Asteroids

Definition Asteroids: relatively small, predominately rocky objects that revolve around the sun Name means “starlike bodies” Sometimes referred to as minor planets or planetoids

Asteroid Basics They move on quite eccentric trajectories Most are found in the asteroid belt between Mars and Jupiter Few are larger than 300 km All the asteroids masses together do not equal the mass of the Earth’s moon

The asteroid Ceres The largest asteroid, at 940 km (1/10,000th the mass of Earth) Discovered in 1801 by Guiseppe Piazzi Has a semimajor axis of 2.8 AU

More Basics Current number of known asteroids is over 200,000. Hundreds of thousands of others may await discover. All but one orbit in the same direction as the planets The asteroids are probably left over material from the solar system’s formation Jupiter’s gravity keeps the asteroids from combining into a larger body

Physical Properties There are several different types C-type (carbonaceous) – darkest in color; found more to the outside of the belt (75 %) S-type - contains silicate or rocky material; found more to the inside of the belt (15 %) The largest ones are roughly spherical, but the smaller ones are irregular

The asteroid Vesta Some Earth meteorites are thought to have originated from Vesta – they are made of basalt. Unique in that it appears to have undergone volcanism

Two other Asteroids to know Pallas – 580 km and Juno – 540 km

Asteroid Observations The Galileo space probe, headed for Jupiter, went through the asteroid belt twice. In had close encounters with Gaspra and Ida

Gaspra v’s Ida Both are S-type. Gaspra is 20 km in size and Ida is 60 km. Ida is more heavily cratered, because it is in a denser part of the asteroid belt. Ida is a billion years old, Gaspra just 200 million years old. Both are thought to be fragments of once large objects.

Gaspra

The neatest thing about Ida… It has a tiny moon, Dactyl! Dactyl is just 1.5 km across. It orbits about 90 km from Ida. It is also an S-type asteroid.

` Asteroid Ida and tiny moon asteroid, Dactyl!

Binary Asteroids Def: two asteroids orbiting one another as they circle the Sun. May be a result of collisions in the belt. Less violent collisions may also be responsible for the binary systems. More violent ones may send asteroids into Earth-crossing orbits.

Binary Asteroid 90 Antiope

The NEAR Spacecraft Stands for Near Earth Asteroid Rendezvous. In 1997, visited the C-type asteroid Mathilde and S-type asteroid Eros. Mathlide was calculated as having a fairly low density, probably a result of being very porous. It rotates every 17.5 days.

Mathilde, Gaspra and Ida (for size comparision)

The NEAR Spacecraft NEAR spent a year around Eros. It was photographed many times. It was found to be more solid (denser) than Mathilde. In February of 2001, NEAR landed on Eros, sending back pictures as it did so.

Eros at sunset

Earth-Crossing Asteroids Most asteroids have a eccentricity of between 0.05 and 0.3. This means that they stay between Mars and Jupiter. Those having an eccentricity of 0.4 or more (few) may intersect Earth’s orbit and are known as Earth-crossing asteroids.

Earth-Crossing Asteroids As of 2004, more than 2600 of these asteroids were known. More than 600 are listed as “potentially hazardous” – more than 150 m in diameter and come within 0.05 AU of Earth. 1994 - 2004, more than 850 asteroids passed within 15 million km of Earth. At least 200 are predicted to pass within that same distance in the next decade.

Earth-Crossing Asteroids Most will eventually hit Earth. During a million year period, our Earth gets struck by about 3 asteroids. On average, 2 will hit water and 1 will hit land. The Moon, Venus and Mars all show evidence of being hit by asteroids also.

Earth-Crossing Asteroids Most are about 1 km in diameter. One 10 km one has been found.

How much of a problem would a 1 km asteroid be if it hit Earth? Catastrophic! Would devastate an area 100 km in diameter. The explosion would be equilvalent to a million megaton nuclear bombs – one hundred times more powerful than all of the nuclear weapons on Earth!

How much of a problem would a 1 km asteroid be if it hit Earth? The shockwave and possible tsunami would affect an even larger area. This is believed to have caused the extinction of the dinosaurs. But, don’t worry! We are watching the skies for “problem” asteroids!

Trojan Asteroids Orbit at and beyond Jupiter’s orbit. Several hundred of these are known.

Holes in the main belt? There are holes in the belt, where fewer asteroid exist, known as Kirkwood gaps (discovered by Daniel Kirkwood, 19th century American astronomor) These exist because of Jupiter’s gravity.

Red – Near Earth Asteroids Green – other asteroids Blue squares – comets Other blue – Trojan asteroids

Dog-Bone Shaped Asteroid 216 Kleopatra