Armagh Astronomer’s Asteroid Discovered to be Double

Nearly 50 years ago, Dr David Andrews, a former Astronomer at the Armagh Observatory, discovered a new minor planet or asteroid. It was discovered on the night of 25th January 1965, and the object eventually received an official number (1727) and the name "Mette", named by the discoverer in honour of his wife, "for her tolerance of my nocturnal working hours and for the many months spent away from home". Very recently, astronomers B. Warner, R. Stephens and A. Harris, working in the USA, have reported CCD photometric observations of the same asteroid, which suggest that the object is a double or binary system. The two components of the system revolve around their common centre of gravity with a period of just over 21 hours, the pair orbiting the Sun in an elliptical orbit with a period of around 2.5 years.

On hearing of the new observations, David Andrews remarked, "I always thought Mette (the asteroid) was slightly elongated, but not double!" In fact, double asteroids are relatively rare, comprising only a few percent of all known asteroids.

The changing brightness of the object, with an amplitude of about one third of a stellar magnitude, indicates that the larger component of the binary, which has a size in the range 5 to 10 kilometres, has a rotation period of about 3 hours and is moderately elongated. The elongation of the primary is unusual, in that the larger components of most known small binary asteroids are more nearly round than elongated. In the case of Mette, the smaller object or secondary appears to be about a fifth the size of the primary.

Mette belongs to a "family" of minor planets known as the Hungaria group, named after the largest member (434) Hungaria. Hungaria was discovered in 1898 by Max Wolf of the University of Heidelberg and was named Hungary following an astronomical meeting in Budapest that year. There are some 5000 known members of the Hungaria group, which orbit the Sun just beyond Mars in rather high-inclination orbits close to the inner edge of the main asteroid belt.

FOR FURTHER INFORMATION PLEASE CONTACT: John McFarland at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; jmfat signarm.ac.uk.

Last Revised: 2013 February 12th