Transit of minor planets

A transit of a minor planet across the Sun takes place when the minor planet passes directly between the Sun and the observer, obscuring a small part of the Sun's disc. During an astronomical transit, the minor planet can be seen as a small black disc moving across the face of the Sun.

Asteroids

Most asteroids orbit in the asteroid belt between Mars and Jupiter, and thus transits of asteroids viewed by an observer on Jupiter would not be uncommon, although the high inclination of many asteroid orbits makes this less common that it would be otherwise. For instance, 4 Vesta transits the Sun for observers on Jupiter on January 4 2044, with an angular diameter of 0.24".

However, it is interesting to consider whether transits of asteroids could be visible from Earth.

The only types of asteroids that can transit the Sun as seen from Earth would be Aten asteroids (including Apohele asteroids and hypothetical Vulcanoid asteroids) and some Apollo asteroids (perhaps including quasi-satellites of Earth). The trouble is, such asteroids are usually quite small, being only a few km in diameter at most, and their angular diameter is too tiny to observe against the Sun.

One example was the May 16 1990 transit of 3838 Epona (the provisional designation was 1986 WA), with a diameter of 2.5 km. At a distance of 0.53 AU from Earth, its angular diameter was only 7 milliseconds of arc, far too small to see.

There are only two transits of asteroids predicted to take place between 2005 and 2020. The asteroid 30825 (1990 TG1) transited on 2005 April 14, and 2101 Adonis transited on 2007 September 24. Neither transit was expected to be observable, due to the small size of the asteroids involved - 30825 (1990 TG1) will have an angular diameter of about 0.05", and 2101 Adonis will have an angular diameter of about 0.005".

On the other hand, if a transit takes place during a very close approach by a near-Earth asteroid, it just might be observable. However, no such asteroid transits have ever been observed up to the present time.

Comets

Comets may also transit the Sun. Halley's Comet transited the Sun on 19 May 1910. Another example of this was comet C/1819 N1 (also known as 1819 II, or the Great Comet of 1819). However, the occurrence of that transit was not known until after it had happened, when the orbit was calculated. Although some observers later claimed to have seen the comet transiting the Sun at the time, these observations seem dubious.

Transits in front of the moon

It is also possible that a minor planet on an earth-crossing orbit can cause a transit in front of the moon. However such events may be extremely rare and only few catalogued minor planets like 2004 FH came closer to earth than the distance of the moon.However such an event may be observable (an asteroid with 25 metres diameter in a distance of 100000 km has an angle diameter of 0.1"), but has been not observed yet, although there are some reports of such events, sometimes classified as UFO-sightings as the mutual Adhara Observatory UFO Lunar Transit on December 1, 1965, which may be mutual observations of such transits.

ee also

* Astronomical transit
* Transit of Mercury
* Transit of Venus

External links

* [http://ssd.jpl.nasa.gov/horizons.html JPL Horizons] (useful for calculating positions of solar system objects)

References

* Martin Hoffmann, "Asteroid transits over the disk of the Sun", Minor Planet Bulletin 16 (1989), 16 [http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1989MPBu...16...11H] (1989MPBu...16...11H)
* John Russell Hind, "On the Transit across the Sun's Disk of the Second or Great Comet of 1819", Monthly Notices of the Royal Astronomical Society 36 (1876), 309 [http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1876MNRAS..36..309H] (1876MNRAS..36..309H)