Asteroids

Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets.

Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater.

They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past.

Asteroids with orbits that bring them within 1.3 AU (121 million miles/195 million kilometers) of the Sun are known as Earth-approaching or near-Earth asteroids (NEAs). It is believed that most NEAs are fragments jarred from the main belt by a combination of asteroid collisions and the gravitational influence of Jupiter. Some NEAs may be the nuclei of dead, short-period comets. The NEA population appears to be representative of most or all asteroid types found in the main belt.

Near Earth Asteroids (NEAs)

NEAs are grouped into three categories, named for famous members of each: 1221 Amor, 1862 Apollo, and 2062 Aten.

Amors: Asteroids which cross Mars' orbit but do not quite reach the orbit of Earth. Eros -- target of the NEAR mission -- is a typical Amor.

Apollos: Asteroids which cross Earth's orbit with a period greater than 1 year. Geographos represents the Apollos.

Atens: Asteroids which cross Earth's orbit with a period less than 1 year. Ra-Shalom is a typical Aten.

In 2004, it was reported that the asteroid 2004 MN4 (later named 99942 Apophis) had as much as a 1 in 37 chance of impacting Earth on 13 April 2029. The asteroid is about 1,300 ft (400 m) across and its impact would cause local or regional devastation. Further study of the orbit of Apophis found that it will miss Earth in 2029 but could strike in 2036. Still more research concluded that Apophis poses little danger of impacting Earth in the 21st century, but the object should be regularly

monitored.

Asteroids are material left over from the formation of the solar system.

One early hypothesis suggested that they were the remains of a planet that was destroyed in a massive collision long ago. Given the variety of asteroids, a single parent body is highly unlikely.

More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across -- less than half the diameter of our Moon.

The first 10 asteroids profiled against the Earth's Moon. From left to right, 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, 5 Astraea, 6 Hebe, 7 Iris, 8 Flora, 9 Metis, and 10 Hygiea.

 Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo:                       C-type (and rarer B-, F-, and G-types)• more than 75% of known asteroids • extremely dark (albedo 0.03)• similar to carbonaceous chondrite meteorites?

•S-type• 15-20% of known asteroids• relatively bright (albedo .10-.22)• metallic nickel-iron mixed with iron- and magnesium-silicates• similar to stony-iron meteorites and ordinary chondrites? M-type• most of the rest• bright (albedo .10-.18)• nickel-iron• similar to iron meteorites?

D- and P-type•Outer edge of main belt, Trojans, and Jupiter’s small moons•Very dark•Ultra-primitive organic compounds

Asteroid Classification

1 Ceres - The largest and first discovered asteroid, by G. Piazzi on January 1, 1801. Ceres comprises over one-third the 2.3 x 1021 kg estimated total mass of all the asteroids. 2 Pallas - The 2nd largest asteroid and second asteroid discovered, by H. Olbers in 1802. 3 Juno - The 3rd asteroid discovered, by K. Harding in 1804. 4 Vesta - The 3rd largest asteroid, Vesta appears to have a basaltic crust overlying an olivine mantle, indicating differentiation has occurred. Imaged by the Hubble Space Telescope in 1995.

Some Notable Asteroids

Much of our understanding about asteroids comes from examining meteorites and observing spectral data from Earth-based telescopes.

In 1991 asteroid 951 Gaspra was visited by the Galileo spacecraft and became the first asteroid to have hi-resolution images taken of it. In 1993 Galileo made a close encounter with asteroid 243 Ida. Both Gaspra and Ida are classified as S-type asteroids composed of metal-rich silicates. In 1997 the spacecraft NEAR made a high-speed close encounter with asteroid 253 Mathilde. This encounter gave scientists the first close-up look of a carbon rich C-type asteroid. NEAR continued on to encouter, orbit, and touch down on asteroid Eros.

Reflectance Spectra

  

Reflection & Emission

Main Belt Asteroids

Primitive versus Differentiated Asteroids

Based on heating by decay of aluminum-26, a short-lived radionuclide.

Gaspra is an S-type asteroid, believed to be composed of a mixture of rocky and metallic minerals.

Gaspra was encountered Oct 29, 1991 by the Galileo spacecraft on its way to Jupiter

951 Gaspra

orbit: 330,000,000 km from the Sun (average) size: 19x12x11 km

   Ida was originally thought to be an S-type asteroid, like Gaspra, composed of nickel-iron and some silicates. But a density of 2.9 is too low for that. Instead, Ida could well have a composition like that of ordinary chondrite meteorites, which are primitive and largely unaltered.

243 Ida and Dactyl

orbit: 428,000,000 km from the Sun (average) size: 58x23 km

Ida was encountered Aug. 28, 1993, by the Galileo spacecraft on its way to Jupiter.

Dactyl (right of Ida) is about 1.6 x 1.2 km, surprisingly round for such a small body. It orbits Ida at approximately 90 km.

orbit: 394,000,000 km from the Sun (average) size: 59 x 47 km

The spacecraft NEAR made a flyby of Mathilde on 27 June 1997

253 Mathilde

As dark as soot, carbonaceous asteroid 253 Mathilde is rich in carbon compounds and other dark substances common in the outer region of the Main Asteroid Belt. Mathilde is like a loose rubble pile with the density of water.

433 Eros

orbit: 172,800,000 km from the Sun (average) size: 33x13x13 km

Eros is an S-type asteroid and was the main target of the NEAR Mission.

NEAR Spacecraft

Mathilde

Asteroids Mathilde and Ida, shown at their correct relative brightness.

MUSES-C (Hayabusa)Asteroid Sample Return Mission

MUSES-C was launched by the M-V launch vehicle into a transfer orbit toward asteroid Itokawa in May 2003, and it arrived at the asteroid in September 2005. Landing took place on November 20, 2005.

After staying in close proximity to the asteroid for about 5 months and performing scientific observations and sample collections, it departed from the asteroid and is returning to earth in the summer of 2010.

MUSES-C Spacecraft

MUSES-CSampling Technique

MINERVA (MIcro/Nano Experimental Robot Vehicle for Asteroid) is a small robot lander whose weight is less than 600g. Although it is a tiny lander,

MINERVA can investigate the surface of ITOKAWA using three small color CCD cameras.

          

              

Unbowed by Robot Loss, Japan's Asteroid Probe Readies For Touchdown By Leonard DavidSenior Space Writerposted: 15 November 200502:41 pm ET

 

Despite a glitch in deploying a mini-robot onto asteroid Itokawa, Japanese space officials plan to proceed in a milestone-making touchdown on the space rock to obtain samples of the object for return to Earth. The problems encountered Nov. 12 with the release by Japan’s Hayabusa space probe of its camera-toting robot highlight the difficulty of this kind of mission. The robot probe, called the MIcro/Nano Experimental Robot Vehicle for Asteroid (MINERVA), was lost upon release from the mother ship Hayabusa.

MINERVA was successfully released, but the device appeared to start drifting away from the asteroid's surface, according to a release from the Japan Aerospace Exploration Agency (JAXA). The robot was expected to land and hop around on the asteroid's surface collecting data with three small color cameras.

Launch 2007Arrival at Vesta 2011Arrival at Ceres 2015