Asteroids and Comets

Eros - Close-up of an Asteroid

The space probe NEAR (for Near-Earth Asteroid Rendezvous) approached the asteroid Eros in February 2001, went into orbit around it, and landed on it on February 12th. From these vantage points the satellite transmitted images of Eros back to Earth so that astronomers now have amazingly sharp pictures of this cosmic lump of rock.

Maximum Length32.7 km
Average "radius"7.3 km
Mass6.69x1018 grams
Average density2.67 grams/cm3
Average distance from Sun1.46 AU
Orbital Period1.76 years

The images below are some of the many received that are helping astronomers understand more about Asteroids. Click here for Eros Asteroid pictures


Binary Asteroids

Astronomers have also been learning about asteroids from the ground. Using adaptive optics techniques (See Chapter 4) they have been able to see several asteroids that have a small companion asteroid orbiting it. Although a few such "Binary" asteroids were known earlier (and others suspected from the existence of double impact craters on Earth and other astronomical objects), few true binary asteroids had been detected until recently.

Comet Flyby

In late September 2001, astronomers obtained pictures of the nucleus of Comet Borrelly. This comet is small compared to more famous comets, but its orbit allows a far better and easier view than of is possible for most other comets. Borrelly circles the Sun every 6.8 years, making its orbit about the size of Jupiter's. Approaching to within about 2200 km (about 1300 miles) pictures show that the nucleus is shaped like a zucchini and is about 8 km (about 5 miles) long. The images show clearly jets of gas and dust erupting from the comet.

Click here for pictures of Comet Borrelly


The Zodiacal Light

Although spacecraft have given us our best views of asteroids, you can see evidence of asteroidal dust with the naked eye. If you live where skies are clear and dark, and if you look to east about an hour before dawn or the west about an hour after sunset, you may see a very faint triangular wedge of light rising from the horizon. This diffuse glow is called Zodiacal Light and is interplanetary dust, illuminated by the Sun. Just as light from the beam of a movie projector makes dust floating in the air of a theater gleam faintly, so too does the Sun's light make tiny dust particles gleam. The light we see is reflected off the tiny particles (technically, a process called scattering). The wedge shape occurs because the dust is mainly in the plane of the Solar System. The picture at the link below shows the Zodiacal light seen at dawn in October 2008 from a dark location in New Mexico. The bright dot in the fuzzy pale wedge of light is the planet Saturn. You may be able to pick out the constellation Leo too (see sketch below image).

Zodiacal Light at dawn in New Mexico

Sketch of Zodiacal Light shown in above image