ABSTRACT
The asteroids, or minor planets, were not always popular members of the Solar System. One irritated German astronomer went so far as to dub them "vermin of the skies", because their trails kept being found on photographic plates exposed for guite different reasons. Today, however, we recognise that they are full of interest. Among them are small bodies which swing well inside the main swarm and may pass close to the Earth. These NEAs, or Near Earth Asteroids, are divided into three classes: 1. Amor asteroids, whose orbits cross that of Mars but not
that of the Earth. 2. Apollo asteroids, whose orbits do cross that of the Earth. 3. Aten asteroids, whose mean distances from the Sun are less
than that of the Earth. Several ApoUo asteroids, including No 3200 (Phaethon) and
156 (Icarus) are distinguished by their close approaches to the Sun. Phaethon can skim by the solar globe at 13,000,000 miles (21 ,000,000 km), and Icarus at 17,000,000 miles (27,000,000 km). Icarus, discovered by Walter Baade in 1949, is a mere 1500 yards (1372 m) across, so that even when fairly close to us it remains faint. But in fact it is not a real "Earth-grazer", because its orbit is inclined at the high angle of 22 degrees, and there is no immediate fear of a collision. (The same is true of Phaethon.) The orbital period of Icarus is 409 days.
