Asteroids, Comets, and Meteoroids Asteroids are small, rocky objects. The name “asteroid” actually means ‘star-like bodies’.

The largest known asteroid is Ceres. Ceres is 940 km across. There were 4000 known asteroids by the mid-1990’s.

The total mass of all these asteroids is less than 1/10 the mass of our Moon. Asteroids generally have very eccentric orbits between the orbits of Mars and Jupiter.

Asteroids were first discovered when astronomers were looking for a planet that fit the “Titius- Bode” law.

Giuseppe Piazzi first detected Ceres in 1801 at an orbit 2.8 A.U.’s from the Sun. This distance was perfect for the expected location of a planet by the T-B law.

Soon thereafter, three more asteroids were discovered: Orbit Diameter Pallas2.8 A.U.580 km Juno2.7 A.U.320 km Vesta3.4 A.U.540 km

Most of the asteroids are confined to the asteroid belt, between 2.1 and 3.3 A.U. from the Sun.

The asteroids may have been a planet at one time, or they may have never formed a planet due to the gravitational pulls of Jupiter (probably the latter).

Most asteroids always stay between the orbits of Mars and Jupiter. Some stray inside Earth’s orbit, the Apollo Asteroids. The Amor Asteroids cross Mars’ orbit.

In 1968, the asteroid Icarus missed Earth by 6 million km. In 1989, asteroid 1989FC passed Earth at a distance of 800,000 km (2 times the distance to the Moon).

In 1991, 1991BA missed by less than half the distance to the Moon at 170,000 km.

The possibility of a collision with an asteroid is a real concern; however, there is not much we could do about such an occurrence.

Statistically, over a million year period there should be 3 Apollo asteroid collisions.

Most Apollo asteroids are about 1 km in size, but one is 10 km.

Collision with even a 1 km asteroid would be catastrophic. It would devastate an area 100 km in diameter.

The impact would be equal to one million 1-megaton nuclear bombs (this is 100 times ALL the nuclear weapons now on Earth). It would also produce a fatal blast wave.

Spaceguard is a systematic search to try and predict potential collisions with Apollo asteroids. The entire staff of Spaceguard is less than the number of employees at the local McDonalds.

Asteroid 3753 follows the yellow path while also orbiting the Sun. Each orbit the yellow path shifts, taking 385 years to complete one cycle. Asteroid 3753’s orbit is inclined to Earth’s, so they can never collide.

There are families of asteroids called “Hirayama Families”. Each family contains 5 to 10 members that all have similar orbits.

They are probably the remains of a single asteroid that broke up after a collision with another asteroid. The remnants then spread out along the orbit.

There are also binary asteroids: two asteroids that orbit each other in addition to orbiting the Sun.

Trojan Asteroids are asteroids that orbit in Jupiter’s orbit. They are locked at LaGrange points with Jupiter (60°) as a result of the synchronism of gravity with the Sun and Jupiter.

The main asteroid belt has gaps called “Kirkwood Gaps”. These are produced by gravitational resonances with planets (much as Saturn’s moons produce gaps in the rings).

There are two basic types of asteroids: C-type asteroids and S-type asteroids.

The C-type asteroids contain lots of carbon (“C” stands for carbonaceous). They are the least reflective asteroids with albedos around 0.05.

The S-type (“S” means silicon) are made up more of rocky materials. Containing less carbon, they are more reflective with albedos of 0.15 to 0.25.

The asteroids at the edge of the asteroid belt that is closest to the Sun are primarily S-type. The asteroids transition gradually to being more C-type as you move out further from the Sun.

The size of asteroids is estimated by the amount of light reflected and the amount of heat radiated. Rare stellar occultations aid in determining size.

Larger asteroids are roughly spherical, while the smaller ones can be highly irregular in shape.

The Jupiter probe Galileo passed through the asteroid belt twice. It passed close to and took pictures of Gaspra in 1991.

Gaspra is irregularly shaped, has a maximum diameter of 20 km, is pitted with craters up to 2 km across, and has a regolith (dust) layer.

On Feb 14, 2000 the NEAR spacecraft became the first artificial satellite of an asteroid when it moved into orbit around the asteroid Eros, a 20 mile long asteroid with a 3 mile crater.

The NEAR spacecraft arrived at Eros (the goddess of love) on February 14, 2000, so here is your valentine!!

Eros is a large, S-type near-earth asteroid that is too small to have enough gravity to form it into a sphere. By comparison, Eros has less than a thousandth Earth's gravity, so a 100 pound object on Earth would weigh about 1 ounce on Eros. A baseball thrown at 22 miles per hour would completely escape into space!

Other spacecraft have been sent to research asteroids as seen in the next slide.