In addition to the eight planets and their satellites there are numerous smaller bodies with irregular orbits and at random orientations to the ecliptic. Unlike planets, these objects do not appear spherical and are usually consist of huge chunk of rocks of irregular shapes. Below describe some of these objects.
Asteroids - Or minor planets, they are usually rocky with lots of metallic content but are too small to be considered as planets. The size could vary from the largest (Ceres) with 933 km in diameter down to the size of a stone. Asteroids can be found in a large region of space anywhere from within the Earth's orbit to beyond Saturn's orbit. However, most could be found at the so-called Main Asteroid Belt, situated
at a region between Mars and Jupiter orbits. Some asteroids' orbits cross that of Earth and a few even impact on Earth in times past.
The largest and the first asteroid, Ceres, was discovered in January 1801. Since then several hundred thousand asteroids have been discovered with thousands more being spotted each year. However, there are only 26 known asteroids with a diameter larger than 200 km. It is thought that a million asteroids of 1 km size may have exist. Some of these are so small (and illusive) that they were not discovered until they approach very near to the Earth! In fact the combine total mass
of asteroids are smaller than that of Moon. Asteroids are designated with a number to indicate the order it is being discovered. Hence the first asteroid is called 1 Ceres.
Potato-shape asteroid 243 Ida and its newly discovered moon, Dactyl. It is the first evidence that satellites of asteroids exist. The images were taken by the Galileo spacecraft, on Auguat 1993. The length of Ida is about 56 km and is situated at the Main Asteroid Belt. With this image, Dactyl is about 1.5 km and is about 100 km away from Ida. The image show numerous degraded craters, indicating that Ida's surface is very old.
951 Gaspra, taken by Galileo spacecraft on October 1991. The irregular asteroid is estimated with dimensions 19 x 12 x 11 km. Gaspra rotates counterclock wise every 7 hours with its north pole at upper left. The groove-like linear features and lots of small craters seem to suggest the asteroid was originated from a larger body by violent collisions.
Comets - Comets are small, irregularly shaped and icy bodies composed of a mixture of non-volatile materials and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them far into space, often beyond the orbit of Pluto. Comets are usually easier to spot when they approach the Sun. This is because as they are near towards Sun, the icy materials melt, forming the comet's coma of gas and dust. Radiation pressure and solar wind from the Sun accelerate materials away from the comet forming comet's tails.
The structure and shapes of tails depend on the size and mass of the escaped materials. Relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is traced as a nearly straight line extending away from the comet opposite the Sun. In addition, a hydrogen envelope also forms as the comet adsorbs ultraviolet light, a feature that has been detected by spacecrafts.
When a comet is moving away from the Sun, it begins to freeze and the tails dissappear. It remains invisible until it approaches the Sun again. A comet usually survive after about 500 passes, after which the ice and gasses are depleted, leaving a rocky object. Meteor shower sometimes occur when Earth passes thru the orbit of a comet. The most famous example is the Perseid meteor shower occurs every year in August when the Earth passes thru the orbit of Comet Swift-Tuttle. Particles left behind from the comet burn in Earth's atmosphere and during peak times more than a hundred streaks of light can be spotted in an hour.
Close-up image of Comet Halley, taken from Giotto spaceraft on March 1986. The images revealed the comet nucleus to be a dark peanut-shaped body with two bright jets spewing material out. Just 14 seconds prior to closest approach, the spacecraft was struck by a large dust particle which knocked the spacecraft off Earth point, and it took approximately 30 minutes for the spacecraft to recover and point its antenna back to Earth and reestablish communications. Giotto was partially damaged during the encounter.
Credit: Giotto, ESA.
This is the first X-ray image from Comet Hyakutake, taken from ROSAT X-ray satellite on March 1996. The strength and rapid changes in intensity of the comet's X-ray emission both surprised and puzzled astronomers. The scale bar on the lower left of the comet indicate a distance of 55 000 km or 10 arcmin. The X-ray emmision could be the result of interaction with the Sun's radiation as most emmisions seem to come from a crescent-shape region of the comet facing sunward.
Credit: ROSAT, Max Planck Institute/NASA.