Asteroids

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Asteroids
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Definition

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

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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 Earths moon

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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

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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 systems formation
• Jupiters gravity keeps the asteroids from
  combining into a larger body

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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

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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

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Two other Asteroids to know

• Pallas 580 km and Juno 540 km

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Asteroid Observations

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

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Gaspra vs 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.

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Gaspra
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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.

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Asteroid Ida and tiny moon asteroid, Dactyl!
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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.

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Binary Asteroid 90 Antiope
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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.

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Mathilde, Gaspra and Ida (for size comparision)
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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.

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Eros at sunset
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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 Earths orbit and are known as
  Earth-crossing asteroids.

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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.

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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.

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Earth-Crossing Asteroids

• Most are about 1 km in diameter.
• One 10 km one has been found.

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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!

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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, dont worry! We are watching the skies for
  problem asteroids!

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Trojan Asteroids

• Orbit at and beyond Jupiters orbit.
• Several hundred of these are known.

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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 Jupiters gravity.

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Red Near Earth Asteroids Green other
asteroids Blue squares comets Other blue
Trojan asteroids
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Dog-Bone Shaped Asteroid 216 Kleopatra