from Goldstone during a 2009 flyby Because of their distance from Earth and their small dimension, minor planets such as asteroids represent a challenge for astronomical instruments to resolve. Even two of the largest objects in the
asteroid belt,
2 Pallas and
4 Vesta, have maximum angular diameters of less than an
arcsecond. With a ground-based
optical telescope, resolution of these objects through the Earth's thick atmosphere can require techniques such as
speckle interferometry or
adaptive optics.
Radio telescopes such as
Arecibo or
Goldstone have been used to observe asteroids. This technique can be used to measure the
Doppler shifts and
radar cross-sections of the bodies, while more detailed studies allow three-dimensional shape models to be built. The first radar detection of a minor planet was
1566 Icarus by JPL astronomer Richard M. Goldstein in June 1968. This was followed by
1685 Toro in 1972. A regular program of radar observation of the asteroid belt asteroids was begun in 1980 at Arecibo. Goldstone joined the effort in 1990. Together, they observed 37 main-belt asteroids between 1980 and 1997. A more direct approach to asteroid study, allowing the object to be examined greater detail, is to send a spacecraft to either make a fly-by or go into orbit. The first such asteroid to be imaged in this manner was
951 Gaspra in 1991 by the
Galileo spacecraft. In 2000, the
NEAR Shoemaker spacecraft went into orbit around
433 Eros after making a fly-by of
253 Mathilde in 1997. ==Objects==