comet nucleus and bright jet of material spewing out

In August 2015, ESA's Rosetta spacecraft captured this view of comet 67P/Churyumov-Gerasimenko and a spectacular jet of material coming from its nucleus. Credit: ESA/Rosetta/MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The Usual Suspects: a Rogues Gallery of Asteroids, Comets and Other Witnesses to History

By Bill Dunford

Feature | November 15, 2018

The story of the solar system is violent, full of both destruction and creation resulting from collisions between objects in space over the course of billions of years. Here’s a lineup of the perpetrators, or at least the survivors and witnesses—various kinds of small bodies that orbit the Sun and sometimes impact planets and each other:

Asteroids | Comets | Kuiper Belt Objects | Centaurs | Trojans | Meteors | Interstellar Visitors

The more that scientists know about these roving characters, including their age, their composition and their movements, the better they’ll be able to piece together the full, tumultuous history of the Sun’s family. [See "Cosmic Detective Work: Why We Care About Space Rocks​"]

animation of rotating, rocky asteroid
This set of images shows the asteroid Bennu rotating for one full revolution. Over a four-hour and 11-minute period on Nov. 2, the PolyCam camera on NASA’s OSIRIS-REx spacecraft acquired a 2.5-millisecond image for every 10 degrees of the asteroid’s rotation. Credit: NASA/Goddard/University of Arizona


  • Rocky, airless worlds, ranging in size from 3-6 feet (1-2 meters) to Vesta—the largest at about 329 miles (530 kilometers) in diameter
  • Numbering in the millions, most orbiting in a belt between Mars and Jupiter
  • Most are very old material, including some left over from the formation of the solar system
  • Made primarily of rock and metal. Many also contain significant amounts of carbon compounds, including organic (carbon-based) molecules, and water bound to minerals
  • Occasionally collide with planets and other bodies

Asteroids are airless, miniature worlds. There are likely several million of them circling the Sun, but the confirmed count now stands at nearly 800,000. Some of them are solid, while others are more like clumps of loosely bound rubble. Many asteroids are irregularly shaped, like potatoes. Because they have little mass, asteroids have weak gravity. They spin around an axis, though some do it so erratically that they appear to tumble through space. Some have other, similarly sized asteroids bound to them, creating a binary asteroid pair, or smaller companions that orbit them as tiny moons.

Most asteroids are found in a region between Mars and Jupiter called the main asteroid belt, an area where Jupiter’s gravity may have prevented material from coalescing into a large body. There are also asteroids with orbits that stray inside the orbit of Mars, with many that cross the orbit of Earth. Wherever they are found, asteroids represent ancient material, surviving from the early days of the solar system’s formation.

We’ve studied asteroids with telescopes, and even visited a few “in person” with robotic spacecraft. Most asteroids are composed mainly of rocky material, along with some clays and metals. Others are made mostly of metal, rich in iron, nickel and cobalt, and may even contain platinum and gold. Some are rich in carbon, giving them a coal-black color. These may contain organic molecules, which are chains of carbon bonded with oxygen, hydrogen and other elements, are essential for creating the building blocks of life. Some asteroids have another component important for life, water, chemically locked up in their minerals.

Scientists speculate that asteroids colliding with the early Earth might have been the source of some of the organic molecules and most of the water found here today.

comet nucleus and coma
In 1986, the European spacecraft Giotto became one of the first spacecraft ever to encounter and photograph the nucleus of a comet, passing and imaging Halley's nucleus as it receded from the sun. Credit: NASA/ESA/Giotto Project


  • Objects made of ice and rock, ranging from hundreds of yards (or meters) to tens of miles (or kilometers) across that develop tails when they come into the inner solar system
  • Objects that could become comets likely number in the billions or trillions
  • Many come from the Kuiper Belt beyond Neptune, orbiting the Sun over the course of decades or centuries
  • Others come from the far more distant Oort Cloud, taking up to thousands of years to complete an orbit
  • Known to contain organic compounds
  • Occasionally collide with planets, other bodies, and the Sun

In addition to the materials contributed to Earth by asteroids, other material arriving on Earth came from comets. Like small asteroids, comets are usually irregularly shaped bodies that orbit the Sun. Unlike most asteroids, comets contain ice.

Each comet has a center called a nucleus, usually a few miles or kilometers across. The nucleus may be coated with dark organic compounds, and contains icy chunks of frozen gases and water, along with bits of rock and lots of fine dust. When a comet’s orbit brings it close to the Sun, the comet heats up and spews gases and dust, which form a thin atmosphere around its nucleus, called a coma. Driven by a constant flow of particles from the Sun called the solar wind, and radiation pressure, material escaping the comet also forms two tails – one made of dust and the other of excited gas (ions). The tails point away from the Sun, sometimes stretching for millions of miles.

There are likely billions of potential comets in the solar system. Astronomers discover new ones all the time. Some comets spend most of their time relatively close to the Sun, completing an orbit in less than 200 years, like Halley’s Comet. These are called short-period comets. Many of them come from a region of the Solar System beyond Neptune called the Kuiper Belt.

Long-period comets are thought to originate in an even more remote realm, the Oort Cloud. Named for astronomer Jan Oort, this region is thought to contain countless icy bodies surrounding the rest of the Solar System in an unimaginably vast, roughly spherical halo. Objects in the Oort Cloud follow orbits so far away that they reach distances thousands of times the distance between the Earth and Sun. Sometimes, the gravity of a passing star can nudge these objects toward the inner Solar System. Objects from the Oort Cloud, like the comet Hale-Bopp, can take thousands or even millions of years to make a single trip around the Sun.

Even after such a long journey, some comets don’t survive their encounter with the inner solar system. Sun-grazing comets are those that get so close to the Sun they break up and evaporate—or even crash right into it.

Two NASA missions — OSIRIS-REx and New Horizons — are about to make history as they venture to two unexplored worlds in our solar system. New Horizons will pass close by a Kuiper Belt Object on Jan. 1, 2019, and OSIRIS-REx will reach the asteroid Bennu in December 2018.

space rock and distant sun
An artist’s impression of a Kuiper Belt Object (KBO) billions of miles from the Sun. In 2015, NASA's New Horizons mission explored a much larger KBO, Pluto. New Horizons will pass close by a small KBO called MU69 on Jan. 1, 2019. Credit: NASA/GSFC

Kuiper Belt Objects (KBOs) and Trans-Neptunian Objects (TNOs)

  • Icy objects found in a vast disk outside the orbit of Neptune
  • Vary in size; the largest include Pluto and Eris
  • May be several hundred thousand bigger than 20 miles (32 kilometers) across

Beyond the orbit of Neptune is a vast disk of objects named for astronomer Gerard Kuiper.

The inner edge of the Kuiper Belt begins at the orbit of Neptune, at about 30 AU from the Sun. (1 AU, or astronomical unit, is the distance from Earth to the Sun.) The inner, main region of the Kuiper belt ends to around 50 AU from the Sun. Overlapping the outer edge of the main part of the Kuiper Belt is a second region called the scattered disk, which continues outward to nearly 1,000 AU, with some bodies on orbits that go even farther beyond.

Kuiper Belt Objects, or KBOs for short, contain ice, with some that are quite large. The most famous example is the dwarf planet Pluto. Scientists estimate there could be several hundred thousand objects bigger than 20 miles (32 kilometers) across—most still undiscovered—in this vast, dimly lit region.

cratered surface and bright band of a ring in the sky
This artist's impression shows how the rings might look from close to the surface of Chariklo. Credit: NASA/JPL


  • May be comet-like in composition
  • Asteroid-like in size
  • Found in orbits that range between Jupiter and Neptune
  • May be Kuiper Belt Objects that migrated into the inner solar system

Centaurs are a special class of object that is comet-like in composition, in that they are icy bodies, and asteroid-like in size, found between Jupiter and Neptune. One Centaur, Chariklo, has the distinction of being the first small body found to have a ring system. With a diameter of about 188 miles (302 kilometers), Chariklo is also the largest known Centaur. It orbits between Saturn and Uranus.

Centaurs are of special interest, because they’re thought to be Kuiper Belt bodies that migrated into the inner solar system.

spacecraft and two groups of asteroids in orbit around the sun
The Jupiter Trojan asteroids orbit ahead and behind the giant planet, as seen in this illustration. NASA's Lucy spacecraft will visit these asteroids, which have never been explored up close before. Credit: NASA


  • Asteroids that share an orbit with a larger planet, but do not collide with it
  • Most significant group is the Jupiter Trojans, perhaps as numerous as the asteroids in the main asteroid belt

Trojan asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit where the gravitational pull from the Sun and the planet are balanced by a Trojan's tendency to otherwise escape the orbit. The Jupiter Trojans form the most significant population of Trojan asteroids. It is thought that they are as numerous as the asteroids in the main asteroid belt. There are Mars and Neptune Trojans, and NASA announced the discovery of an Earth Trojan in 2011.

small streaking meteor in starry sky above trees
In this 30 second exposure, a meteor streaks across the sky during the annual Perseid meteor shower in 2016 in Spruce Knob, West Virginia. Credit: NASA/Bill Ingalls

Meteoroids, Meteors, Meteorites and Dust

  • Meteoroids are “rocks” found in space that range in size from dust grains to small asteroids
  • When meteoroids enter Earth’s atmosphere (or that of another planet, like Mars) at high speed and burn up, the fireballs are called meteors (“shooting stars”)
  • When a meteoroid survives its trip through the atmosphere and lands on the ground, it’s called a meteorite

There are many dust grains and other small objects in space, usually made of rock, metal or a combination of the two. Most of this material likely comes from dust ejected from comets and asteroids, or was blasted out of them during impacts or collisions. The larger of these objects are called meteoroids, and the dust grains and small particles may be called cosmic dust or micrometeoroids.

Sometimes meteoroids encounter the Earth, and because of the high speeds of planetary bodies relative to each other, they slam into our atmosphere at thousands of miles per hour. A meteoroid that enters the atmosphere creates a flash of light called a meteor. The pressure and heat meteoroids generate as they push through the air at high speed cause them to glow and blaze a streak of light across the sky. People often refer to them as “shooting stars.” Most burn up completely before touching the ground, but some make it all the way down intact. These are called meteorites.

Not surprisingly, objects that fall from the sky are pretty interesting to scientists. Although most meteorites originated on unknown asteroids, some meteorites can be traced all the way back to the places where they came from, including Mars, the Moon, and the asteroid Vesta.

For example, NASA’s Dawn mission to the asteroid belt confirmed that howardite, eucrite and diogenite meteorites (or "HEDs") likely come from Vesta. Dawn visited Vesta and found it to be a large asteroid or dwarf planet whose growth was interrupted. HED meteorites are actually pretty common. These little bits of space on Earth allow solar system explorers to study Vesta and other far flung worlds in their own laboratories.

However, there is one major problem with meteorites, scientifically: you can’t always be sure how much it might have been affected or contaminated by processes here on Earth. That’s why the deep space expeditions that NASA and other space agencies are now undertaking to visit asteroids and comets up close carry so much weight among space explorers.

long rock
Artist's concept of interstellar object1I/2017 U1 ('Oumuamua) as it passed through the solar system after its discovery in October 2017. The aspect ratio of up to 10:1 is unlike that of any object seen in our own solar system. Credit: European Southern Observatory / M. Kornmesser

Interstellar Visitors

  • Only one known so far
  • The one example may be a comet-like object

The most mysterious character in the entire rogues gallery of space objects must be ‘Oumuamua (pronounced oh MOO-uh MOO-uh), which is Hawaiian for “a messenger from afar arriving first.”

It’s the first confirmed object from another star to visit the solar system. This interstellar interloper appears to be a rocky, cigar-shaped object with a somewhat reddish hue, up to one-quarter mile (400 meters) long and highly-elongated—maybe as much as 10 times as long as it is wide. While first characterized as an asteroid, some scientists believe it may be more like a comet that has had most of its volatiles (such as ice and frozen gases) stripped away over time.

Observations suggest it had been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years before its chance encounter with our star system. In any case, we’ll probably never get a close look; ‘Oumuamua is leaving as suddenly as it arrived. After swinging around the Sun, it is now set to travel beyond Saturn’s orbit in January 2019. As it leaves the solar system, ‘Oumuamua will head in the direction of the constellation Pegasus.

No one knows exactly how it first began its journey, but preliminary orbital calculations suggest that ‘Oumuamua came from the general direction of the bright star Vega. However, it took so long for ‘Oumuamua to make the journey, even at the speed of about 59,000 miles per hour or 26.4 kilometers per second, that Vega was not near that position when the ‘Oumuamua was there about 300,000 years ago.

Astronomers estimate that an interstellar object similar to ‘Oumuamua passes through the inner solar system about once per year, but they are so faint and hard to spot that we’ve missed them until now.

No one knows exactly how many more might be out there.