MINOR MEMBERS OF THE SOLAR SYSTEM: Asteroids
Dec 18, 2015
• Images of three asteroids, taken during spacecraft flybys, shown to scale (Mathilde is 59 km wide and 47 km high)
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 LawPlanet Calculated distance True distance
Mercury (0+4)/10 = 0.4 0.39
Venus (3+4)/10 = 0.7 0.73
Earth (6+4)/10 = 1.0 1.00
Mars (12+4)/10 = 1.6 1.5
? (24+4)/10 = 2.8
Jupiter (48+4)/10 = 5.2 5.2
Saturn (96+4)/10 = 10.0 9.6
Uranus (192+4)/10 = 19.6 19.2
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
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
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 1.017 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