Summary

Ceres, the first asteroid (minor planet) to be discovered in the Solar
System, has held the record as the largest known object of its kind for
two centuries.

However, recent observations at the European Southern Observatory with
the world’s first operational virtual telescope ‘Astrovirtel’ have
determined that the newly discovered distant asteroid “2001 KX76” is
significantly larger, with a diameter of 1200 km, possibly even 1400 km.

PR Photo 27a/01: Image of “2001 KX76, obtained with the MPG/ESO 2.2-m
telescope at La Silla.

PR Photo 27b/01: Relative sizes of the largest known Kuiper Belt Objects
and Pluto.

Astrovirtel provides decisive data about 2001 KX76

ESO PR Photo 27a/01

Caption: ESO PR Photo 27a/01 shows a reproduction of a colour composite
image, based on three exposures with the Wide Field Imager (WFI) at the
MPG/ESO 2.2-m telescope at the La Silla Observatory.

By combining data from the world’s first operational “virtual telescope”,
Astrovirtel, with that from a conventional telescope at the European
Southern Observatory (ESO) at La Silla (Chile), European astronomers [2]
have determined the size of the newly found, remote asteroid, 2001 KX76.

Their measurements indicate that this icy rock has a diameter of at least
1200 km and is therefore larger than any other known asteroid in the Solar
System. The previous record-holder, the asteroid Ceres, was also the first
object of its type to be discovered — by the Italian astronomer Giuseppe
Piazzi on January 1, 1801. Its diameter is about 950 km, relegating it to
second place after holding the asteroid size record for two hundred years.

This conclusion is based on data from Astrovirtel, which has been operating
at the ESO Headquarters in Garching (Germany) for about one year. This
advanced prototype science tool which in effect mimics a telescope provides
astronomers with access to a wide variety of high-quality data.

The first scientific results from Astrovirtel have allowed a substantial
improvement of the accuracy of the computed orbit for 2001 KX76. It is now
possible to confirm that this object is just outside that of the most remote
known major planet Pluto. Further analysis carried out by the team seems to
indicate that the orbit of 2001 KX76 is very similar to that of Pluto.

Asteroid 2001 KX76 is even larger than Pluto’s moon Charon (diameter 1150
km), adding fuel to the discussions concerning Pluto’s status as a “major”
or “minor” planet. The new data show that 2001 KX76 is about half the size
of Pluto (diameter about 2300 km) and this increases the likelihood that
there are other bodies still to be discovered in the outer Solar System that
are similar in size to Pluto.

Observations of 2001 KX76

On July 2 2001, a group of American astronomers lead by Robert Millis
(Lowell Observatory, Flagstaff, Arizona) announced the discovery of a
seemingly rather large so-called Kuiper Belt Object, designated 2001 KX76.

Objects of this type are icy planetary bodies that orbit beyond planet
Neptune in the distant region of the Solar System known as the Kuiper Belt.
More than 400 such objects are currently known and they are believed to be
remnants of the formation of the Solar System and consequently amongst the
most primitive and least-evolved objects available for study in the Solar
System.

The first observations of 2001 KX76 were quite sparse, so the initial
estimates of the size of the new asteroid were very uncertain. However, it
did look large, possibly about the same size as the largest known asteroid,
Ceres, the diameter of which had earlier been measured at about 950 km.

A team of German, Finnish and Swedish astronomers [2] took the initiative
to carry out a more accurate measurement of the size of 2001 KX76 within a
unique collaboration between “Astrovirtel” and a conventional ESO telescope
at the La Silla Observatory in Chile. The results show that this object is
definitely the largest Kuiper Belt Object so far discovered.

Determining the size of a distant asteroid

ESO PR Photo 27b/01

Caption: ESO PR Photo 27b/01 shows the relative sizes of the largest
known Kuiper Belt Objects, together with the outermost major planet,
Pluto and its moon, Charon.

In order to measure the size of any asteroid, it is necessary first to
determine its orbit around the Sun, which gives its present distance from
the Earth. The next step is to estimate its “albedo”, i.e. the percentage
of incident sunlight reflected from its surface. From these numbers and
the measured, apparent brightness of the asteroid (as seen from the Earth),
its diameter can finally be derived.

To determine the orbit of 2001 KX76, the group used “Astrovirtel” to apply
automatic search software to scan through “old” photographic plates obtained
with various astronomical telescopes, as well as recent CCD observations
made with the ESO Wide Field Imager (WFI) at the MPG/ESO 2.2 m telescope on
La Silla (Chile).

The search was successful: the astronomers were able to find several
photographic plates on which faint images of 2001 KX76 could be
identified — some of these plates had been obtained as early as 1982. The
exact sky positions were measured and with accurate positional data now
available over a time span of no less than 18 years, the team was able to
compute the first, high-precision orbit of 2001 KX76 [3]. This also allowed
to determine that the current distance from the Earth which turned out to
be about 6.5 billion km, corresponding to 43 times the distance of the
Earth from the Sun, or nearly one-and-a-half times farther from the Sun
than Neptune.

Combining this with a realistic assumption for the albedo of 2001 KX76 of
7 percent (corresponding to the albedo of another well-observed Kuiper Belt
Object, Varuna, and comparable to that of our own Moon), a diameter of no
less than 1200 km results. Assuming instead an albedo of 2001 KX76 of only
4 percent — a typical value for icy cometary nuclei — leads to the even
larger (although less likely) value of 1400 km.

A real name for 2001 KX76

Thanks to the work of this group of astronomers, the orbit of 2001 KX76 may
now be considered relatively secure and it may therefore soon receive a real
name.

Following astronomical tradition, the discoverers have the right to make a
suggestion. The current custom dictates that a Kuiper Belt Object must be
given a mythological name associated with creation. The name must then be
confirmed by the International Astronomical Union (IAU) through its
Committee on Small Body Nomenclature before becoming official.

With a little bit of luck …

The observations made with ESO’s Wide Field Imager were crucial for this
work to succeed in that they allowed the object’s path to be tracked back in
time. However, luck admittedly also played a key role. “These observations
were originally made for a completely different project”, says Gerhard Hahn,
team-leader for the project. “And we found the image of 2001 KX76 right at
the edge of the WFI frames”.

Jenni Virtanen, another member of the team, agrees: “And if we hadn’t used
our powerful methods to improve the orbit we would still be searching
through the archives.”

Arno Gnaedig, a German amateur astronomer and team member, performed the new
and accurate position measurements and also calculated the new orbit on his
home computer: “To me this is a wonderful example of the fruitful
collaboration that can take place between well-equipped amateur astronomers
and professional astronomers”, he says. “The Web and the access to ‘virtual
observatories’ means that amateur astronomers — located far from any ‘real’
professional telescopes — can also make important contributions”.

Following this success, the group is currently working on a study of the
long-term orbital evolution of 2001 KX76, accounting for orbital
uncertainties, in order to investigate the dynamical behaviour, and its
relationship to both Pluto and Neptune.

The Astrovirtel co-ordinator, Piero Benvenuti, comments: “These results are
thrilling for more than one reason. The latest in modern astronomical
technology combined with a novel scientific procedure have been able to
produce results that would otherwise have been very difficult to achieve. I
am very delighted to see the first important scientific results materialise
from our work with Astrovirtel”.

The ‘Virtual Observatory’ concept, for which “Astrovirtel” is a prototype,
is the start of a new era in astronomy. A larger study project called the
‘Astrophysical Virtual Observatory’ is now about to start within the Fifth
EC Framework programme as a collaboration between ESO, ESA (ST-ECF), the
University of Edinburgh (UK), CDS (Strasbourg, France), CNRS (Paris, France)
and the University of Manchester (UK).

Notes

[1] This is a joint Press Release by the Space Telescope European
Coordinating Facility (ST-ECF) and the European Southern Observatory (ESO).
Credit to “ESA, ESO, Astrovirtel and Gerhard Hahn (German Aerospace Center,
DLR)”.

[2] Members of the group of scientists involved in these observations are:
Gerhard Hahn (German Aerospace Center, DLR, Berlin), Claes-Ingvar Lagerkvist
(Uppsala University, Sweden; http://www.astro.uu.se/planet/asteroid/), Karri
Muinonen, Jukka Piironen and Jenni Virtanen (University of Helsinki,
Finland; http://www.astro.helsinki.fi/~spa/), Andreas Doppler and Arno
Gnaedig (Archenhold Sternwarte, Berlin, Germany) and Francesco Pierfederici
(ST-ECF/ESO).

[3] Acknowledgment: Observations from Siding Spring Observatory (Digitized
Sky Survey 1), and NEAT/JPL were also used in the orbit determination.

Appendix: More about Astrovirtel

Astrovirtel is a collaboration between Europe’s astronomy organisation, the
European Southern Observatory (ESO), and the European Space Agency (ESA). It
is the first virtual astronomical telescope dedicated to data mining and
provides an interface between the scientists and the huge amounts of data
stored in scientific archives. This interface partly consists of a
combination of the development of special software tools that incorporate
advanced data query methods and the dedicated support of archive
astronomers.

Astrovirtel is a response to the rapid developments currently taking place
in the fields of telescope and detector construction, computer hardware,
data processing, archiving, and telescope operation.

Astronomical data archives increasingly resemble virtual gold mines of
information. Nowadays astronomical telescopes can image increasingly large
areas of the sky. They use an ever greater variety of different instruments
and are equipped with ever-larger detectors. The quantity of astronomical
data collected is thus rising dramatically, generating a corresponding
increase in potentially interesting research projects. Astrovirtel
facilitates such projects by enabling astronomers to access these archives.

Astrovirtel is supported by the European Commission (EC) within the “Access
to Research Infrastructures” action under the “Improving Human Potential &
the Socio-economic Knowledge Base” of the EC Fifth Framework Programme.

Astrovirtel website: http://www.stecf.org/astrovirtel

See also ESO PR 09/00.