What Are They?

• Discovery: Asteroids are not visible to the naked eye; The first one was discovered in 1801 by Giuseppe Piazzi, who saw an object between the orbits of Mars and Jupiter in his search for an extra planet that he thought should be there; It was Ceres, the largest asteroid; They are assigned a number in order of discovery, and a name chosen by the discoverer.
• Appearance in the sky: They show up as moving objects, like planets but often even on much smaller time scales, and they are much fainter (smaller); A few observations and Kepler's laws give their orbits.
• What are they? Leftover scraps of planetesimals, debris and fragments of would-be planets; Their orbits were disrupted by each other and by Jupiter, and by now there are not even enough of them to make up a terrestrial-size planet.

  Location and Properties

• Asteroid belt: Most of the asteroids, many millions (billions?), are located between Mars & Jupiter (2.1–3.3 AU, with gaps due to resonances with Jupiter); Despite the number, they are not very crowded anymore, and major collisions only occur every 100,000 yr or so.
• Apollo asteroids: Eccentric orbits that could hit the Earth; Hundreds of potentially hazardous ones are known (some make the news each year), and there are systematic searches.
• Trojan asteroids: Orbits in 1:1 resonance with Jupiter's at the Lagrange points, although some are tilted; Could be as numerous as those in the asteroid belt.
• Captured ones: Some end up orbiting planets as their moons (Mars' two moons and some of Jupiter's, for example).
• Size and shape: At least 200 larger than 100 km, more than 100,000 (maybe a million) larger than 1 km; We know their size from flyby pictures, or guess it from the amount of reflected light and estimate of their albedo; Only Ceres is roughly round, the others are not large enough; We estimate the shape using their rotation.
• Mass and composition: We can find their mass if they have an orbiting object (natural or artificial); From their density and spectroscopy, we find that some are loosely packed, some are dense rocky/metallic objects, some contain carbon compounds.
• Types: Many, like Vesta, have suffered collisions and heating to the point of melting; Others, like Ceres, have not been modified since formation and can tell us more about the early Solar System; We estimate their age and history from the appearance of the surface if we can see it, or from their shape, composition and density.

NEO's (Near Earth Objects)

• What they are: Asteroids or comets whose orbits pass within 130 million miles of the Sun; Mostly asteroids in the inner Solar System, with eccentric orbits that could hit the Earth; Some small asteroids actually share our orbit.
• PHO's: Potentially Hazardous Objects, the ones whose orbits come to within 0.05 AU from the Earth's; For example, many Apollo asteroids; There may be about 1000 of diameter at least 1 km. The ESA is studying a mission to crash on an asteroid and test the possibility of deflecting it, "Don Quijote".
• Next close call: On April 13, 2029, asteroid 2004 MN4 is predicted to pass within 5.7 Earth radii from us.

Survey Projects and Exploration

• From here: Our best photographs are from the Hubble Space Telescope, but the most extensive surveys are carried out by four or five telescopes around the world that systematically scan the sky electronically for asteroids and comets; The main projects are LINEAR and Spacewatch; Congress has mandated, and NASA made it its goal, to locate and track 90% of NEO's larger than 1 km by 2008.
• By spacecraft: The Galileo probe has photographed Gaspra and others; The NEAR spacecraft flew by Mathilde, then orbited and landed on Eros in 2001; The Japanese probe Hayabusa orbited asteroid Itokawa in 2005 and will return samples of asteroid material; In 2007 the Dawn mission to Ceres and Vesta was launched.

Reason for interest: They give us information on the early solar system; Possible mining for minerals?

  Special Asteroids

• Largest ones: The largest one is Ceres (950-km diameter, almost round, 2.8 AU from the Sun); Other large ones are 2 Pallas and 4 Vesta (more than 500 km across), 3 Juno and 45 Eugenia (more than 200 km).
• 433 Eros: Well known now because it was orbited and landed on by the NEAR spacecraft, now inactive on its surface; 34 km long, not a differentiated body; It has few small craters and many boulders, because seismic shaking by collisions makes the material shift and fill in older craters.
• Double Ones: Many asteroids are known to have moons of their own (like Ida, 53 km, + Dactyl; or Eugenia, 215 km across) or to be binaries (like Antiope; a Trojan one, 617 Patroclus, 105 and 95 km, a Near-Earth one, Hermes, 300-450 m each, and a centaur; Two pairs are possible contact binaries, and some like 87 Sylvia and 45 Eugenia are triple systems!); Looking for asteroid moons from Earth is like looking for a firefly around a candle 150 miles away! Optically, must be done using adaptive optics.

 Other Aspects and Open Questions

• Yarkovsky effect: The surface of an asteroid is heated by the Sun during the day, and then cools off during the night. This means that the asteroid tends to emit more heat from its "afternoon side", and less from the opposite side. The recoil from the afternoon side is therefore larger as well, and the overall effect is that a tiny, non-gravitational force acts on the asteroid. This provides a way to measure the mass, by observing an asteroid's orbit over many revolutions.
• Questions: How exactly did asteroids of different types form? How do craters work on them?
• Vulcanoids? A population of small (up to 60 km across?) asteroid-like objects inside Mercury's orbit has been theorized; Searches has been carried out from Earth and (in 2004) by a rocket, but so far none have been found.