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A Little Bit of Rock

Beyond the Planets - Comets, Asteroids and Meteors

When you mention the Solar System, most people think of the Sun and the nine planets. The Sun and planets certainly account for almost all the mass, but there's even more to our Solar System than that!

Large numbers of asteroids and comets also circle the Sun. Meteors rain down on the Earth's upper atmosphere constantly; when the Earth passes through the denser parts of the meteor distribution, we're treated to a stunning meteor shower down here on Earth.

The study of asteroids, meteors and comets is a vibrant one! Over the past few years we have begun tracking and cataloguing Near Earth Asteroids (NEAs), rocky bodies a few kilometers across whose orbits cross that of Earth. Since many scientists believe that an asteroid impact may have been the cause of the mass extinctions that wiped out the dinosaurs, focusing our attention and technology toward the NEAs seems only prudent. While an impact of a large asteroid with Earth is statistically incredibly unlikely, if we wait long enough (where 'long' is measured in millions of years) it's bound to happen.

The past few years have also seen a rapid increase in our discovery and understanding of "Trans-Neptunian Objects" (TNOs); larger, rather icy bodies that orbit the Sun near the 3:2 resonance with Neptune. For each three orbits Neptune makes around the Sun, the TNOs orbit twice. Remember, Neptune's orbit is 165 years! If we take three times Neptune's orbital period and divide by two, we get the orbital period of these objects - 247.5 years. Sound familiar? That's Pluto's orbit as well! If Pluto is around the same size, and in the same orbit as the TNOs, is it actually a planet unto itself, or just the largest member (so far) of the TNO class of objects? The status of Pluto as a full-fledged planet is an area of vigorous debate in the planetary science community right now!

The TNOs are now believed to mark the beginning of the Kuiper Belt in our Solar System, the home of the short period comets. Comets have an incredibly rich history, and the careful study of their orbits and characteristics allowed early scientists to take incredible leaps forward in understanding the Solar System and gravity---the force that holds the whole Solar System together. These relatively small bodies, composed mostly of water-, carbon-dioxide- and methane-ice with organic molecules as well, orbit on very eccentric ellipses about the Sun, and visit the inner Solar System only very briefly on each orbit about the Sun (tens of years, to tens of thousands of years). Recent scientific investigations of comets have led many to believe that Earth's oceans are the result of comet bombardment early in the Solar System's history. Some believe that comets delivered organic matter to the Earth, contributing one of the essential building blocks to the development of life on Earth. Recent measurements of a dying star indicate a vast swarm of comets near its outer edges, in direct agreement with our current understanding of the formation of our Solar System. Does this imply that comets are typically present, or created, when stars like our Sun are formed? If comets are essential to the existence of oceans and life on our planet, might they also be a critical link in the development of life around other stars?

The Asteroids

An amazing mathematical curiosity called the Titius-Bode Law, first recognized by Titius in 1766 and published by Bode in 1772, still fascinates many astronomers today. Here is how it works. For each planet whose distance was known to astronomers in the 18th century, write down a number 3, like this:

MercuryVenus Earth Mars ? Jupiter Saturn Uranus
33 33 3 333
Now, multiply each 3 by the following values of 2n:
2-infinity 20 2122 23 24 25 26
(In other words, multiply each 3 by:)
0 1 248 1632 64
You get
0 3 61224 4896 192
Two more steps to go! Now to each of the numbers above, add the number 4:
4 7 101628 52100 196
We're almost done Finally divide each number above by 10:
0.4 5.2 10.0 19.6

If you're a student of the solar system, many of these numbers should look familiar. They are nearly identical to the average distance of each planet from the Sun in Astronomical Units (AU)! 1 AU is the distance of the Earth from the Sun, averaged over one year. Mercury is at nearly 0.4 AU from the Sun, and Venus is nearly 0.7 AU. Mars orbits the Sun at a distance very nearly 1.6 AU. Jupiter is found 5.2 AU out, Saturn nearly 10 AU, and Uranus was discovered at the distance expected from the Tituis-Bode Law, namely 19.6 AU.

But wait, there's a number in the series between Mars and Jupiter! Astronomers in the 18th century took this as very strong evidence that an unknown planet must be located about 2.8 AU from the Sun. The search was on for the missing planet! On January 1, 1801, Piazzi discovered Ceres. Since Ceres is rather dim and its orbit was not well known right away, Piazzi was unable to continue his observations. Carl Friedrich Gauss, an incredibly talented physicist, astronomer and mathematician, calculated an orbit based on the original observations, and Ceres was observed again exactly one year later, on Jan. 1, 1802, by Franz von Zach. Ceres orbits the Sun once every 4.6 years and is less than 1000 km across.

The discovery of Ceres was quickly followed by the discovery of many more small objects at about the same distance from the Sun as Ceres. Instead of a missing planet, astronomers had found a thin swarm of much smaller, irregularly shaped objects orbiting in a band around the Sun, and Ceres is the largest among them. These objects are asteroids, orbiting in what is now known as the Main Asteroid Belt.

asteroid belt in relation to the planets
Caption: The orbits of the Earth, Mars and Jupiter are shown in relation to the Main Asteroid Belt. Nearly 10,000 Main Belt Asteroids have been catalogued, with hundreds more added each year. Hundreds of thousands of asteroids are believed to inhabit this region. The smallest may never be directly detected from Earth.

The search for a missing planet, motivated by a number in Bode's Law that didn't correspond to any of the known planets, led to the discovery of the asteroid belt, and a new understanding of the Solar System. The Sun and the nine planets orbiting the Sun are not the only members of our Solar System family. Tens of thousands of asteroids (also known as Minor Planets) also spin with us. It is not yet known whether they are remnants of a small planet that was destroyed by violent collisions with other bodies, or primitive planetesimals from the earliest period of our Solar System that couldn't coalesce to form a single planet due to the perturbing influence of nearby Jupiter.

Scientists still don't know why Bode's Law works. It's not explained by any well-known physics the way Kepler's Laws of Planetary Motion are explained by Newton's Laws of Universal Gravitation. It seems to be "ad hoc" - in other words, a rule that was made up in order to fit the data. The fascinating part for astronomers is why it works at all! Is Bode's Law connected to some fundamental dynamics at work near the birth of the Solar System? It would seem to be - similar relationships can be written down describing the distances of Saturn's satellites from Saturn, and Jupiter's satellites from Jupiter. But the rationale behind these relationships awaits a complete explanation in the framework of gravity and Solar System formation. Maybe a future astronomer will put together a complete picture that explains Bode's Law. Maybe that astronomer of the future will be you!

How Dense is the Asteroid Belt?

Taken together, the hundreds of thousands of pebble-sized and larger objects located in the Main Asteroid Belt possess a total mass less than that of Earth's Moon. Spread out along a ring around the Sun at 2.8 AU, the actual distribution of asteroids is very, very thin. There is a lot of space between the individual asteroids. Space probes sent out to visit the outer planets easily pass through the asteroid belt unharmed by collisions with a significant asteroid. In fact, the orbits of probes that have recently encountered asteroids have been planned very carefully to make sure that they will come close enough to a large asteroid to get detailed information on it.

In 1993, the Galileo space probed passed within 10,000 km of the asteroid Ida. An image from this encounter is shown below. Scientists plan and schedule close asteroid flybys like this in order to learn about the history of the asteroid belt. In the image, it is clear that Ida is irregularly shaped - more like a lumpy potato than the nearly spherical major planets. (This image also reveals a surprise! Ida has a natural satellite, or moon, that orbits it!) Our current understanding of planet formation starts with a swarm of planetesimals - possibly similar in shape, size and composition to Ida - attracting each other through their mutual gravity and sticking together in a process called 'accretion'. Once enough planetesimals have joined together, the mass of the object reaches a critical value where self-gravity causes differentiation to occur. Differentiation causes the heavier elements to sink to the center of the object, and causes the object as a whole to assume a spherical shape.

Ida and its moon
Caption: August 28, 1993 Galileo spacecraft encountered Ida (left) and its moon (right). This is one of the series of images from which the moon was discovered. The surface of Ida is covered with numerous craters of many different sizes, just like the surface of the Earth's Moon, Mercury, and most solid Solar System objects.

If Ida is the remnant of an early planet that was obliterated by a collision, then its surface may show signs of differentiation. If it is an ancient planetesimal, unchanged since the early formation stage of the Solar System, then there should be no evidence of differentiation. Analysis is still continuing on data from Ida, to try to answer this question.

How Close do Asteroids Come to Earth?

Tracking and cataloguing Near Earth Asteroids (NEAs) may someday be essential to our survival. Some scientists believe that several mass extinctions on Earth may have been the result of climate changes due to dust being kicked up into the atmosphere following a large asteroid impact. We have ample evidence that impacts have occurred throughout the inner Solar System (just look at the surface of the Moon and Mercury). While the era of heavy cratering is behind us, and the likelihood of a major asteroid impact on the Earth is extremely unlikely in the next several hundred years, it is clear that such an impact could have devastating consequences. Read more about cratering.

Asteroids with orbits that approach the Earth, passing inside the perihelion of Mars, are known as Amor asteroids. Those that cross the orbit of Earth are Apollo asteroids. A third group of Near Earth Asteroids that cross the Earth's orbit but do not travel further from the Sun than the Earth's aphelion are known as Aten asteroids. (Aphelion: the point at which a solar system body is farthest from the sun in its orbit. Perihelion: the point in its orbit at which it is closest to the sun.) Calculations show that the Amors, Apollos, and Atens have been in their present orbits for 10-100 million years.

Starting in the 1990s, NASA made a commitment to detect and track 90% of all the large asteroids that come close to the Earth's orbit. This work is done with images that are analyzed by computer to find previously undetected objects. A recent analysis of all detected NEAs suggests that there are fewer large Earth-crossing asteroids than was once thought. New estimates put the number between 500 and 1000 asteroids larger than 1.0 km which cross the Earth's orbit. The majority of these are on orbits that are inclined to the Earth's orbital plane in such a way that they will never collide with the Earth.

By tracking the known NEAs and discovering those that are lurking in our neighborhood, scientists are able to project their orbits far into the future and identify potentially hazardous asteroids in plenty of time for the Earth as a whole to react.

Whether or not they hit us, the Near Earth Asteroids are important. We study these objects to find out what they're made of, to study the origin and evolution of the Solar System, and to determine what resources might be available for us in space activities - NEAs that come near enough to the Earth may one day be mined for raw materials for building and manufacturing in space in the near future. And, as we'll see below, the NEAs are the source of many of the meteors we see in the skies.

There are also asteroids that share Jupiter's orbit of 5.2 AU, 60 degrees ahead of and behind the planet. These are called the Trojan asteroids. The position they occupy, forming an equilateral triangle with the Sun and Jupiter, is a special, stable point in the gravitational field called a Lagrange point. An asteroid has been found at one of the Lagrange points of Mars as well. Recently, some teams have begun to search for asteroids in the Lagrange points of Earth's orbit. It is not clear exactly how the Lagrange points of Jupiter were populated with the Trojan asteroids, but once they are in place, the relatively minor perturbations they suffer from other planets do not provide enough energy to release them from the stable point.

graph showing the rapid increase in the number of known NEAs
versus time
Caption: The increase in our knowledge of all Near Earth Asteroids, from 1980 to the present. Large NEAs (greater than 1.0 km) are shown in red. The increase in detections since 1997 is the result of NASA's effort to detect 90% of the larger NEAs.
For the most recent data: see the JPL CNEOS site

The Comets

About once a year a naked-eye comet is discovered which gives us an incredible sky show for several weeks before fading back into obscurity. Comets have visited our skies in this way for as long as we have a historical record, but they remained mysterious objects until the last few hundred years. There is a lot of exciting science going on right now in the cometary field!

Image of Comet halley nucleus
Caption: Giotto's close encounter with Comet Halley in 1986. This image shows the irregularly-shaped nucleus, which is quite dark. We can also see outgassing, where gas and dust escape from the comet's surface into space. This outgassing generates the comet's coma.

Comets orbit the Sun on extremely elliptical paths. Since Kepler's Laws tell us that the line between the Sun and any body in orbit about the Sun must sweep out equal areas in equal times, this means that comets spend a very small fraction of their lives in a quick jaunt through the inner Solar System. Most of their lives are spent in the outer Solar System, as members of the Kuiper Belt or the Oort Cloud. During this time the gas and dust that make up the comet are frozen solid. Only as a comet approaches the Sun and warms up are the brilliant coma and tail produced. The nucleus of a comet is usually between 1 km and 50 km across. The coma, on the other hand, can reach a size of several thousand km across. The coma of the great comet of 1811 (one of the largest comets observed to date) briefly reached the same apparent size in the sky as the Sun! As a comet approaches the inner Solar System it can form a very long tail. The great comet of 1843 generated a tail that was about 250 million km long (well beyond the distance from the Sun to the planet Mars!).

graphic of the location of the Kuiper Belt
Caption: The Kuiper Belt is now believed to be the reservoir of the short period comets. The Kuiper Belt begins just beyond the orbit of Neptune, and extends out beyond the orbit of Pluto. In fact, Pluto's realm is shared by an ever-growing legion of minor icy planets known as Plutinos (little Plutos).

Short period comets (less than 200 years) come from the Kuiper Belt. On the other hand, Oort Cloud comets may have periods of many thousands of years. The Oort Cloud is immense, and trillions of comets are thought to reside here. The total mass of comets here is estimated to be 40 times the mass of the Earth. Individual comets are tens of millions of km apart. The edge of the Oort Cloud may reach half as far as the nearest star, or perhaps even further! Objects this far from the Sun are only weakly bound by the Sun's gravity. Perturbations from nearby stars can thus affect their orbits, possibly kicking some into the inner Solar System. Others may become unbound and travel through interstellar space. Since the Sun orbits the center of the Milky Way Galaxy, it encounters different stars and giant clouds of molecular gas and dust on its way. Every 300 million years or so, our Oort cloud may be dramatically redistributed by a close approach of such a cloud. The tidal force of the Milky Way core also deforms the Oort Cloud of our star, as well as other stars. It's possible that comets in the Oort Cloud may jump from star to star!

Recently, evidence has been mounting that comets played a pivotal role in the development of life on Earth. Some scientists believe that a rain of comets is the source for virtually all the water in the oceans on Earth. Other researchers, called astrochemists, have found evidence that organic molecules might have first developed in the slushy interiors of comets as they warmed when approaching the Sun. It's possible that the stuff of life was delivered to Earth in the form of crashing comets! If comets really can jump from star to star, the building blocks for life on Earth may even have come from another Solar System entirely. Very deep stuff!

The SWAS satellite telescope has recently delivered another cometary surprise: a Sun-like star that is a little more massive than the Sun, and hence a little more evolved, shows strong evidence of a swarm of comets in the outer regions. Our current view of Solar System formation includes a model for what the comets are (frozen remnants of the very early Solar System) and how they got out to such great distances (slingshots around the giant planets). Finding a large cloud of comets around a similar star lends us confidence that we have the overall picture of star formation and Solar System formation about right. If comets turn out to be essential to the development of life on our planet, then finding comets encircling other stars is one more independent piece of evidence that life may be common in our Galaxy.

graphic showing the shape and location of the Oort Cloud
Caption: The home of long period comets is the Oort Cloud, a spherical halo encircling our Sun and planets, containing millions of comets, and reaching out a significant fraction of the distance to the nearest star, Alpha Centauri.


Directly related to the asteroids and comets, the solar system also contains other "little bits of rock" known as meteoroids. We refer to these as meteors when they are seen streaking through the sky, and meteorites if they survive their journey and reach the surface of the Earth.

Meteors are also known as shooting stars, or falling stars. As a meteor passes through the atmosphere it is heated by friction and may glow for several seconds before it burns up. You may see several per hour on an average dark, clear night. Meteor showers are seen at certain times of year, when the Earth's journey around the Sun takes it through an orbiting stream of debris from a comet that has broken up. The most prominent annual meteor showers include the Lyrids in April, the Perseids in July/August, the Orionids in October, the Leonids in November, and the Geminids in December. During such a shower, you might see several hundred shooting stars in an hour.

Meteors generally enter the atmosphere at speeds of 10-70 km/sec. As they pass through the atmosphere they suffer heating, ablation, and high pressure, which often cause them to break into fragments. A meteorite is called a 'fall' if it was seen to hit the Earth, or a 'find' if it is discovered after the fact, with no witnesses to its arrival.

It is estimated that anywhere from 100 to 3000 tons of "extraterrestrial" material--ranging from small dust particles to boulders that weigh several tons--enter the Earth's atmosphere each day. Over 100 meteorites hit the Earth each year. An asteroid that is 1 km in diameter or larger collides with the Earth about once every 1.5 to 2 million years.

About 50,000 years ago, an iron meteor between 30 and 50 meters in diameter came to Earth near Winslow, Arizona, forming a crater 1200 meters across and 200 meters deep. Perhaps you can visit Meteor Crater (also known as Barringer Crater) on your next visit to the southwest.

An even more spectacular meteor event occurred in 1908, in a (fortunately) remote area in western Siberia called Tunguska. Here, a meteor probably about 60 meters across disintegrated in mid-air and then crashed to the ground as a collection of loosely-bound pieces. Because it broke up, no crater was formed, yet it flattened trees in an area 50 km in diameter, and the sound of the explosion could be heard half-way around the world.

As far as we know, only one human has ever been directly injured by a meteor. The lucky woman was Ann Hodges, of Sylacauga, Alabama, who suffered minor damage in 1954 when a 20 kg meteorite crashed through the roof of her house. This must have made a great story to tell the neighbors.

The Asteroid-Meteor Connection

Meteor showers are often associated with comets, while individual meteors often originate in asteroids. How do we know? Spectroscopy!

Astronomers can find out the surface composition of the asteroids by studying their absorption bands in the near infrared. We now know there are several types of asteroid:

  • the C asteroids are the darkest, rich in carbon and silicates. They form 60% of asteroids and are found mostly in the outer regions of the Main Asteroid Belt;
  • the S asteroids are rocky, generally small, very abundant among the Apollo-Amors and in the inner regions of the Belt. They make up 30% of the total number of known asteroids; and form 90% of the meteorites found on the Earth
  • the M asteroids are entirely metallic, made of iron and nickel.

    iron meteorite
    chondrite meteorite

  • the U asteroids (for the ever-popular 'Unknown') display a complex spectrum and are probably of mixed type.
Most meteorites are rocky, suggesting that they were formed in the outermost layers of an asteroid. About five percent of meteorite 'falls' contain iron and nickel. A third class of meteorites are darker, containing carbon. Inside some of these meteorites we find spherical grains of lighter rock known as "chondrules", giving these meteorites the classification "carbonaceous chondrites". These may come from the most ancient material in the asteroid belt, either from the most pristine of the planetesimals, or from the surfaces of asteroids that have never melted and differentiated. Many chondrites are also discovered without the carbonaceous covering.

Thus, the categories of meteorites recovered on Earth can be linked directly to the various types of asteroids seen in space. Carbonaceous chondrites originate from the C asteroids, and some of the stony and stony/iron meteorites from the S asteroids. The metallic meteorites, mixtures of iron and nickel, originate in the M asteroids or the cores of differentiated asteroids.

Finally, some small meteorites are identified as 'achondrites', similar to terrestrial basalts. To date, two dozen of these meteorites have been found to contain material identified as Moon rock, while around 18 others contain material from Mars!

photo of the famous meteorite ALH84001

The most famous meteor from Mars is named ALH84001, and was kicked into space by a major asteroid impact on the red planet about 15 million years ago to eventually come to rest in Antarctica 13,000 years ago. In 1996, David McKay from NASA's Johnson Space Center and his colleagues announced that they'd found organic compounds in ALH84001 - possible evidence of life on Mars. However, this claim remains controversial, and there are several interpretations of these results that do not require them to be organic in origin. Carl Sagan used to say that "extraordinary claims require extraordinary evidence," and the evidence in this case is not yet secure. The best way to get a truly uncontaminated sample from Mars would be to go there and dig one up. That's the goal of robotic sample-return missions being proposed by NASA, ESA, and several other space agencies!

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