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Discovery of a Satellite Around Asteroid 3671 Dionysus

ESO Education and Public Relations Dept.

Press Release 08/97

For immediate release: 22 July 1997
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Text and photos with all links are available on the ESO Website at URL:

http://www.eso.org/outreach/press-rel/pr-1997/
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DISCOVERY OF A SATELLITE AROUND A NEAR-EARTH ASTEROID

In the course of the major observational programme of asteroids by the
Institute of Planetary Exploration of the German Aerospace Research
Establishment (DLR) [1] in Berlin, two of the staff astronomers, Stefano
Mottola and Gerhard Hahn, have discovered a small satellite (moon) orbiting
the asteroid (3671) Dionysus.

The new measurements were obtained with the DLR CCD Camera attached
at the 60-cm Bochum telescope at the ESO La Silla Observatory in Chile.

This is only the second known case of an asteroid with a moon.

Moons and planets

Until recently, natural satellites were only known around the major planets.
The Moon orbits the Earth, there are two tiny moons around Mars, each of the
giant planets Jupiter, Saturn, Uranus and Neptune has many more, and even
the smallest and outermost, Pluto, is accompanied by one [2].

However, the new discovery now strengthens the belief of many astronomers
that some, perhaps even a substantial number of the many thousands of minor
planets (asteroids) in the solar system may also possess their own moons.
The first discovery of a satellite orbiting an asteroid was made by the NASA
Galileo spacecraft, whose imagery, obtained during a fly-by of asteroid
(253) Ida in August 1993, unveiled a small moon that has since been given
the name Dactyl.

(3671) Dionysus: an Earth-crossing asteroid

In the framework of the DLR asteroid monitoring programme, image
sequences are acquired to measure an asteroid's brightness variations
caused by the changing amount of sunlight reflected from the asteroid's
illuminated surface as it spins, due to its irregular shape. The brightness
variations may be used to derive the asteroid's rotational properties, such
as speed of rotation and spin axis orientation.

Asteroid Dionysus [3] was put on the observing list because it belongs to a
special class of asteroids, the members of which occasionally come very
close to the Earth and have a small, but non-negligible chance of colliding
with our planet. Most of these objects move in highly elliptical orbits that
lie partly inside, partly outside that of the Earth. They are accordingly
referred to as `Earth-crossing asteroids' or Apollo-type asteroids, after
the proto-type of this group, (1862) Apollo, that was discovered in 1932 by
Karl Reinmuth in Heidelberg [4].

The orbital characteristics of Dionysus lead to moderately close approaches
to the Earth every 13 years, with the one in 1997 being the first since its
discovery that is favourable for extensive observations. On July 6, 1997, it
passed within 17 million km of our planet. At that time it was visible from
the southern hemisphere with a moderately-sized telescope as a relatively
fast-moving object.

The strange lightcurve of asteroid (3671) Dionysus

The first observations of the brightness of this asteroid in late May 1997
showed a `normal' two-maxima-two-minima lightcurve (change of
brightness with time), typical of rotating non-spherical bodies. The period
of rotation was 2.7 hours, i.e., this asteroid spins almost nine times as fast
as the Earth.

However lightcurves observed on two subsequent nights were strikingly
different from the previous ones. In both cases a deeper and shifted dip
was seen, indicative of an attenuation -- an additional dimming of the
sunlight reflected by the asteroid, cf. ESO Press Photo 20/97.

The observers hypothesised that these lightcurve features were due to an
eclipse by an unknown object moving in an orbit around (3671) Dionysus,
thereby covering part of the illuminated surface of the asteroid at regular
time intervals [5]. Fortunately, this hypothesis can be checked, because the
phenomenon should then repeat itself periodically.

Accordingly, the DLR scientists made a prediction for the next occurences
of dips in the lightcurve, based on the time difference between the two
observed events.

Confirmation of the satellite

Contacts were made with observers located at other observatories, in order
to secure lightcurve coverage over a longer period of time than was possible
from La Silla alone. As a result, a series of lightcurve measurements were
performed from June 3 to 9 in close cooperation with Petr Pravec and Lenka
Sarounova working at the Ondrejov Observatory, near Prague in the Czech
Republic.

Luckily, the weather conditions were favourable at both sites and the dips
in the lightcurve were indeed observed at the predicted times.

Based on the four well observed events, it was then possible to determine a
period of 1.155 days for their occurence. Thus, the hypothesis of a
satellite orbiting around Dionysus was confirmed. As a result, the
International Astronomical Union's Minor Planet Center located in
Cambridge (MA, USA) promptly gave a provisional designation to the new
satellite -- S/1997 (3671) 1.

How big is Dionysus?

Meanwhile, in Hawaii, the world's largest infrared telescope was being
trained on Dionysus to obtain information about its size and composition.
Alan Harris, also a scientist from the DLR in Berlin, and John Davies from
the Joint Astronomy Centre in Hilo, Hawaii, observed the thermal infrared
radiation emitted by Dionysus with the 3.8-m United Kingdom Infrared
Telescope (UKIRT) situated on Mauna Kea. Similar observations over a
broader spectral range were also made by the European Space Agency's
orbiting Infrared Space Observatory.

The thermal or "heat" radiation emitted by an asteroid depends on its size
and the amount of sunlight it absorbs (darker bodies being warmer). In the
case of Dionysus the measured radiation was much weaker than expected,
indicating that the asteroid has an intrinsically bright (reflective)
surface and is only about 1 km in diameter. This is much smaller than (253)
Ida, the only other asteroid known to have a moon, which is about 60 km
across.

Further observations

Eventually it should be possible to determine the orbital radius of the
satellite, its size and the inclination of its orbital plane. In order to
obtain the data necessary for these determinations, observations will be
continued during the present period of good visibility that lasts until
September-October 1997. For this reason the discoverers have initiated an
international observation campaign devoted to the study of this intriguing
object and now involving astronomers from many countries.

How common are such satellites?

Satellites in orbit around small bodies in the solar system -- asteroids and
cometary nuclei -- have been predicted on theoretical grounds for a long
time, even though there is no consensus among planetary scientists about
the actual numbers of such systems.

Hints about the existence of asteroid satellites also come from the presence
of double impact craters on the Moon and other planetary surfaces. This
suggests that the projectiles forming these craters were `double' asteroids.
Moreover, measurements obtained when an asteroid passes in front of a
relatively bright star (a so-called `occultation') have on a few occasions
shown features which could be interpreted as due to the presence of a
satellite. However, because of the difficult nature of such measurements, it
has never been possible to draw unambiguous conclusions.

The existence of double asteroids was invoked earlier by Petr Pravec and
Gerhard Hahn to explain the unusual features observed in the lightcurves of
two other Earth-approaching asteroids 1991 VH and 1994 AW1. In the case
of Dionysus, however, it is possible to predict eclipse events and to confirm
them by subsequent measurements.

There is therefore mounting evidence that asteroid binary systems might be
comparatively common. Observational programmes like the present one by
the DLR and Ondrejov groups will help to verify this possibility.

Where to find additional information

Detailed and up-to-date information about (3671) Dionysus can be found in
the Web at the following URL: http://earn.dlr.de/dionysus.

Notes:

[1] This institute and its parent organisation are known in Germany as
Institut fuer Planetenerkundung and Deutsche Forschungsanstalt fuer Luft-
und Raumfahrt e.V. (DLR).

[2] See ESO Press Release 09/94 of 18 May 1994.

[3] Asteroids are small solid planetary bodies revolving around the Sun in
orbits that are mostly located in the so-called Main Asteroid Belt, confined
between the orbits of Mars and Jupiter. Most of them are thought to be
fragments derived from catastrophic, past collisions between larger
asteroids. By mid-1997, the orbits of about 8000 asteroids in the solar
system were sufficiently well known to allow them to be officially numbered
by the rules of the International Astronomical Union. (3671) Dionysus was
discovered in 1984 at the Palomar Observatory (California, USA) and is
named after the Greek god of wine.

[4] The gravitational influence of the giant planet Jupiter can modify the
orbits of asteroids located in particular regions of the Main Belt (the
effect is refered to as `orbital perturbations'). As a result, the orbit of
an asteroid may `cross' that of a major planet, and eventually it may become
a NEO, i.e. a near-Earth object. The orbits of NEO's are highly unstable
over times comparable to the age of the solar system. This instability can
result in a collision with one of the terrestrial (inner) planets, or with
the Sun, or in the ejection of the asteroid out of the solar system. The
present orbit of (3671) Dionysus is such that this object is not likely to
collide with the Earth in the foreseeable future.

[5] The method of analyzing the lightcurve of Dionysus consists of
`removing' (subtracting) the normal short-period brightness variations due
to rotation of the asteroid and plotting the residuals against time, cf.
Press Photo 20/97. The residual lightcurve shows a clear resemblance with
typical lightcurves of eclipsing binary stellar systems (in which two stars
move around each other, producing mutual eclipses) and leads to a model
of two bodies revolving around a common gravitational centre, in an orbital
plane containing both the Earth and the Sun.

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Information from the European Southern Observatory

ESO Press Photo 20/97

For immediate release: 22 July 1997


Lightcurves of Asteroid (3671) Dionysus

This figure shows the lightcurve data from observations of asteroid (3671)
Dionysus, made from ESO on June 8th, 1997. These observations
confirm that this asteroid is accompanied by a small moon (natural
satellite).

In this diagramme, the abscissa indicates the time and the ordinate the
light intensity (brightness), expressed on the logarithmic magnitude scale.
The three curves have been shifted vertically by an arbitrary amount so as
not to overlap.

The upper curve shows the `normal', periodic light variation due to the 2.7
hour rotation of the irregularly shaped asteroid. This curve has been
derived by fourier analysis of photometric observations taken during the
period June 1-16.

The middle curve displays the observations taken on June 8th, revealing
an eclipse event.

The bottom curve represents the difference between the observed curve
(middle) and the average curve (upper). This procedure `removes' the
light variations caused by the rotation of the asteroid. The minimum caused
by an eclipse in the double asteroid system is now clearly seen. The
similarity with the lightcurve of a partial eclipse in a double stellar system
is striking.


This is the caption to ESO PR Photo 20/97 [GIF, 10k] which accompanies
ESO Press Release 08/97 (21 July 1997). It may be reproduced, if credit
is given to the European Southern Observatory.

Copyright ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
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