Asteroid transit

An asteroid transit in front of the sun occurs when a small planetoid crosses the straight line connecting the sun and the earth . Seen from the earth, the planetoid covers a small part of the solar disk and moves over it as a small black disk.

observation

Asteroid transit is relatively difficult to observe and is not necessarily common, even if there are many asteroids. Most asteroids orbit the sun in the asteroid belt , which is between the orbits of Mars and Jupiter . Since the earth orbits the sun at a smaller distance than Mars, the orbits of these asteroids do not cross the straight line connecting the earth and the sun. Consequently, they cannot be observed in front of the solar disk from Earth either.

Furthermore, the size of the asteroids is usually very small and usually a few kilometers in diameter at most. On the other hand, the distance between the earth and an asteroid in transit is relatively large. The asteroid thus only covers a very small part of the solar disk and is extremely difficult to make out against the bright rest of the solar disk. So the larger the asteroid and the smaller its distance to Earth during its passage, the greater the probability that it will just be observable. However, no such asteroid passages have been observed to date. For this reason, the passages of alleged intramercurial planets observed again and again in the 19th century can not be interpreted as asteroid transits.

An additional complication for an observation is that the orbital plane of an asteroid often has an angle to the orbital plane of the earth, whereby the frequency of the necessary constellation, earth and asteroid and sun form a straight line, is considerably reduced.

Examples of asteroid transits

On May 16, 1990, the asteroid 3838 Epona with a diameter of 2.5 kilometers (provisional designation 1986 WA) passed in front of the sun at a distance of 0.53 AU from Earth. Seen from earth, the angular diameter of this planetoid was only 7 milli-arcseconds , which is too small even for observation with large telescopes.

For the period between 2005 and 2020, only two planetoid passages have been predicted so far. Asteroid 30825 (TG1 1990) passed in front of the Sun on April 14, 2005 and had an angular diameter of approximately 0.05 ". On September 24, 2007, 2101 Adonis passed in front of the Sun with an angular diameter of approximately 0.005". Because of the small angular diameter, it was not expected to be able to observe these passages.

Types of asteroids that could pass in front of the Sun when viewed from Earth include Aten-class asteroids, including the Apohele-class asteroids and the hypothetical intramericurial vulcanoids, as well as some Apollo-class asteroids, possibly including Earth's quasi-satellites .

In contrast, asteroid transits should occur significantly more frequently for an observer on Jupiter, since Jupiter's orbit is outside the asteroid belt. For example, the asteroid 4 Vesta will pass a possible observer on Jupiter on January 4th 2044 as an object with an angular diameter of 0.24 "across the solar disk.

Comets

Also comets can pass in front of the sun, such as Halley's comet on May 19, 1910. This transit was, however, not predicted. Only after the transit had already taken place and the comet's orbit had been calculated, this transit could be confirmed retrospectively. Still, some observers later claimed to have seen the comet in transit. However, these observations appear doubtful.

Other examples of transit are Comet C / 1819 N1 (aka 1819 II or the Great Comet of 1819) and C / 1882 R1 ( Great September Comet ).

Comparison with a solar eclipse

In principle, an asteroid transit is the same process as a solar eclipse , in which the moon crosses the straight line connecting the earth and the sun, only that the moon orbits the earth, is much larger and much closer, which means that it is much larger and much closer to the earth during such a passage Seen from the earth, it can, but does not have to, cover the entire solar disk (see also umbra and penumbra ) and also does not cause a solar eclipse with every new moon, since its plane around the earth also has a small angle to the plane of the earth around the sun.

Mercury and Venus transits phenomenologically correspond to asteroid transits , as they cover only a small part of the solar disk, but the angular diameters of Mercury and Venus (60 "and 13", respectively) are significantly larger than those of asteroids during a transit (between 0 , 01 "and 0.0001").

Transits in front of the moon

In principle, it is possible for asteroids to pass in front of the moon on orbits that cross the earth's orbit. Such events are likely to be very rare, as very rarely do asteroids come closer to Earth than the moon. To date, only a few such approaches are known that did not involve lunar transit. An asteroid transit in front of the moon should be observable with a larger telescope, since an asteroid 25 meters in diameter at 100,000 km has an apparent diameter of 0.1 ". So far, no asteroid transit in front of the moon has been demonstrably observed, it but there are individual observation reports that have been classified as UFO reports.