February 7, 2000

Letters to the Editor
Sky & Telescope

Dear Mr. Editor

Asteroids at Their Brightest

Contrary to what many observers assume, an asteroid does not attain its maximum possible brightness when it reaches opposition at the perihelion of its orbit (March issue, page 108). About 60 percent of asteroids are brightest when opposition comes at the orbit's node closest to the Sun. In other words, it's more important for the asteroid to be near the ecliptic than near perihelion. For the other 40 percent, peak brightness comes some place between that node and perihelion. The reason is the "opposition effect," which makes any solid object reflect more light to us when it is directly opposite the Sun.

This is why Vesta will be so bright, magnitude 5.4, around its opposition in mid-July. Vesta will be only 11° past its node but 41° from perihelion. The very brightest oppositions of Vesta are those that take place at its descending node, around July 5th, when it reaches magnitude 5.2.

Eduardo Vila-Echagüe
Santiago, Chile