User login
We use telescopes to
collect lots of light and
focus it onto detectors
using mirrors or lenses.
Large modern telescopes,
like the Keck Telescope
shown below, can collect
6,000,000 times as much
light as the human eye.
Keck Telescope
Callisto is the second largest moon of the planet Jupiter, and was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System.
Although about the same diameter of the planet Mercury, Callisto is only about a third of its mass. This means it only has a density of around 1.8 times that of water.
The surface of Callisto is heavily cratered and extremely old. It does not show any sign of volcanic activity or plate tectonics, and is thought to have evolved primarily under the influence of asteroid impacts.
Callisto is composed of equal amounts of rock and ices, and is surrounded by an extremely thin atmosphere composed of carbon dioxide and possibly, small traces of oxygen.
| Facts and Figures | Io | Europa | Ganymede | Callisto |
| Distance from Jupiter (KM) | 421,600 | 670,900 | 1,070,000 | 1,883,000 |
| Radius (KM) | 1,815 | 1,569 | 2,631 | 2,400 |
| Mass (Earth Masses) | 0.015 | 0.008 | 0.025 | 0.018 |
| Time of Orbit (Earth Days) | 1.77 | 3.55 | 7.15 | 16.69 |
| Day Length (Earth Days) | 1.77 | 3.55 | 7.15 | 16.69 |
