The Asteroid Belt, Asteroids, & Comets
The Asteroid Belt, Asteroids, & Comets
Asteroid Belt
What is the asteroid belt?
Why does it exist?
Objects in the main belt are very widely spaced with extremely rare collisions.
Some asteroids aren’t solid but piles of loose rubble.
Total mass of the main belt is less than 5% of the mass of Earth’s moon.
About half of the mass is contained in just four objects:
Ceres
Pallas
Vesta
Hygiea
Lagrangian Points
Points where the gravity of the sun and a planet balance out so that a third object will orbit the sun while remaining stationary relative to the planet.
L1, L2, and L3 tend to be unstable.
L4 and L5 are much more stable.
Orbital Resonance
Relates to Jupiter.
Dwarf Planet Ceres
Ceres: the only dwarf planet in the asteroid belt.
Bright spots = patches of very salty frozen water that welled up from the interior, indicating on-going cryovolcanism.
Contains ∼1/3 of all the mass in the asteroid belt.
*Discovered in 1801 and declared a planet at first.
*The discovery of many other objects in the same orbit caused it to be down-graded to a dwarf planet.
Fundamental Parameters
We can measure chemical composition using spectroscopy.
Most asteroids are carbon-rich rocks; a few (mostly closer to the sun) are mainly metallic.
This observation is consistent with a nebular-collapse formation of the solar system.
Objects in the Asteroid Belt
Vesta: a rocky planetesimal left over from the formation of the solar system!
Comets
Comets formed beyond the frost line.
They are the icy counterparts to asteroids.
The nucleus of a comet is the solid, icy body.
The coma is a cloud of heated gas surrounding the nucleus.
Comas only form when a comet is within 3-5 AU from the sun, formed from sublimated surface ice.
Tails only form when a comet is within 1 AU from the sun.
Comet Tails
The plasma tail is pushed by solar wind and points directly away from the sun at all times.
The dust tail is pushed by radiation pressure (sunlight itself). It also points away from the sun but curves behind the comet in its orbit.
Collecting Stardust
NASA’s Stardust satellite flew through the coma of comet Wild 2 in 2004.
The satellite scooped up material from the coma and returned it to Earth in 2006.
Deep Impact Satellite
A probe delivered by NASA’s Deep Impact satellite was deliberately crashed into the comet Tempel-1 in 2004, and the main satellite took an image.
Spectroscopic analysis of the impact showed the interior of the comet was more rocky than expected.
Extinct Comets
Extinct comets have expelled most of their volatile gases due to many repeat trips around the sun.
They have no material left to form tails.
Many “asteroids” in eccentric orbits outside the main belt were originally comets that ran out of ices.
Meteor Showers
Meteor showers are caused by Earth passing through debris left behind in a comet’s orbit.
Example: the Leonid meteor shower occurs when Earth passes through the orbit of the comet Temple-Tuttle.
Impact Events
Comets must cross the orbit of Jupiter before reaching the inner solar system.
Jupiter’s enormous gravity well does a good job of “vacuuming up” most planet-striking objects.
Comet Shoemaker-Levy 9 struck Jupiter in 1994.
Impacts on Earth
Meteor Crater in Arizona
The Chelyabinsk superbolide (or “airburst”: a meteor that fragments while passing through the atmosphere).
Tunguska Event:
A larger superbolide that hit Earth in 1908.
It was the largest, most powerful space impact recorded in human history.
It was strong enough to have leveled cities but occurred in sparsely inhabited eastern Siberia instead.
Where Do Comets Come From?
Comets come from two distinct populations:
Short-period comets (less than 200 years) come from the Kuiper belt.
Long-period comets (200 to 1,000s of years) come from the Oort cloud.
Kuiper Belt Objects like Pluto are, in many ways, just enormous comets in orbits that do not take them into the inner solar system.
Only a tiny minority of comets cross into the inner solar system.