Other Solar System Bodies Moons Asteroids Comets Meteors and Meteorites

Moons Large natural objects that revolve around planets are called satellites, or moons. Several planets have more than one moon. The most famous satellite is the moon that orbits the earth. It has a diameter that is ¼ that of the earth. It has been visited many times. It has no atmosphere. Its surface has many hills and valleys, as well as craters.

The moons of other planets were discovered after the invention of the telescope. In 1610, Galileo looked at Jupiter through his telescope and became the first person to see the four moons.

Planetary Moon Count PlanetNumber of Known Moons (1998) Mercury0 Venus0 Earth1 Mars2 Jupiter16 Saturn18 Uranus17 Neptune8 (Pluto)1

Phases of the Moon The moon is illuminated by the sun, but the illuminated side does not always face the earth, which mean that we see different amounts of the lit side as the moon orbits the earth. Over a period of about four weeks, the amount of illuminated surface of the moon we see (phases) follows a predictable pattern.

The eight phases of the moon make up the lunar cycle. The lunar cycle starts with a new moon. The moon is not visible from earth as the side that is illuminated is facing away. After this phase, the illuminated portion of the moon increases (waxes). A full moon appears as a lit circle in the sky with the illuminated surface facing the earth. The second half of the lunar cycle has the illuminated portion decreasing (waning) until the moon can no longer be seen, starting the cycle over again.

Eclipses Eclipses are spectacular astronomical events that occur when the position of one celestial object blocks the view of another celestial object from earth.

Solar Eclipses When the moon is aligned between earth and the sun, it blocks the sun from being observed from earth – a solar eclipse. The sun has a diameter 400 times greater than the moon, but it is also 400 times farther from the earth so it appears to be the same size in the sky

A partial eclipse occurs when the moon does not cover the entire sun.

Lunar Eclipses A lunar eclipse occurs when the earth is positioned between the sun and the moon. This casts a shadow on the moon. A total lunar eclipse is when the entire moon passes through the earth’s shadow. A partial eclipse is when only part of the moon passes through the earth’s shadow.

Partial Eclipse Total Eclipse

Asteroids Voyager 2 journeys to the outer planets making tremendous observations and measurements that have changed our view of the solar system. The trip was not without risk though. The craft had to pass through a region of millions of asteroids located between Mars and Jupiter. These objects are also known as minor planets. These irregularly shaped bodies are made of carbonaceous rock or silicate rock. Ceres are the largest asteroid, with a diameter of 1000 km.

Comets Periodically, comets are visible to the unaided eye, but are normally detected by telescopes. These objects are made up of mainly dust and ice. Scientists have labeled them “dirty snowballs”. Occasionally a comet gets bumped from its orbit by the gravitational pull of several objects. This causes the comet to fall toward the sun. Material begins to evaporate from its surface, formaing tails that can be thousands of kilometers longs. The tail always points away from the sun because it is pushed by the solar wind.

Halley’s Comet When it passed through the inner solar system in 1986, Comet Halley was observed by five space probes. The European Giotto Spacecraft showed the first ever close-up pictures of a comet. This one had a nucleus that measured 16km by 8 km.

Meteors Everyday, earth is bombarded by thousands of dust and rock fragments from space. When they enter the atmosphere, friction causes these particles to heat up and vaporize. If the fragments are large enough, however, it sometimes burns up, generating enough light to make it visible. These are called meteors (shooting stars).

Meteor over Northwest De Moines

Meteorites Some fragments are either large enough or tough enough that remnants survive and hit the earth’s surface. These are meteorites. Scientists use these fragments to study extraterrestrial material.