Astronomy Most asteroids seen between orbits of Mars and Jupiter

Press, 31 October 1979, Page 30

Astronomy Most asteroids seen between orbits of Mars and Jupiter

The facilities at the Mount John University Observatory are freely available to astronomers throughout New Zealand. Typical of these visiting astronomers are A. C. Gilmore and P. M. Kilmartin, who are co-directors of the comet and minor planet section of the Royal Astronomical Society of New Zealand . Normally based in Wellington. this husband and wife team try to obtain very accurate positions of minor planets, especially those that are inaccessible from the observatories of the Northern Hemisphere. For observations of the fainter objects, particularly dark clear skies are essential. This is what brings them to Mount John to use one of the 61cm telescopes.

Minor planets, or asteroids. are small planet-like bodies found mainly in the region between the orbits of Mars and Jupiter. In size they range from Ceres, with a diameter of 1003 km. to mere fragments of rock less than a kilometre across. The first asteroid was discovered in 1801. At first they could be discovered only by searching a region of the sky visually and comparing the telescopic view with the best star charts available. If an object not on the chart was seen it was checked for movement. If this was detected then it could be either an asteroid or a comet. Observations spread over a few weeks were necessary to obtain accurate positions over sufficient time to permit an orbit to be calculated. The advent of photogranhy enabled photos of a large area of the sky to be obtained. If the telescone was carefully guided over a long exposure the stars appeared on the plates as small, round images. Any asteroid that was present showed a short trail because of its motion. If the asteroid was close to Earth its trail was longer because of its more rapid motion. Thus a plate, taken in Janua-v, 1977, by Gilmore and Kilmartin showed the rapid motion of Icarus, which pa««es close to Earth. When the obiect is very faint the preceding process is reversed. Knowing the position of the asteroid and its calculated motion the telescone is set on the asteroid so that its I'ght is concentrated on the one spot on the plate. Its image being on the one snot is r ound whereas the S’ars, because of their different motions, are seen as trails.

L’ r ge numbers of asteroids are discovered by p ' to-raphy. This made it d' t.ruh to keen track of ev n those tha* were known, let alone to keep track of all the new discoveries. In fact discoveries became a nui-sn-ee until 1946 when modem computers greatly

speeded the calculation of the orbits and the perturbations on them by the major planets. At the same time new discoveries increased in numbers. Before orbits can be calculated it is necessary to obtain sufficient accurate positions. Precise positions of the brighter and better known asteroids are used to obtain improved determinations of some fundamental elements of astronomy, such as the position of the equator and equinox. These observations assist in correcting the observed positions in star catalogues.

Asteroids are too small to enable us to see any features on their surfaces. They shine only by reflecting sunlight so that any variation in brightness must be due to changes in the reflectance of an asteroid. Reflectance depends on two factors, the apparent size of the body and its albedo, that is, the fraction of the incident sunlight reflected. The albedo changes if the asteroid has dark and light patches, like the Moon.

Asteroids have been observed to vary in brightness in periods that range from two to 32 hours with the majority having periods of about six hours. These variations show that the apparent size of the asteroid changes not that they physically change in size since they are solid lumps of rock. The reason for these variations is that in shape the asteroids are mainly irregular and as it rotates so its apparent size appears to change. The periods found are those of the rotation of the asteroids.

The diameters of asteroids are measured by observations of the polarisation of the reflected sunlight. When the diameter and apparent brightness

are known it is possible to determine the albedo. This shows that they fall into two classes, termed S and C. the S cla- have a higher albedo showing that they are similar to silicate rocks. Asteroids Of this class lie mainly closest to the Sun and appear to have surfaces similar to the inner planets. Asteroids of the C class appear very dark. They are thought to have a high carbon content as they are quite black. In fact they are the darkest objects known in the solar system. These two distinct classes of asteroids seem to support present theories that originally asteroids formed in their present region out of the gas in the solar system left after the Sun first formed. Naturally over the ages they have suffered many collisions but they are no longer considered to be the fragments left from the disruption of one or two larger planets that originally formed in the region between Mars and Jupiter.

The asteroid, Vesta, comes to opposition on November 4, when its magnitude is 6.5. That places it just below naked eye visibility but an easv object with binoculars. It can be found in the constellation Cetus, where, on November 30, it will be very slightly south-east of the famous variable star, Mira Ceti. Mira is close to maximum brightness and easily visible to the naked eye.

The evening sky in November is without bright planets, apart from Venus, visible low in the west during evening twilight. The mornihg sky sees Mars and Jupiter visible in the east for about an hour before dawn. Saturn, rising somewhat later, will only be seen against the brightening dawn sky.