Key points: What asteroids, comets, meteors, and Kuiper Belt objects are; where they come from; why Pluto was demoted from planet status
Several types of material are "left over" from the formation of the Solar System:
Asteroids or "Minor Planets"
The illustration to the left is from the highly fanciful classic children's story "The Little Prince" by Antione de Saint-Exupery, and shows the hero gardening on "his" asteroid B-612. From http://www.poetryfountain.com/littleprince.html. Asteroids generally orbit between Mars and Jupiter. About 100 of them have diameters larger than 100 km. The largest is Ceres, 1000 km in diameter and comprising about 30% of the total mass of the asteroid belt. All large asteroids are round because the force of gravity overwhelms the chemical bonding forces in rocks.
However, most asteroids are small enough (< 10 km in diameter) that they can retain irregular spherical shapes -- their self-gravity is not strong enough to force them into spheres.
|Picture of the asteroid Gaspra from the Galileo spacecraft|
|Picture of Ida (with its tiny moon) from Galileo.|
|Many asteroids appear to be "rubble piles" that have had so many collisions that broke them apart that they are just a bunch of pieces loosely held together. This image of Asteroid Itokawa from the Japanese Hayabusa spacecraft certainly likes it could be a rubble pile. Credit ISAS, JAXA, APOD http://apod.nasa.gov/apod/ap051228.html|
Large asteroids are pulled into round shapes by gravity:
|In early 2015, the Dawn Spacecraft visited Ceres and took a full set of images as it rotated (about 9 hours). The white spots were a surprise (see closeup to right) (both from http://dawn.jpl.nasa.gov/multimedia/Ceres_Awaits_Dawn.asp). They seem to be salts left where water seeped up from ice in the interior and evaporated, revealing the icy origin of this asteroid. It lies near but outside the ice line of the Solar System.|
Most asteroids have slightly elliptical orbits lying about 2.2-3.3 times the distance of the earth from the sun (this distance is called an astronomical unit, or AU). These are called Main Belt asteroids. (From U. Tenn. Ast 161, http://csep10.phys.utk.edu/astr161/lect/solarsys/revolution.html)
This distance is about where small objects in the young Solar System would settle because of the evaporation of ice in many of them - that it is, it marks the frost line.
|The distribution of orbital radii shows sharply
confined zones, due to the influence of Jupiter's gravitational field. Jupiter's
gravitational effects are also probably responsible for the presence of the asteroid belt
at all - the continual changes of asteroid orbits by the gravity from Jupiter would have
kept the asteroids colliding with each other and made it impossible for them to coalesce
into a single small planet.
|However, a few asteroids have very
eccentric orbits that cross the earth's orbit -- they are called
"Apollos" or Earth-crossing asteroids. (in recent years asteroids
have passed within 100,000 km of the earth!)
These objects illustrate the potential for asteroids being captured by planets or even colliding with planets. In fact, we believe there are nearly 1000 Apollo asteroids large enough (at least a kilometer in diameter) to cause widespread destruction if they hit earth; however, they all appear to be on orbits that avoid us at least for several centuries (whew!).
Are they the remains of a smashed planet?
More likely is that they are remnants of material that never formed a planet. The strong gravitational force of Jupiter would prevent matter from accreting and forming a planet by continually upsetting the orbits of objects in the asteroid zone, causing a high rate of collisions..
The composition of asteroids is studied by looking at the spectra of light they reflect: rocks, minerals, ices, and metals all have distinct patterns of absorbing or reflecting light.
Three broad categories
|Meteors and Meteorites
(From Terry Platt, http://leonids.hq.nasa.gov/leonids/gallery/files/10.html)
When a small piece of asteroid material gets captured by the gravity of the earth, it hits the upper atmosphere at very high speed and vaporizes, appearing as a meteor. A meteorite is the part of a meteor that reaches the ground (and survives). Meteorites have proven to be an invaluable resource in understanding the Solar System some are relatively pristine examples of materials left over from the formation of the Solar System. If the meteorite is large enough, it will leave a crater where it smashed into the earth.
The dramatic meteor movie to the right starts with a cloudy sky, then a bright flash from the meteor, and finally a bit of residue from the meteor drifts away on the wind.
Meteor craters are found all over the earth (as well as on other planets):
However, as is evident from the pictures above, on Earth they quickly erode away and become much less evident than they are on other bodies with no atmosphere and no weather. The ages for meteorites are usually between 4.55 to 4.65 billion years, which we equate with the age of the Solar System.
A number of meteorites appear to be pieces of asteroids freed in collisions, and by studying them in comparison with asteroids of similar composition, we can probe directly what asteroids are made of.
Meteor showers occur roughly 10 times a year. In these events, the rate at which meteors are observed increases dramatically over a period of a few days. The rates can even exceed 100 meteors per hour. Some meteor showers can be matched up with debris left along a cometary orbit that the earth crosses at the time of the shower..
Comet Hale Bopp (From T. and D. Hallas, http://www.astrophoto.com/index.htm)
|A comet is actually a small, icy body that brightens considerably and produces a magnificent tail when it gets close to the sun. The solid portion of the comet is called the nucleus and is typically 1 to 10 km in diameter. (Fred Burger, http://www2.jpl.nasa.gov/comet/hyakutake/burger5.html )|
|As comets orbit within the solar system, they appear to move against the pattern of stars. (from http://antwrp.gsfc.nasa.gov/apod/image/0007/?M=A/)|
|Probes visited Comet Halley on its
last trip near the earth. The nucleus was surprisingly dark. The dark material is likely
to be similar to tar left behind when lighter substances evaporate. The tail starts from
sites where the crust is broken and icy material is evaporating rapidly into space. To the
right is an image of the nucleus of Comet Hartley 2, in a more quiescent state so the surface
is more visible. (left, from http://science.nasa.gov/newhome/headlines/ast20oct98_1.htm
right from EPOXI team, JPL, NASA, via APOD: http://antwrp.gsfc.nasa.gov/apod/ap040103.html)
|The animation to the left of the data from Comet Hale Bopp makes even
clearer what is happening. The comet nucleus is like a giant pinwheel, ejecting material
into space from an icy region that is exposed. This next animation shows how it might look
if we were really close (warning: large file (5Mb)) (reload page to restart lecture animations)
(animation from STScI http://hubblesource.stsci.edu/sources/video/clips/details/hale-bopp.php)
As the comet ages, it eventually runs out of icy material and becomes an "asteroid".
This closeup (left) of Comet 67P/Churyumov-Gerasimenko taken by the European Space Agency Rosetta spacecraft shows the very rough terrain of the comet surface and how gas and dust is escaping from its interior through cracks and vents in the crust (from http://www.esa.int/Our_Activities/Space_Science/Rosetta/). Rosetta's lander Philae had a hard time settling in because of the weak gravity of the comet but it has given incredible closeup views of the fractured comet surface (right). This comet was selected because it has exhausted most of its ice and is well on the way to becoming an asteroid - this choice was to prevent escaping material from damaging the spacecraft.
Jan Oort, a Dutch astronomer, noticed that the aphelia (most distant point from the sun) of comet orbits describe a large spherical cloud around the orbits of the planets. Although we have never detected objects in this cloud, we think Oort's idea is correct. We believe that most comets come from this "Oort Cloud."
Not all comets originate in the Oort cloud -- others appear to come from another region closer to the sun lying between roughly Neptune and some distance beyond Pluto. We have started to find a large number of objects in this region, called the Kuiper Belt.
cloud and Kuiper Belt from Univ. of Michigan, D. Yeomans http://www.solstation.com/stars/oort.htm and The Essential Cosmic Perspective, Bennett et al.
The Oort Cloud and Kuiper Belt, and hence comets, are remnants of the cloud from which the sun and planets formed.
Two forms of gravitational interaction play important roles in perturbing comets toward the inner solar system:
1) As the sun orbits the center of Milky Way, the varying gravitational force of nearby stars can cause a comet to adopt a new orbit which brings it close to the sun and planets.
2) A comet passing into the inner Solar System can pass close enough to a planet (Jupiter is the most effective at this) to have its orbit strongly modified.
As comets move from the outer Solar System towards the sun, they may experience strong gravitational forces, especially from Jupiter. Such interactions can result in:
Comets typically have very elongated orbits which are not confined to the plane of the Solar System like the planets and normal asteroids.
Cometary orbits come in two types:
-- short period with periods of less than 200 years, making a number of returns over history
-- long period with periods of 200 years and up and only come "once" within human records
(this division is arbitrary; it can be very difficult to tell whether a comet has a very elongated, elliptical orbit or an orbit this is not closed such as a parabola or hyperbola)
|Halley's Comet is the most famous short-period comet: Here is its orbit|
Comet Hale-Bopp is a long period comet: its orbit projects back to far beyond the orbit of Pluto.
Many of the long period comets may be entering the inner Solar System for the first time.
For the ride of your life, drive a comet! Use this link to get it:http://www.windows.ucar.edu/tour/link=/comets/comet_model_interactive.html
As a comet moves into the inner Solar System, the sun begins to heat its outer layers. These layers melt and evaporate to form the coma and eventually also the comet's tail.
|The comet's tail has two components -- a dust tail and a ion tail made of glowing gas. (from http://www2.jpl.nasa.gov/basics/bsf1-3.html, http://images.astronet.ru/pubd/2000/12/27/0001164430/halebopp_gleason.jpg)|
|Comet tails always point away from the sun. Why? Radiation from the sun -- light--hits the dust particles and pushes them outward. (from Wiphu Rujopakarn)|
Each time a comet passes through the inner Solar System, some fraction of it is evaporated away. The solid particles that are lost by the comet still follow roughly in its orbit..
A few comets have even been observed to disintegrate as they passed close to the sun, with the debris generally following the orbit of the former comet. The leftover dust from comet tails produces meteor showers, if the earth passes through the dust trail as it orbits the sun. The picture at 24 microns from the Spitzer Telescope below right summarizes this behavior. The comet Schassmann-Wahmann 3 is breaking up and each of the fragments has a tail, while they all move along the trail of debris left along the orbit by previous passages. (from NASA/JPL, W. Reach).
Kuiper Belt Objects
|Over the last decade, we have started to discover objects mostly outside the orbit of Pluto that are thought to contain mostly icy material and could be thought of as a reservoir of short period comet nuclei (although the ones found so far are much bigger than typical comet nuclei). Pluto is thought to be closely related to them in origin and composition. This zone of objects is called the Kuiper Belt after former UA planetary astronomer Gerard Kuiper. Identify the Kuiper Belt Object in this computerized blink comparison. (from Dave Jewitt, http://www.ifa.hawaii.edu/faculty/jewitt/kb.html)|
|In fact, in
2005 an object a bit larger than Pluto was discovered, in an orbit more than
twice as large (97 AU) and one that is even more tilted than Pluto's
compared with the orbits of the large planets. The new object, named Eris, even has a moon! Seven more objects are known that are at least
half as big as Pluto. The discovery of so many large Kuiper Belt
Objects is what brought Pluto's status as a planet, as opposed to a member
of the Kuiper Belt, into serious question (image of Eris from
|This figure (from
the known large Kuiper Belt Objects with their satellites. It shows why
there was a problem in continuing to treat Pluto as a unique object,
fundamentally different from the rest.
|Here is an artist's guess at how one of these icy worlds (top row third from the left) might look. (European Southern Observatory, http://apod.nasa.gov/apod/ap121226.html)|
In fact, on August 24, 2006 astronomers voted to strip Pluto of its planet status, so it is now considered as one of the larger members of the Kuiper Belt
|We are beginning to get a peek at a class of objects still farther out, with the discovery of Sedna, nearly as large as Pluto and on a very elliptical orbit that takes 10,000 years for a circuit. Right now, Sedna is about 86 AU from the sun (Pluto is only half so far on average), but it gets up to 800 AU from the sun. Sedna gets nearly as far out as the Oort cloud and therefore supports its existence. Here is a quick look at the whole system. Robert Hurt, http://www.spitzer.caltech.edu/Media/releases/ssc2004-05/ssc2004-05v1.shtml|
Test your understanding before going on
Child's book from,http://www.gasolinealleyantiques.com/cartoon/disney.htm
The Great Nebula in Andromeda, from Hoover Dam (G. Rieke)
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hypertext G. H. Rieke
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