MINOR MEMBERS OF THE SOLAR SYSTEM: Asteroids

Images of three asteroids, taken during spacecraft flybys, shown to scale (Mathilde is 59 km wide and 47 km high)

General Thesis Asteroids and Comets are relics of the early stages of accretion of the solar nebula

I: History of Discovery A “gap” in the known planetary system between Mars (1.5 AU) and Jupiter (5.2 AU) The Titius-Bode law (purely numerical relation) “predicted” a planet in the gap between Mars and Jupiter

Titius-Bode Law PlanetCalculated distanceTrue distance Mercury(0+4)/10 = Venus(3+4)/10 = Earth(6+4)/10 = Mars(12+4)/10 = ?(24+4)/10 = 2.8 Jupiter(48+4)/10 = Saturn(96+4)/10 = Uranus(192+4)/10 =

Asteroid discoveries Discoveries: Ceres 1801, Pallas 1802, Juno 1804, Vesta 1807 Largest is Ceres (940 km diameter) Photography (after 1890’s) allowed discovery of many more asteroids As many as several million with diameters of 1 km or more Total mass ~1/20 mass of Moon

II: Classification of Asteroids Belt Asteroids Semimajor axes 2.2–3.3 AU Periods 3.3–6 years Families of asteroids (similar orbits, surface appearance) may be fragments of a single asteroid produced by collisions Some gaps in belt caused by resonances with Jupiter

Belt asteroids

Trojans Orbit at Lagrangian Points (60° ahead and behind Jupiter) Stable orbits – the asteroids will not be swept up by Jupiter May be several thousand in number Size: Most are a few km, some are >100 km

Trojan Asteroids

Earth-approaching May be several thousand >1 km diameter Radar images are available for several that approached Earth Amor Have orbits crossing Mars’s orbit Perihelion distances between and 1.4 AU (ie between Earth and Mars)

Apollo Cross Earth’s orbit but have semimajor axes greater than 1.0 AU (elliptical orbits!) Aten Have orbits with semimajor axes of less than 1.0 AU (inside Earth’s orbit!)

Centaurs Orbit beyond Jupiter eg Hidalgo: A = 5.9 AU Chiron: A= 13.7 AU (beyond Saturn) Are these asteroids or comets?

Kuiper Belt Asteroids? Beyond Neptune (~ 40 AU) a growing number of small bodies have been discovered Are these asteroids or comets? Distinction may be unimportant – all bodies out here are ice-balls like comets! The Kuiper Belt A belt of many orbiting icy chunks Perturbations alter orbit of Kuiper Belt object and can send it into inner solar system  comet !

Several space missions have brought back close-up pictures of asteroids: –Galileo spacecraft flew by Gaspra and Ida –NEAR spacecraft flew by Mathilde.

Origin of the Asteroids Debris left over from formation of solar system. (Accretion process limited by tidal effects of Jupiter). Total mass (1/20 mass of moon) is too low to have been remnants of a planet. Collisions between asteroids produce smaller fragments (families of asteroids) and meteoroids, some of which fall on Earth.

IV: Collisions between Asteroids and the Earth? Evidence that impact of ~10 km asteroid 65 million years ago, at end of Cretaceous Period –led to the extinction of the dinosaurs –enhanced Iridium in layer at K-T (Cretaceous-tertiary) boundary in sediments worldwide Chicxulub crater (200 km) in Yucatan, Mexico Tunguska event (30 June 1908) in Siberia, may have been an impact with a 100,000 ton body

Watch Out! Estimated that impacts like the K-T impact occur about every 100 million years. Results could be catastrophic to civilization!