Geol 1420 Chapter 14.1

Asteroids are mostly small celestial bodies found primarily in the asteroid belt between Mars and Jupiter.

S-Group Asteroids
  - Comprise around one-fifth of known asteroids.
  - Characterized as stony and shine fairly bright.
  - Composed of silicate minerals and a metallic nickel-iron mixture.
  - Example:
    - Didymos: A near-Earth binary asteroid, it spent some time in the asteroid belt. Notably, its moonlet Dimorphos was impacted by NASA's DART mission in 2022 and is slated for follow-up by ESA's Hera mission in 2026.
M-Group Asteroids
  - Constitute a small minority of known asteroids and are stony, metallic, and carbon-rich.
  - Appear from very dark to very bright, with similar spectra but varied compositions.
  - Example:
    - Lutetia: Visited by ESA's Rosetta mission in 2010.
C-Group Asteroids
  - Make up around three-quarters of known asteroids.
  - Carbon-rich and often dark as coal.
  - Represent some of the oldest materials in the Solar System.
  - Example:
    - Ceres: The largest asteroid in the belt, visited by NASA's Dawn mission in 2015.

Gaspra and Ida:
- Two small S-type asteroids visited by the Galileo probe.
- Gaspra appears in false color but is actually gray; Ida has a small moon called Dactyl.
Mathilde:
- Low-density asteroid, likely not solid.
Itokawa:
- Considered a typical “rubble pile” asteroid.
Eros:
- Contains many craters, some of which are filled with debris from more recent impacts.
Vesta and Ceres:
- Vesta features deep grooves whereas Ceres contains intriguing bright spots within its craters.

These are asteroids with highly eccentric orbits that cross Earth's orbit, raising concerns about potential collisions.
As of current records, 21,083 Apollo asteroids have been discovered, with 2,130 designated as potentially hazardous due to their size.

Bennu:
- Currently regarded as the most dangerous known asteroid to Earth.
- It carries a 1-in-2700 chance of impacting Earth on September 24, 2182.
2024 YR4:
- Initially labeled a “city killer” for its potential to strike Earth in 2032, but later calculations indicated a strike on the Moon, and by March 25, 2026, astronomers fully ruled out any impact risk.

Comets that approach the Sun closely enough to be detectable from Earth generally have highly eccentric orbits.

Comets exhibit a range of structural components:
  - Nucleus: The solid core of the comet.
  - Coma: A cloud of gas and dust that surrounds the nucleus when a comet is near the Sun.
  - Tails of the Comet:
    - Ion Tail: Always points directly away from the sun.
    - Dust Tail: Curves slightly as the comet moves ahead in its orbit.
Comet tails develop as the comet approaches the Sun and dissipate as it moves away.

One of the most recognized comets; has a periodicity of 76 years and has been observed since ancient times.
Recent notable appearances:
- 1910 (visible to the naked eye).
- 1986 (observed with telescopes).
Although Halley's Comet has a shorter period than most comets, its orbit is not aligned with the plane of the solar system, likely due to past interactions with larger celestial bodies.

Typical comet masses range from $10^{12}$ to $10^{16}$ kg.
Each time a comet approaches the Sun, material is lost; Halley’s Comet is expected to last another 40,000 years before it disintegrates.

Stardust Mission:
- Successfully flew through the tail of comet Wild -2, collecting dust particles for analysis back on Earth.
Deep Impact Mission:
- Involved slamming a projectile into comet Tempel 1 and studying the expelled material to analyze the comet's composition.
Rosetta Mission:
- Entered orbit around comet 67P/Churyumov-Gerasimenko and deployed the lander Philae to its surface.

Most comets that enter the inner solar system originate from the Kuiper belt, which exists outside the orbit of Neptune.
Occasionally, comets emerge from the far larger Oort cloud where they wander into the inner solar system.