Gary DeLeo
Part 1
Asteroids, often called "minor planets," are smaller bodies that orbit our sun. The orbits of over 40,000 of these objects have been found to date with sizes ranging from about 600 miles down to less than one mile across. Most of these are "Main Belt Asteroids," concentrated between the orbits of Jupiter and Mars. Although all but a few appear only as points of light in telescopes, their irregular shapes lead to brightness variations as they rotate, revealing rotational periods typically in the range of 5 - 15 hours.
Part 2
The first asteroid was discovered by Giuseppe Piazzi in 1801, and named "Ceres". At close to 600 miles across, 1 Ceres is the largest asteroid (asteroid names are preceded by a number assigned sequentially in order of orbit determination). A day on Ceres is about nine hours long. On the other hand, asteroids can be much smaller. Asteroid 3352 McAuliffe is about two miles across and rotates with a period of six hours.
Part 3
Several asteroids have been resolved using Earth- or space-based instruments, and four of these have been viewed close up by passing or orbiting spacecraft. While en route to the planet Jupiter, the Galileo spacecraft passed within 1000 miles of Gaspra and 1500 miles of Ida in 1991 and 1993, respectively. The NEAR (Near Earth Asteroid Rendezvous) spacecraft passed within about 700 miles of Mathilde in 1997on its way to Eros. NEAR was placed in orbit about Eros in 2000, and soft landed on the asteroid surface at the end of the mission in 2001. This was the first time a vehicle has landed on an asteroid.