13.1 Asteroids

Introduction to Asteroids

• Asteroids are primarily found in the broad region between Mars and Jupiter, known as the asteroid belt.

• They are too small to be seen without a telescope; their discovery began in the early 19th century.

Discovery and Orbits of the Asteroids

• Early Discoveries (Learning Objective 1: Outline the story of the discovery of asteroids and describe their typical orbits)

  • 1801: Sicilian astronomer Giovanni Piazzi discovered the first asteroid, Ceres, while searching for a missing planet between Mars and Jupiter.

  • Ceres orbits at $2.8$ AU from the Sun and was initially considered a planet, then reclassified as an asteroid, and is now classified as a dwarf planet.

  • This discovery was quickly followed by three other similar objects, revealing a group of smaller objects rather than a single planet.

  • By 1890: Over 300 minor planets (asteroids) had been discovered.

  • Technological Advancements: Max Wolf introduced astronomical photography in 1890, significantly accelerating discoveries. In the 21st century, computer-driven electronic cameras and space-based cameras led to over a million well-determined asteroid orbits.

• Asteroid Naming Conventions

  • Asteroids are assigned a number (order of discovery) and often a name.

  • Initially named after Greek and Roman goddesses and other female names. Later, names of colleagues, friends, and places were used.

• Largest Asteroids

  • Ceres (1): Nearly $1000$ km in diameter, the largest asteroid, now classified as a dwarf planet.

  • Pallas (2) and Vesta (4): About $500$ km in diameter.

  • The number of asteroids increases rapidly with decreasing size; for instance, there are about $100$ times more objects $10$ km across than $100$ km across.

• Typical Orbits

  • Asteroids revolve around the Sun in the same direction as planets.

  • Most orbits lie near the plane of the Earth and other planets.

  • The majority (over $75$%) reside in the asteroid belt between Mars and Jupiter, with orbital periods between $3.3$ to $6$ years.

  • Spacing: Despite their vast numbers, asteroids are not closely spaced in the belt; the typical spacing between $1$-km objects is several million kilometers. Spacecraft like Galileo, Cassini, and Rosetta have safely traversed the belt.

  • Some asteroids exist in the inner solar system, and others, like the Trojans and Greeks, orbit along Jupiter's path, controlled by its gravity.

• Asteroid Families

  • 1918: Kiyotsugu Hirayama discovered asteroid families, groups with similar orbital characteristics.

  • These families likely resulted from the breakup of a larger body or the collision of two asteroids.

  • Fragments from collisions spread out, forming families whose members often have similar compositions, suggesting a common origin.

Composition and Classification

• Reflectivity and Spectral Studies (Learning Objective 2: Describe the composition and classification of the various types of asteroids)

  • Asteroids vary significantly in darkness; most are very dark (reflectivity $3-4$%), while others have $15$% reflectivity.

  • Astronomers use the spectrum of reflected light to infer chemical composition.

• C-type Asteroids (Carbonaceous)

  • Composition: Primitive bodies (little chemical change since solar system formation) composed of silicates mixed with dark, organic carbon compounds.

  • Appearance: Very dark.

  • Location: Prevalent in the outer part of the asteroid belt.

  • Examples: Ceres and Pallas are primitive C-type asteroids.

  • Spacecraft Visits: Bennu and Ryugu (C-types) were visited by spacecraft in 2020-2021 for surface contact.

• S-type Asteroids (Stony/Silicate)

  • Composition: Silicate-based composition, lacking the dark carbon compounds found in C-types.

  • Appearance: Higher reflectivity and clearer spectral signatures of silicate minerals.

  • Origin: Also chemically primitive, but likely formed in different locations than C-types.

• M-type Asteroids (Metallic)

  • Composition: Primarily metal.

  • Numbers: Less numerous than C-type or S-type.

  • Identification: Spectroscopic identification is difficult, but radar confirms metal presence for large M-types like Psyche.

  • Origin: Thought to be fragments of the metallic cores of larger parent bodies that differentiated (heavier metals sank to the center) and then shattered in collisions.

  • Resource Potential: Even a $1$-km M-type asteroid could supply vast amounts of industrial metals.

  • Mission: NASA's Psyche mission, launched in 2023, is targeting this asteroid.

• Other Differentiated Asteroids

  • Some asteroids show signs of early heating and differentiation, with basaltic surfaces similar to the Moon and Mars.

  • Vesta is a notable example, discussed in more detail below.

• Distribution: Different classes of asteroids are found at varying distances from the Sun, which helps reconstruct properties of the early solar nebula.

Vesta: A Differentiated Asteroid

• Characteristics: Orbits in the inner asteroid belt at $2.4$ AU, with a high reflectivity ($30$%), making it the brightest asteroid (visible to the unaided eye).

• Composition: Its surface is covered with basalt, indicating it is a differentiated object that was once volcanically active, despite its $500$-km diameter.

• Meteorites: Meteorites from Vesta's surface, identified by comparing their spectra, have been found on Earth.

• Age: Lava flows from these meteorites are dated to $4.4$ to $4.5$ billion years, consistent with intense, short-lived volcanic activity on a small object.

• Mission: The Dawn spacecraft orbited Vesta for about a year.

Asteroids Up Close: Spacecraft Missions

• Galileo Spacecraft (Learning Objective 3: Discuss what was learned from spacecraft missions to several asteroids)

  • Targets: Gaspra and Ida (main-belt S-type asteroids).

  • Findings: Both were long and irregular (battered potato shape), consistent with collision fragments.

  • Craters: Crater counts estimated their age at about $200$ million years.

  • Dactyl: The discovery of Ida's moon, Dactyl ($1.5$ km diameter), allowed scientists to determine Ida's mass and density ($2.5$ g/cm$^3$) using Kepler's laws, matching primitive rocks. Many other asteroid moons have since been discovered.

• Moons of Mars: Phobos and Deimos are likely captured asteroids due to their irregular, elongated, and heavily cratered shapes, resembling other small asteroids. Outer moons of Jupiter and Saturn are also likely captured asteroids.

• NEAR-Shoemaker Spacecraft

  • Target: Eros (S-type asteroid).

  • Mission: Orbited Eros for a year, mapping its surface and interior composition.

  • Findings: Eros is made of some of the most chemically primitive materials. Its uniform density (similar to Earth's crust) and grooves indicate it's a cracked but solid rock, not a rubble pile.

  • Surface: Extensive loose surface material (up to $100$ m deep) with scattered, half-buried boulders.

  • Landing: The spacecraft successfully soft-landed on Eros, continuing chemical analysis for another week.

• Hayabusa 1 Mission

  • Target: Itokawa (small S-type asteroid, about $500$ meters long).

  • Findings: Elongated, appears to be the result of a collision, almost no impact craters but abundant boulders (a rubble pile).

  • Sample Return: Successfully collected a few dust grains that had been exposed for about $8$ million years and returned them to Earth.

• Hayabusa-2 and OSIRIS-REx Missions (Advanced Sample Returns)

  • Hayabusa-2 Target: Ryugu (small, dark, rubble-pile asteroid). Returned samples successfully in December 2020.

  • OSIRIS-REx Target: Bennu (small, dark, rubble-pile asteroid with a small chance of colliding with Earth in the next century). Returned a larger sample in 2023.

  • Key Finding (Bennu): Its composition indicated it was part of a larger object that had liquid water and many organic compounds, including various amino acids (precursors of life on Earth). These chemicals survived and were preserved.

• Dawn Mission

  • Targets: Ceres and Vesta (the two largest main-belt asteroids).

  • Findings (General): Both are heavily cratered, implying old surfaces.

  • Findings (Vesta): Located large impact craters that ejected the basaltic meteorites identified as coming from Vesta.

  • Findings (Ceres): Surface covered with craters similar to lunar highlands. Presence of very bright white spots (primarily salt) associated with central peaks of large craters. Data suggests Ceres has (or had) a subsurface ocean with occasional surface eruptions, including the $4$-km tall ice volcano, Ahuna Mons.

• Lucy Mission

  • Targets: Trojan asteroids (two large clouds preceding and following Jupiter in its orbit).

  • Significance: First mission to explore this category; some speculate they formed separately from other asteroids.

  • Journey: Launched in 2021 on a $12$-year journey to visit eight asteroids, using remote instruments to characterize composition and geology.

  • Nomenclature: Trojan asteroids are named for participants in the Trojan Wars; Greek heroes for those preceding Jupiter, Trojan heroes for those following.

Interstellar Visitors

• ‘Oumuamua (2017 Discovery)

  • Nature: Discovered as an interstellar asteroid, traveling too fast to be part of the Sun's family.

  • Orbit: Hyperbolic orbit, rapidly leaving the inner solar system.

  • Shape: Highly elongated, approximately cylindrical, with nominal dimensions of about $200$ meters in length and $35$ meters across, making it a very extreme natural object.

  • Significance: Its discovery was not unexpected, as planetary systems are common, and ejected debris from other systems was theorized. This confirmed their existence.

• Interstellar Comet (2019 Discovery)

  • Another interstellar object, a comet, was discovered entering our solar system in 2019.