Asteroids, remnants of our solar system's early days, offer clues about its formation. These rocky bodies, mostly found in the asteroid belt between Mars and Jupiter, come in various types and compositions. Some even venture close to Earth, sparking scientific interest and exploration.

Space agencies have sent missions to study asteroids up close. These missions have revealed fascinating details about asteroid composition, structure, and history. From rubble piles to water-bearing minerals, each discovery brings us closer to understanding our cosmic neighborhood.

Asteroid Characteristics and Exploration

Discovery and orbits of asteroids

• Asteroids first discovered in early 19th century
  • Giuseppe Piazzi discovered first asteroid, Ceres, in 1801
  • Several more asteroids discovered in following years (Pallas, Juno, Vesta)
• Most asteroids orbit Sun in asteroid belt between Mars and Jupiter
  • Asteroid belt located approximately 2.1 to 3.3 AU from Sun (1 AU = Earth-Sun distance)
  • Asteroids in belt have orbital periods ranging from about 3 to 6 years
  • Kirkwood gaps: regions in the asteroid belt with fewer asteroids due to orbital resonances with Jupiter
• Some asteroids, called near-Earth asteroids (NEAs), have orbits that bring them close to Earth
  • NEAs classified into three main categories based on orbits: Atens, Apollos, and Amors
  • Aten asteroids have orbits entirely within Earth's orbit
  • Apollo asteroids have orbits that cross Earth's orbit
  • Amor asteroids have orbits entirely outside Earth's orbit but still come close
• Trojan asteroids share same orbit as a planet, typically Jupiter
  • Trojans located at L4 and L5 Lagrange points, stable gravitational regions 60° ahead of and behind planet in its orbit (Greek camp and Trojan camp)

Types and compositions of asteroids

• C-type (carbonaceous) asteroids
  • Make up about 75% of known asteroids
  • Composed of clay and silicate rocks, with some organic compounds (carbon, water)
  • Relatively low albedo (reflectivity) of 0.03 to 0.09
  • Mostly found in outer regions of asteroid belt
• S-type (silicaceous) asteroids
  • Make up about 17% of known asteroids
  • Composed of silicate materials and nickel-iron
  • Relatively high albedo compared to C-type asteroids, ranging from 0.10 to 0.22
  • Mostly found in inner regions of asteroid belt
• M-type (metallic) asteroids
  • Make up about 8% of known asteroids
  • Composed mainly of nickel-iron
  • Relatively high albedo, similar to S-type asteroids
  • Mostly found in middle region of asteroid belt
• Other less common types include V-type (basaltic), E-type (enstatite), P-type (primitive)
• Spectroscopy is used to determine asteroid composition by analyzing reflected light

Formation and Evolution of Asteroids

• Asteroids are remnants of planetesimals, small bodies that formed in the early solar system
• Some asteroids collide and break apart, creating smaller fragments
• Fragments that reach Earth's surface are called meteorites, providing valuable information about asteroid composition
• Impact craters on asteroids and other bodies provide evidence of past collisions

Spacecraft missions to asteroids

• NASA's Near Earth Asteroid Rendezvous (NEAR) Shoemaker mission (1996-2001)
  • Studied near-Earth asteroid 433 Eros
  • Discovered Eros has density similar to Earth's crust, suggesting solid composition
  • Found evidence of layer of regolith (loose surface material) on asteroid's surface
• NASA's Dawn mission (2007-2018)
  • Studied dwarf planet Ceres and asteroid Vesta
  • Discovered Ceres has differentiated interior with rocky core and icy mantle
  • Found evidence of past water activity on Ceres, including bright salt deposits in craters
  • Revealed Vesta has differentiated interior and history of volcanic activity
• Japan's Hayabusa mission (2003-2010)
  • Studied and sampled near-Earth asteroid 25143 Itokawa
  • Discovered Itokawa is a "rubble pile" asteroid, composed of numerous small rocks held together by gravity
  • Successfully returned small amount of asteroid material to Earth for analysis
• NASA's OSIRIS-REx mission (2016-2023)
  • Studying and sampling near-Earth asteroid 101955 Bennu
  • Found evidence of water-bearing minerals on Bennu's surface
  • Successfully collected sample of Bennu's surface material in October 2020, with sample scheduled to return to Earth in September 2023