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Crater
A depression formed by the impact of a meteorite. There are craters on many planets and moons. Craters are very well preserved on the Moon and other planets and moons without atmospheres. They are less well preserved on the Earth because rain and rivers have eroded them. One of the best-preserved on Earth is Meteor Crater, in the desert of Arizona.
 Crater—Inset shows Mimas, a small moon of Saturn that has a very large impact crater compared to its size. If the crater was any larger, it might have destroyed the entire moon.
Crust
The outermost layer of a planet. (See also: Earth.)
D
Deep Space Network (DSN)
A worldwide network of radio telescopes that track space
probes. There are three clusters
of telescopes: one in the Mojave Desert of California, one near Madrid in Spain, and one near Canberra, Australia. The network allows communication with probes in space at all times, even though the Earth is spinning.
Deimos
One of the two moons of Mars.
 Crater—The cratered surface of the Moon.
Deneb
The common name for the star Alpha Cygni, a supergiant star some 60,000 times as bright as the Sun. It lies about 1,500 light- years away and is the farthest bright (magnitude 1) star that can be seen with the naked eye. It makes up part of the Summer Triangle of stars.
Diamond ring
The effect seen immediately after a solar eclipse, when the first rays of the Sun appear like a diamond jewel set against the ring of the corona.
Dione
The fourth largest moon of Saturn, some 1,120km across. It is probably a rocky moon with an icy surface (see: Moons).
Disk
The side of a planet or star that can be seen against the sky.
Dog star
(See: Sirius.)
Dust
The name given to tiny particles
of solid material that occur in space. Dust is thought to make
up about 10% of all matter. It is responsible for making distant stars look redder and fainter than they otherwise would (just as haze on Earth reduces the distance
you can see from a hilltop). It is believed that much of the dust contains carbon. It may have formed in the cool, outer regions of supernovas and red giant stars.
(See also: Accretion and Coma.)
Dwarf star
The name for a small star like the Sun. Most stars are dwarf stars.
Dwarf star
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