Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 34: Asteroids and Meteoriods

Key Ideas:

Asteroids: Small bodies in the inner solar system; Asteroid Belt between Mars and Jupiter; Orbits are strongly influenced by Jupiter; Made of rock, metal, or a mix of the two
Meteoroids: Bits of rock and metal orbiting the Sun; Seen as meteors or collected as meteorites
Earth impacts: Craters on Earth

Discovery of Asteroids

Many noticed the large gap between the orbits of Mars and Jupiter: Searches started in earnest in the late 1700s to find a planet in this region
In 1801 Giuseppe Piazzi discovered Ceres at 2.8 AU, which was originally classified as a planet: Many more, smaller objects were then discovered and called "asteroids" (star shaped) by Herschel
About a hundred were known by 1900

Asteroids Today

At present: >20,000 asteroids with known orbits and names; >60,000 with insufficient orbit data; When you know its orbit, you can name it
Asteroid Sizes: Only ~100 are >140 km across; About 100,000 >1km across; There are probably over 1 million undiscovered asteroids 1km or larger

Ceres

Originally classified as a planet in 1801: About 900 km in diameter
Demoted to asteroid ¹Ceres around 1851: Called ¹Ceres because it was the first asteroid discovered
Promoted to Dwarf Planet in 2006

The Asteroid Belt

90% of Asteroids are in the Asteroid Belt: Lies between about 2.1 and 3.2 AU; Orbits can be tilted up to 15 degrees (a few up to 30 degrees); Some orbits are fairly eccentric (e=0.15); About 5 million km apart on average; Total mass in asteroids is only ~0.0008 M_Earth, Ceres is about 1/3 or this total; About enough for a mid-sized rocky moon; Jupiter's gravity likely prevented these bodies from merging into a planet

Kirkwood Gaps

Gaps in the asteroid population are at mean motion orbital resonance with Jupiter:: Gap at 2.5 AU is the 3:1 resonance (3 orbits for every Jupiter orbit); Main Belt is between the 2:1 and 4:1 resonances; Asteroids that were in these resonances had their eccentricity increased until they collided with another body
Also get confining resonances:: Asteroids confined to specific orbits; 3:2 (Hildas), 7:2 (Floras), and 1:1 (Trojans)
Caused by the slow inward migration of Jupiter

Asteroid Exploration

NASA's Dawn Mission was launched in September 2007: Visits Vesta in 2011, 2012; Visits Ceres in 2015

Asteroid Shapes

Asteroids are irregular in shape: Too small for gravity to make them spherical; Even the largest, Ceres, is only semi-round
Rotate as they orbit: Most have rotation peroids of about 9 hours; Extreme range from <3 hours to many weeks, probably reflecting different collision histories

Composition

Asteroids are classified by their colors: C-type: dark in color, probably composed of carbonaceous materials (~75%); S-type: reddish in color, probably stony or stony iron (~16%); M-type: bluer than S-type, probably iron-rich
Rest are in the 'other' category

Monoliths or Rubble Piles?

Some asteroids are clearly solid: Densites of 3-5 g/cc, like solid rock/metals; Heavily cratered surfaces and dusty regoliths
Others appear to be rubble piles: Lower in density (1-2 g/cc); Loose aggregates of rock held together by mutual gravity; Formerly solid but shattered by impacts?

Clues to Asteroid Origins

Silicate and Iron rich asteroids are probably fragments of larger, differentiated bodies: Parents were hot enough to differentiate into silicate mantles and iron cores; Got shattered by collisions into smaller pieces
Carbonaceous asteroids may be the remnants of more primordial material that never got differentiated

Meteoroids and their Progeny

Meteoroids:: Chunks of rock and iron smaller than asteroids orbiting the Sun; Sizes range from grains to 100 m across
Meteor:: Streak of light when a meteoroid enters the Earth's atmosphere; Most are tiny grains
Meteorite:: Any remnant that reaches the ground intact

Types of Meteorites

Stony Meteorite: (92%): Silicate rocks; Fragments of S-type asteroids
Iron Meteorite: (6%): Iron chunks; Fragments of M-type asteroids
Carbonaceous Chondrites:: Carbon-rich, with complex carbon compounds; Fragments of C-type asteroids

Origin of Meteorites

Orbits of some meteors have been traced back after their entry into the atmosphere: Some originate in the main asteroid belt; Those making "meteor showers" are trails of debris left behind by passing comets; Rare meteors have been found that have been knocked off the Moon or Mars
Among the oldest rocks in the solar system (radiometric ages of 4.6 +/- 0.1 Gyr)

Near Earth Asteroids

Asteroids whose orbit is close to the Earth
Orbits range from 0.98 to 1.3 AU
Several 1000 are known
Many searches actively catalog and track these objects
A subset are classified as Potentially Hazardous Objects: Come within 0.05 AU of Earth; Are at least 150m in diameter

Giant Impacts on the Earth

Impact Earth's surface at 10-30 km/hour
Craters are typically 10x larger than the impactor
Shockwaves far exceed the strength of typical meteor material (except maybe an iron core)
Special minerals can form at the extreme temperatures and pressures of impact

End of Civilization?

Small collisions occur often, but almost always go unnoticed
A 100m asteroid could destroy a 'city-sized' area: Occur about every 10,000 years
A 1km asteroid would cause extreme, widespread destruction: Occur about every 0.5 million years
A 10km asteroid would be an 'extinction level event': Last such event was 65 million years ago (K-T event)

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.