Figure courtesy of NASA/Goddard Space Flight Center.
The key points about comets that you should take away from the last several lectures are as follows:
Asteroids are smaller, irregularly shaped objects whose orbital semi-major axis (average distance from the Sun) generally lies between the orbits of Mars and Jupiter. They are made of rock and metal, so they have a higher density than comets.
Figure courtesy of NASA/Goddard Space Flight Center.
Asteroids also represent relatively unaltered material from the early solar system, although this material is left-over in the inner solar system. They too provide us with a window into the conditions in which the planets formed.
There are several different types of asteroids. It is important to study each group to truly understand the different populations and how/why they vary:
The different asteroid types are located in different parts of the asteroid belt. The S-types are closer to the Sun, while the C-types are farther from the Sun. The M-types are believed to be the metal cores of larger objects that were impacted and broken apart.
In addition to the asteroids in the Asteroid Belt, there are some other groups elsewhere in the solar system. The Trojan Asteroids are located at the same orbital distance as Jupiter, and they cluster slightly ahead of and slightly behind Jupiter in its orbit. These objects resemble the C-type asteroids of the Main Belt. The Near Earth Objects have orbits that cross the orbit of the Earth. There is a big push to discover more of these objects as we would like to be forewarned if there is an asteroid on a collision course with Earth.
Asteroids have been visited by several spacecraft, including the recent Galileo and NEAR missions.
Galileo imaged the asteroid Ida in 1994 (on its way to Jupiter), and discovered that it has a small moon, Dactyl. Image courtesy of NASA/JPL.
This set of images compares Mathilde (left), which was observed by the NEAR spacecraft, and Gaspra and Ida, which were observed by Galileo. All three objects are shown on the same scale, although Mathilde's brightness has been stretched for easy viewing; it is actually much darker than the other two objects. Image courtesy of NASA.
Eros in the only asteroid on which we ever landed a spacecraft (even though it was not designed as a lander mission!). NEAR landed on Eros, a Near-Earth asteroid, at the end of its nominal mission. This was not only a tremendous feat of engineering, but it provided us with a wealth of information about asteroids UP CLOSE.
NEAR image of Eros courtesy of NASA.
NEAR image of Eros courtesy of NASA.
NEAR image of Eros courtesy of NASA.
This is the last image of asteroid 433 Eros received from NEAR Shoemaker. Taken from a range of 120 meters (394 feet), it measures 6 meters (20 feet) across. What we can see of the rock at the top of image measures 4 meters (12 feet) across. The streaky lines at the bottom indicate loss of signal as the spacecraft touched down on the asteroid during transmission of this image. Image courtesy of NASA.
The two moons of Mars, Phobos and Deimos, may be captured asteroids. They are extremely small! The diameter of Deimos is roughly 15 km, or the distance from NMSU to Dona Ana. Phobos is more heavily cratered, and contains one very large crater whose impactor probably nearly broke the satellite apart. Deimos has a smoother appearance due to partial filling in of some craters.
Phobos (top) and Deimos (bottom) images taken by the Viking orbiter, courtesy of NASA.
A meteor is a tiny solid object that enters the Earth's atmosphere from space. When they enter the Earth's atmosphere at high speeds, the friction they encounter (caused by the atmosphere) causes them to burn up at altitudes ~ 100 km above the Earth's surface. These are often called "shooting stars" by people on Earth, who see a streak of light across the sky, but they are not stars at all. Rather, they are small particles being burned and disintegrated in the Earth's atmosphere.
From our discussion of comets, we learned that a comet has a life cycle, and undergoes many changes as it approaches and then leaves the vicinity of the Sun. Each time a comet passes near the Sun, it leaves behind a large trail of dust behind it. These dust particles orbit the Sun in roughly the same orbit as the comet. When the Earth passes through these debris trails, we see an increase in meteor activity over a short time. This is known as a meteor shower. The next meteor shower will take place on April 22 if you look to the East before dawn (see full story here).
This is a photograph of the 2001 Leonids as seen from Australia.
Meteorites are pieces of interstellar debris that survive passage through the Earth's atmosphere and land on the surface. These "stones from heaven" represent the only first-hand evidence we have about the conditions of the formation and early history of our solar system. They are therefore very valuable!
Meteorites are found either by observations of the fireball in the sky (and then searching on the ground at the location where it should have landed), or by someone happening across a rock that appears unusual or extraterrestrial in origin. Either scenario requires follow-up analysis at a laboratory, where a true determination can be made regarding a rock's status as a meteorite.
Image courtesy of the ANSMET program.
Image courtesy of the ANSMET program.
Image courtesy of the ANSMET program.
Image courtesy of the ANSMET program.