More than 4 billion years ago, the dust swirling around the newborn Sun gathered into bigger and bigger rocks and ultimately into the planets and moons, collecting ice and gases as they grew. But not all of the rocks and ice went into building these worlds. Billions of pieces remained free. These are the asteroids and comets, and the smaller chunks called meteoroids.

Scientists study all of these kinds of objects for clues to the development of the Solar System and of life, itself. Compared to the planets, which have undergone dramatic transformations, asteroids and comets have remained in relatively pristine condition since the beginning of planet formation and can therefore help scientists understand how the Solar System evolved.

Researchers have also uncovered evidence that comets and asteroids delivered complex organic molecules to Earth (and other worlds) that may have served as a starter kit for life. And it appears likely that these chunks of rock and ice provided at least a portion of the water that fills our oceans and the atmosphere that existed before life emerged. So we may have comets and asteroids to thank for life’s origin. But there’s a dark side to their visits as well. Ask the dinosaurs.

Near-Earth Objects

It is almost certain that the dinosaurs died out in the aftermath of a collision with a large asteroid or comet 65 million years ago, along with most of the rest of the creatures who lived at that time. It’s possible that other mass extinctions were tied to other, similar collisions.

Astrophysicist Neil deGrasse Tyson, director of New York’s Hayden Planetarium, hopes we’ll be able to prevent a catastrophe like that from happening to us. "I don’t want to be the laughing stock of the Galaxy when they find out that a species that had a space program and the intelligence to stop asteroid impact just simply went extinct," he said in a recent speech. "That’d just be embarrassing."

"NASA is guarding against any unknown impacts," Don Yeomans, manager of NASA’s Near-Earth Object Program office at JPL, assures us. "NASA’s charter for near-Earth objects is to find 90% of the kilometer and larger near-Earth objects. Those are the ones that would be a global problem if they were to hit. And we’ve almost accomplished that. We’re tracking them, and none of them are a problem.

"Congress has asked NASA to establish the next generation of search," he continued, "which is to look for the 140-meter and larger objects. A hundred forty meters (around 460 feet) is about at the point where these things can easily punch through the atmosphere and cause tsunamis, regional damage."

Yeomans acknowledges that "long-period comets are the wild cards." These are comets falling into the inner Solar System for the first time, or on such long orbits that thousands or millions of years go by between visits. We may not have an opportunity to track such a comet and establish that it’s a threat to us in time to deal with it effectively. But this kind of event has a very low probability. If astronomers do detect a relatively large asteroid or comet headed for a collision with Earth, chances are good that there will be plenty of time to prepare a way to deflect it.

"It’s statistically unlikely we’re going to get taken by surprise by anything big," Yeomans said. "By the time a decade rolls around, we’re likely to have most of the large, dangerous objects discovered and tracked."

Asteroids, comets and meteoroids

Most of the objects NASA is tracking found their way into near-Earth space from an abandoned planetary construction site between the orbits of Mars and Jupiter. There, you can find Ceres, a planet-in-progress (now dubbed a dwarf planet) that never got to finish gathering the building blocks in its orbit because of the disruptive influence of Jupiter’s gravity. Millions of rocks remain there, ranging from dust motes to mega-mountains the size of large islands on Earth. Some of these rocks (commonly, the ones larger than about 10 meters across, though there is no official cutoff) are called asteroids, and this region of space is known as the asteroid belt. At about 950 km (590 miles) in diameter, Ceres is by far the largest object in the asteroid belt and constitutes about a third of the belt’s total mass.

Another group of objects formed further from the Sun, in an enormous region stretching from just inside Jupiter’s orbit to beyond Neptune. Untold numbers of those objects went into the construction of Jupiter, Saturn, Uranus, and Neptune, but billions of them were left over after the giant planets were completed. These objects, which tend to have a much higher proportion of ice than the ones that formed closer to the Sun, are the comets. Those that formed past Neptune’s orbit, safely out of the gravitational reach of the four giant planets, constitute the Kuiper belt, sort of a bigger, icier version of the asteroid belt.

The comets that buzzed around the giant planets without being captured found the speed and direction of their orbits changed by the planets’ gravity, in just the same way that a "gravity assist" from one planet can help our spacecraft to reach another. Without mission controllers to guide their trajectories, however, these comets were scattered haphazardly. Some flew sunward and crashed into the terrestrial planets—Mercury, Venus, Earth (and its battered moon), and Mars—during an era known as the Late Heavy Bombardment. Others were flung clear out of the Solar System into interstellar space.

A small fraction, but still numbering in the billions, were flung just far enough so that they were still—just barely—under the Sun’s gravitational influence, but were also susceptible to the gravity of the Galaxy and the occasional passing star. These formed the Oort cloud, a spherical shell of comets surrounding the rest of the Solar System. Though very sparsely populated, it occupies such a vast region of space that a beam of starlight entering the outer reaches of the cloud takes more than a year to emerge from the inner edge.

On occasion, gravity changes a comet’s orbit in a way that brings it to the inner Solar System. When it gets within about three times Earth’s distance from the Sun (just about midway between the orbits of Mars and Jupiter), the Sun’s warmth vaporizes some of the comet’s ice. The vapor lifts dust particles and forms a great cloud called a "coma," which can span 100,000 km (62,000 miles). The pressure of sunlight pushes the very tiny dust grains into a dust tail, while the solar wind of charged particles drives ionized gas molecules into an ion tail. These tails, always pointing away from the Sun, can extend as much as 150 million km (93 million miles). They can make a comet, for a short but glorious time, the largest object in the Solar System, dwarfing even the Sun.

Some asteroids are solid masses of iron and nickel, but they are a minority. "Most asteroids and comets are rubble piles," according to JPL cometary scientist Paul Weissman (for whom an asteroid was named in 1986). Collisions break them into pieces, which reassemble weakly under the influence of their own small levels of gravity.

Comets, which used to be thought of as "dirty snowballs" are now more commonly thought of as "icy dirt balls," containing a large proportion of rocky material. "You might compare them to a powdery rock that breaks apart easily in your hand," Weissman said. A dramatic demonstration of such a comet appeared in 1994, when comet Shoemaker-Levy 9 fragmented into 21 pieces before crashing into Jupiter. "There’s a comet you could easily break apart with your fingertips," Don Yeomans said. "It was just that fragile."

Meteoroids range from tiny flecks of dust ejected by comets to boulders several meters across, sometimes chipped off of asteroids that have collided with each other. If a meteoroid falls through Earth’s atmosphere, it is called a meteor. If it survives to hit the ground, it is called a meteorite. Some 400 tons of meteoroids enter Earth’s atmosphere every day, typically ranging in size from dust grains to basketballs.

Meteorites are sometimes called the "poor man’s space probe" because they spare us the expense of leaving the planet to collect extraterrestrial samples. Though they fall all over the Earth, most are found in Antarctica because they’re easier to spot on the ice, there’s little erosion, and glacial movement tends to concentrate them.

The discovery of complex organic molecules in some meteorites, such as the amino acids that make up proteins, has led a number of scientists to suspect that the origin of life on Earth might be traced to deliveries of building blocks from space. And a mysterious discrepancy between the pattern of isotopes (forms of the same element containing different numbers of neutrons) found in certain meteorites and that of Earth rocks may point scientists to a long-sought explanation of how grains of dust in the early Solar System began to form planets.

While most meteorites are thought to come from asteroids, at least 50 are known to have come from the Moon and at least 40 are deemed very likely to have come from Mars, blasted off of those worlds by large, crater-forming impacts. One Martian meteorite (called ALH84001) contains structures that look like tiny fossils and has other characteristics that could possibly have a biological origin, prompting a heated scientific controversy over whether it constitutes evidence of Martian life. Meanwhile, our rovers on Mars have found numerous meteorites on the surface of that planet, supporting the ample evidence provided by craters on other planets and moons that meteoroids have fallen everywhere.

Galileo’s successors have used advanced telescopes to study the asteroid belt and, more recently, the Kuiper belt. And during the last couple of decades, they’ve taken the final step of sending spacecraft to rendezvous with comets and asteroids. See the Exploration Timeline for some highlights.

Asteroid and Comet Exploration Timeline

1986: Five spacecraft fly by comet Halley, several close enough to find a potato-shaped nucleus about 15 km (9 miles) long, containing about 1/3 ice, 1/3 silicates, and 1/3 organic substances. About 80% of the ice is water ice and about 15% is frozen carbon monoxide.

1991: En route to Jupiter, the Galileo spacecraft takes the first detailed photos of an asteroid: Gaspra, an irregularly shaped object about 19 × 12 × 11 km (12 × 7.5 × 7 miles).

Gaspra imaged by the Galileo spacecraft in 1992. Subtle color variations have been exaggerated

1993: Galileo flies by asteroid Ida and makes the first observation of an asteroid with its own moon (Dactyl).

Asteroid 243 Ida and its moon, Dactyl, imaged by the Galileo spacecraft in 1993. The color is "enhanced" in the sense that the CCD camera is sensitive to near-infrared wavelengths of light beyond human vision. A "natural" color picture of this asteroid would appear mostly gray.

1994: Galileo provides the only direct observations of comet Shoemaker-Levy 9’s collision with Jupiter.

In this series of pictures taken by Galileo, one of the fragments of comet Shoemaker-Levy 9 explodes as it hits Jupiter’s night side.

1997: NASA’s Near Earth Asteroid Rendezvous (NEAR) spacecraft flies by asteroid Mathilde.

Mosaic of asteroid 253 Mathilde, constructed from 4 images taken by the NEAR spacecraft in 1997.

1999: NASA’s Deep Space 1 flies within about 26 km (16 miles) of asteroid Braille.

2001: Deep Space 1 flies by comet Borrelly. NASA’s NEAR probe (later renamed NEAR-Shoemaker) becomes the first spacecraft to orbit and then land on an asteroid, which is called 433 Eros.

Asteroid Eros, site of the first orbiting and landing of a spacecraft on an asteroid.

2002: Stardust flies by asteroid Annefrank.

2005: NASA’s Deep Impact fires an impactor into comet Tempel 1 so the exploded material could be analyzed.

Deep Impact’s projectile hits comet Tempel 1. (Credit: NASA/JPL-Caltech/UMD)

2006: NASA’s Stardust, which collected samples of comet Wild 2 in 2004, delivers the samples to Earth, along with a mystery: Comets must form in cold environments where ice is stable, but researchers found some particles that form in hot surroundings, suggesting that they formed near either the young Sun or another star. The samples are stored at the Astromaterials Curation Labs at Johnson Space Center.

Dust grains from comet Wild 2 captured in the aerogel delivered to Earth by the Stardust spacecraft [Poster 1] [Poster 2]

2006: NASA launches New Horizons, scheduled to arrive at Pluto in 2015 and to explore other Kuiper Belt Objects from 2016 to 2020.

2007: NASA launches its Dawn spacecraft, intended to orbit asteroid Vesta in 2011 and 2012, and dwarf-planet Ceres in 2015.

2009: NASA launches WISE, which will map the entire sky in infrared light. It will also observe hundreds of thousands of asteroids in the main belt and hundreds of near-Earth asteroids. The infrared observations will allow astronomers to estimate the diameters for these asteroids.

2010: Japan’s Hayabusa (“peregrine falcon” in Japanese), which in 2005 may have been the first spacecraft to collect samples from an asteroid (Itokawa), though its success is uncertain, is scheduled to deliver whatever it gathered to Earth. It revealed Itokawa's structure to be a pile of rubble.

2010: The Deep Impact spacecraft flies past the active comet Hartley 2.

2011: The Stardust spacecraft, now called Stardust-NExT, flies by comet Tempel 1 to image the crater made by the Deep Impact mission and parts of the comet previously unseen.

2014: The European Space Agency’s Rosetta spacecraft (with some instruments contributed by NASA), which was launched in 2004, arrives at comet Churyumov-Gerasimenko. It is scheduled to orbit the comet, release a small lander, and escort the comet into the inner Solar System to observe its transformation from a small, frozen object into an active comet with coma and tails.

 

A collision with a large asteroid or comet is thought to have led to the extinction of the dinosaurs and most other species on Earth.

The asteroid belt’s dwarf planet Ceres, imaged by the Hubble Space Telescope. The Dawn spacecraft is on its way to explore Ceres and an asteroid called Vesta.

Comets are thought to have accreted from particles of rock and ice. Click here for an animation of the process.

The Kuiper belt and Oort cloud are two major repositories of comets. A third may reside in the outskirts of the asteroid belt.

Meteorites like the Murchison Meteorite are thought to have delivered organic material which may have kick-started the development of life on Earth—and possibly on other worlds as well.