Small Bodies of the Solar System

Part I: Asteroids & Meteorites

A Missing Planet?By the 1700’s, it became clear to astronomers that there was something peculiar about the layout of the Solar System.

The orbits of the terrestrial planets appear to form a pattern, but there was something missing:

Mercury 0.4 AU

Venus 0.7 AU (0.3 AU farther out)

Earth 1.0 AU (0.3 AU farther out)

Mars 1.5 AU (0.5 AU farther out)

??? 2.0 AU (0.5 AU farther out)

A Missing Planet?Working in from the jovian planets suggested another pattern with the same sort of problem:

Uranus 19.2 AU

Saturn 9.5 AU (half of 19.2 = 9.6 ~ 9.5)Jupiter 5.2 AU (half of 9.5 = 4.75 ~ 5.2)

??? 2.6 AU (half of 5.2 = 2.6)

There was no planet at a distance of 2 – 2.5 AU.

Was a new planet waiting to be discovered?

A New Family of ObjectsIn 1801-2, Ceres and Pallas were discovered at an average distance of 2.8 AU from the Sun.

This led to some confusion – why were there two planets orbiting the Sun at the same distance?

Over the next few years, it became clear that there was a large family of objects orbiting between Mars and Jupiter.

Eventually, it became clear that these objects were much smaller and orbited the Sun together. They became known as the asteroids.

Asteroid FamiliesToday, nearly 500,000 asteroids have been found. They fall into several different families according to their orbits.

Main belt: orbits outside Mars’ orbit and inside Jupiter’s orbit (vast majority of asteroids)

Trojans: orbit same as Jupiter, but lead or trail Jupiter by 60 degrees (~2000 members)

Atens, Apollos, Amors: orbits that are similar to, or cross Earth’s or Mars’ orbit (~3500 members)

About 13,000 asteroids have names, the rest have serial numbers.

Asteroid Families

Members of the main belt are shown in white.

The Trojan asteroids are shown in green.

Other smaller families are in orange, red, green, and blue.

Jupiter

Earth

A Failed PlanetIf you lumped all the asteroids together into 1 body the total mass is about 4% of the Moon’s mass.

The young Jupiter stirred up the planetesimals in this region of the Solar System, spurring collisions and preventing a single object from forming outside the orbit of Mars.

Over the history of the Solar System, those objects that did form were ejected by Jupiter or sent on orbits that eventually collided with the Sun or a planet. Not much is left in the asteroid belt…

Flying MountainsThe asteroids are generally rocky in composition, but some appear to contain some water ice.

Small asteroids never got hot enough to melt and never differentiated. They are mixtures of rock and metal and other compounds throughout.

Large asteroids retained enough heat to melt. They have metal cores and rocky exteriors just like the much larger terrestrial planets.

Not Really CrowdedTypical distances between asteroids is millions of kilometers. Unlike what is portrayed in sci-fi films, the asteroid belt in our Solar System is not a navigational hazard.

In fact, spacecraft have to be carefully directed to fly close enough to an asteroid to get a decent picture.

Otherwise, the tiny asteroid just looks like a point of light (aster + oid = Latin star-like).

A Near-Earth AsteroidIn 2000, the NASA spacecraft NEAR orbited the asteroid Eros. This asteroid crosses Mars’ orbit but is not a direct “threat” to Earth.

NEAR orbited Eros for one Earth year and then soft-landed on the surface in 2001.

Images sent back revealed a body that is heavily cratered and probably fractured in its interior.

A Rubble PileIn 2005, the Japanese spacecraft Hayabusareturned images of the asteroid Itokawa. This asteroid has the most “rubbly” surface yet seen.

Impacts blasted the asteroid apart and its weak gravity slowly pulled it back together.

In June 2010, dust from Itokawa was brought to Earth.

Dawn: A Mission to the Big OnesIn 2007, the NASA spacecraft Dawn was launched to orbit and explore the two largest members of the Main Belt.

Dawn will orbit Vesta in 2011-2012 and thenorbit Ceres in 2015-?.

Dawn: A Mission to the Big OnesVesta is much bigger than all previously-visited asteroids. Geology should be quite different!

Dawn: A Mission to the Big OnesDawn arrived in orbit around Vesta in July 2011 and began mapping.

Further information at http://dawn.jpl.nasa.gov

Meteoroids, meteors, meteoritesThree names for the same object:

In space, it’s a meteoroid.

In the atmosphere, it’s a meteor.

On the ground, it’s a meteorite.

These names apply to Earth or any other world.

Types of MeteoritesMeteorites are separated into three classes:• Stones, composed mostly of silicate rock are

further divided into two more groups• Chondrites, very primitive unmelted mixtures,

very old (dating back to birth of Solar System)• Achondrites, rocky material from the crust of a

differentiated world (asteroid or larger)• Irons, nearly pure iron-nickel metal, pieces of the

core of a shattered asteroid• Stony-Irons, unusual mixtures of metal and rocky

glass, possibly from mantle-core boundary?

Allende MeteoriteIn 1969, several thousand small stony meteorites landed on and around the small town of Allende, in northern Mexico.

Analysis of the meteorite showed it was primitive, having formed before the planets!

The small spheres inside were once liquid drops of rock.

The parent asteroid never differentiated, so Allende is a “fossil” from the earliest days of the Solar System.

Meteors and MeteoritesOn an average dark night, you can see a few meteors every hour. These are due to random bits of interplanetary dust that strike the top of Earth’s atmosphere. The flash is caused by frictional heating of air.

Most meteors do not reach the ground. They burn up high in the atmosphere.

An object that does reaches the ground is a meteorite.

Heads Up!And just to round out the discussion of meteorites, here are a few spectacular examples of falls.

The Peekskill meteor was witnessed by thousands on the evening of October 9, 1992.

The meteorite was found under the car it had struck.

Heads Up! Part IISeptember 15, 2007: A multi-ton meteorite fell near Carancas, Peru creating a large impact crater in the mud. Pieces found by local villagers are of an ordinary stony meteorite.

Heads Up! Part IIIOctober 6, 2008: Richard Kowalski of Tucson, AZ discovers a small meteoroid approaching Earth.

Later observations shows it will impact Earth over Sudan two days later. This is the first time an object was seen before it impacted Earth.

December 6, 2008: Peter Jenniskens’ team find fragments of the object. Here’s fragment #15…

Part II: Comets

Comets: Cosmic Show-offsEven the largest asteroids are difficult to see in the night sky.

Comets, on the other hand, are show-offs.

Anatomy of a Comet“Comet” means “hairy star”. The heart of a comet

is its nucleus, a dusty ball of ice 1-100 km across. The visible part of the comet is dust and gas that sublimates from the heated ices of the nucleus.

The head of the comet is known as the coma. Comas can be larger than 100,000 km across.

The tail or tails can stretch for millions of kilometers. Comet Hyakutake in 1996

Comet TailsComets have two tails:The plasma tail is gas that points straight out from the Sun, blown by the solar wind. The blue glow is due to light emitted by carbon, oxygen and hydrogen.

The dust tail is also pushed out the comet’s nucleus, by pressure of sunlight. It is white-yellow due to reflected sunlight.

Comet Hale-Boppin 1996

The Nucleus of Comet HalleyLike asteroids, comets have only recently been seen up close. The Giotto mission (1986) snapped the first images of Comet Halley’s nucleus.

Streams of gas and dust flowing off the nucleus.

The nucleus is 16 km long.

The Nucleus of Comet HartleyMost recently (November 2010), the EPOXI craft flew past the nucleus of Comet Hartley.

Streams of gas and dust flowing off the nucleus.

The nucleus is 2 km long.

Comet OrbitsComets have very ellipticalorbits.

This takes them close to the Sun and then far away.

Ices and dust evaporate from the nucleus to form the coma and tails during times of strong solar heating.

At other times, the nucleus is cold and quiet and there is no coma or tail.

Comet ReservoirsTwo populations :

Kuiper Belt

Formed in place; Fragments from collisions

Oort Cloud

Icy planetesimals thrown outward during formation of jovian planets

Frozen HistoryComets have never felt very high temperatures so the ices and rock that formed them are mostly unchanged from the earliest days of the Solar System.

Like meteorites, understanding comets and collecting samples of comet dust provides a window on the formation of the Solar System.

Preliminary results from several recent comet missions (Stardust, Deep Impact) make it clear that planet formation was a very messy process.

Part III: Kuiper Belt Objects

Pluto and the Kuiper BeltPluto was discovered in 1930. It was found during a search for “Planet X”, so it came to be called a planet. (The asteroid story repeating itself…)

It wasn’t until 1992 that the second small, icy world orbiting beyond Neptune was found : 1992 QB1.

Today, astronomers have found more than 1000 small, icy worlds orbiting beyond Neptune. These objects are commonly called Kuiper Belt objectsor KBOs.

In 2005, Eris was found. Eris is about the size of Pluto. Other large KBOs are likely out there, too.

The Kuiper Belt and Others

The Kuiper Belt and OthersThe largest objects beyond Neptune...

What is Pluto Like?Pluto is the last major piece of the Solar System that has not been explored, so we don’t really know much about it. But we can make some educated guesses with the help of telescope observations.

Triton, moon of Neptune, is most likely a captured Kuiper Belt object. So Pluto may have a similar surface.

What is Pluto Like?

After the flyby through the Pluto system, the spacecraft will be sent to a 2nd KBO if one can be found in the direction of New Horizon’s motion.

We won’t really know what the surface of Pluto is like until the arrival of New Horizons.

Launched in January 2006, this small spacecraft will reach Pluto in 2015.