brett Gladman and his collaborators at two ESO telescopes in exactly this period.

Just before sunrise on July 25, he used the Wide Field Imager (WFI) on the MPG/ESO 2.2-m telescope at La Silla to search for S/1999 J 1.

This camera has a comparatively large field-of-view, about 0.5 x 0.5 deg2, or about the size of the full moon. This was comfortably larger than the estimated uncertainty in the object's predicted position at the time of the observation. And indeed, S/1999 J 1 was spotted not too far from that location, weakly visible in the glare of the nearby waning moon.

Detailed observations at Paranal
Only three days later, in the early morning hours of July 28, the small object was again imaged, this time from the 8.2-m VLT ANTU telescope at Paranal. Brett Gladman and Hermann Boehnhardt, now knowing exactly where to look in the sky, used the FORS-1 multi-mode instrument to obtain exposures of S/1999 J 1 through several optical filters.

The great light-collecting power of this telescope resulted in excellent images while S/1999 J 1 was moving across the sky. These observations definitively confirmed the "recovery" of the object and also provided an accurate determination of its brightness and colour, cf. IAU Circular 7472, published on August 3.

From accurate positional measurements on these exposures and the earlier ones from La Silla, Gareth Williams of the Minor Planet Center was able to substantially improve the computation of the orbit of S/1999 J 1 around Jupiter. It was found (IAU Circular 7469) to move in a somewhat elliptical orbit around Jupiter with a period of just over 2 years (768 days) and at a mean distance of 24.2 million kilometres from the planet.

The nature of S/1999 J 1
This means that S/1999 J 1 belongs to the class of "irregular satellites" which move on non-circular and inclined orbits around the planet. They are believed to have been captured onto their current orbits after the planet was formed.

S/1999 J 1 is one of the outermost moons of Jupiter known so far. The eight previously-known "irregular satellites" are split into two groups of four. The four members of the more distant group (Ananke, Carme, Pasiphae and Sinope) move on retrograde orbits, i.e. clockwise as seen from above the solar system (opposite to the motion of all major planets, including the Earth). The newly improved orbit of S/1999 J 1 shows it to be a fifth member of this retrograde cluster.

The brightness of S/1999 J 1, as measured on the VLT images, indicates that it must be comparatively small, with a diameter of the order of 10 - 15 kilometres (the smallest Jovian moon known so far). However, an accurate value can only be deduced once the reflectivity of its surface is known.

The colour is very slightly red. This appears to favour the possibility that it is a captured asteroid (minor planet), rather than a cometary nucleus, but additional work is needed to cast more light on this.

When more observations of S/1999 J 1 become available, the discoverers will propose a name, from Greek mythology according to astronomical tradition, to be approved by the IAU Working Group for Planetary System Nomenclature.