VARIOUS ASTEROIDS

There is a variety of other asteroids which have been imaged either optically or by using radar. They represent at least a hundred thousand other asteroids, most of which have not been observed, let alone named.

4769 Castalia

Castalia was discovered on 9th August 1989 by Eleanor F. Helin (USA), and determined to be a Near Earth Asteroid - in this case, Castalia actually crosses Earth's orbit. When Castalia came within 6 million kilometres of Earth during 1989 the Arecibo radio telescope in Puerto Rico bounced radar signals off the asteroid. This allowed a computer-generated 3D model to be built, which showed that Castalia is a "contact asteroid". The two parts of Castalia were originally separate asteroids that came together gently under the influence of their mutual gravity. Castalia has a length of 1.8 kilometres, but the point of contact is less than 200 metres across. The image here shows the computer model of Castalia from different viewpoints.

Different viewpoints of the radar-based computer model of 4769 Castalia.

Geographos

The National Geographic Society sponsored Palomar Observatory to conduct a sky survey. In 1951 Geographos was discovered by the Palomar Observatory and named to honour the Society. Geographos turned out to be a Near Earth Asteroid, and it crosses Earth's orbit. A collision is unlikely.

Radar studies of the small asteroid during 1994, when Geographos approached Earth within 5 million kilometres (its closest approach for 200 years), showed that the asteroid is very elongated. The ratio of its length to its width is the largest of any Solar System object. No other object is so narrow compared to its length. Geographos has a length of about 5 kilometres, but a width of less than 2 kilometres.

Geographos is a good example of the many thousands of Near Earth Asteroids that have yet to be detected. There are probably hundreds larger than Geographos, and hundreds of thousands that are smaller.

Radar image of Geographos, the most elongated body in the Solar System.

Hubble asteroids

Asteroids are often regarded as a nuisance because they get in the way of astronomical observations. Long exposures of distant galaxies often end up with the signature of an asteroid crossing the target object.

A long search by astronomers Karl Stapelfeldt and Robin Evans through 28,000 images in the archives of the Hubble Space Telescope resulted in the discovery of about a hundred more asteroids. In many cases, the Hubble image is the only image of the asteroid. The asteroid may never be seen again.

An asteroid with an estimated width of 2 kilometres was caught in this Hubble exposure. At the time, the asteroid had a distance of 139 million kilometres from Earth, and 250 million kilometres from the Sun. The asteroid trail (in blue) is curved due to the slow rotation of the Hubble Space Telescope.

A small asteroid leaves its trail in a Hubble Space Telescope image.

An asteroid trail crossing a Hubble image of a galaxy.

A double exposure caught this asteroid 300 million kilometres from Earth. The first exposure shows the asteroid trail in blue, the second in red. The large white shape is a background galaxy. The asteroid, unnamed, is estimated to have a diameter of only 0.8 kilometres.

A small asteroid leaves a straight trail across a Hubble image.

One of the faintest asteroids hidden in the Hubble archives is shown here. The multi-coloured trail is a result of the asteroid being caught in different exposures. The straightness of the trail means an accurate distance determination cannot be made, but an estimate of the distance is 330 million kilometres. The asteroid, with a width of about 0.8 kilometres, will probably never be seen again.

During Hubble imaging of the galaxy NGC 4548, in the constellation of Coma Berenices, an asteroid revealed itself by leaving a trail across the outer part of the galaxy. As the galaxy view was obtained with multiple exposures, so the asteroid left multiple trails. The trails have gaps in them - Hubble orbits the Earth, and could not in this case maintain constant viewing of the galaxy. This asteroid, with a diameter of 1.6 kilometres, was caught when over 400 million kilometres from Earth.

An asteroid trail superimposed on the galaxy NGC 4548.

The asteroid trail shown here was left by a small asteroid that may regularly cross the orbit of Mars. The background stars lie near the centre of our galaxy at a distance of about 25,000 light years (over 230,000 million million kilometres). The asteroid, however, is "local" in astronomical terms, with a distance of only a few hundred million kilometres. This asteroid is too faint to be seen with ground-based telescopes, and Hubble will not see it again. This asteroid will rejoin the many thousands of other asteroids that are unseen and unknown. Perhaps in centuries to come it will one day be seen again.

An unnamed asteroid leaves a fleeting trail against the background stars of our galaxy.