Asteroids and Comets

The Asteroid Vesta. Image from NASA.
The Asteroid Vesta. Image from NASA.

What are Asteroids?

Asteroids are rocky, airless worlds that orbit our sun, but are too small to be called planets. Tens of thousands of these “minor planets” are gathered in the main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. Asteroids that pass close to Earth are called Near-Earth Objects (NEOs).

Asteroids, sometimes called minor planets, are small, rocky fragments left over from the formation of our solar system about 4.6 billion years ago. Most of this ancient space rubble can be found orbiting the sun between Mars and Jupiter. Asteroids range in size from Ceres, about 952 km (592 miles) in diameter, to bodies that are less than 1 km (0.6 mile) across. The total mass of all the asteroids is less than that of Earth’s Moon. Even with more than one-half million asteroids known (and there are probably many more), they are still much more widely separated than sometimes seen in Hollywood movies: on average, their separation is in excess of 1-3 million km (depending on how one calculates it).

Early in the history of the solar system, the formation of Jupiter brought an end to the formation of planetary bodies in the gap between Mars and Jupiter and caused the small bodies that occupied this region to collide with one another, fragmenting them into the asteroids we observe today. This region, called the asteroid belt or simply the main belt, may contain millions of asteroids. Because asteroids have remained mostly unchanged for billions of years, studies of them could tell us a great deal about the early solar system.

Nearly all asteroids are irregularly shaped, though a few are nearly spherical, and are often pitted or cratered. As they revolve around the sun in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go. More than 150 asteroids are known to have a small companion moon (some have two moons). There are also binary (double) asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.

The three broad composition classes of asteroids are C-, S- and M-types. The C-type asteroids (carbonaceous) are most common. They probably consist of clay and silicate rocks and are dark in appearance. C-type asteroids are among the most ancient objects in our solar system. The S-types (silicaceous) are made up of silicate (stony) materials and nickel-iron. M-types (metallic) are made up of nickel-iron. The asteroids’ compositional differences are related to how far from the sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface. One such asteroid, Vesta, survives to this day.

Jupiter’s massive gravity and occasional close encounters with Mars or another object changed the asteroids’ orbits, knocking them out of the main belt and hurling them into space in both directions towards or away from the sun, across the orbits of the planets. Stray asteroids and asteroid fragments have slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth.

Scientists monitor asteroids whose paths intersect Earth’s orbit. These are Near Earth Objects (NEOs) that may pose an impact danger. Besides optical observations, radar is a valuable tool in detecting and monitoring potential impact hazards. By bouncing transmitted signals off objects, images and information can be derived from the echoes, such as the asteroid’s orbit, rotation, size, shape, and metal concentration.

The U.S. is the most active and successful country operating a survey and detection program for discovering NEOs.

NASA space missions have flown by and observed asteroids. The Galileo spacecraft flew by asteroids Gaspra in 1991 and Ida in 1993; the NEAR-Shoemaker mission studied asteroids Mathilde and Eros; and Deep Space 1 and Stardust both had close encounters with asteroids.

In 2005, the Japanese spacecraft Hayabusa landed on the near-Earth asteroid Itokawa in order to collect samples. Hayabusa returned to Earth in June 2010, and the tiny asteroid particles collected in the capsule are currently being examined. Hayabusa was the first spacecraft to successfully land, take off and collect samples from the surface of an asteroid.

NASA’s Dawn mission (launched September 2007) is on a 3-billion-km (1.7-billion-mile) journey to the asteroid belt, and is planned to orbit the asteroids Vesta and Ceres. Vesta and Ceres are sometimes called baby planets — their growth was interrupted by the formation of Jupiter, and they followed different evolutionary paths. Scientists hope to characterize the conditions and processes of the solar system’s earliest epoch by studying these two very different large asteroids.

Comet Hale Bopp. Photo Credit: Science Daily
Comet Hale Bopp. Photo Credit: Science Daily

What are Comets?

Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. When a comet’s orbit brings it close to the sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the sun for millions of kilometers.

In the distant past, people were both awed and alarmed by comets, perceiving them as “long-haired” stars that appeared unpredictably and unannounced in the sky. To some ancient observers, an elongated comet looked like a fiery sword blazing across the night sky. Chinese astronomers kept extensive records for centuries, including illustrations of characteristic types of comet tails. They recorded the times of cometary appearances and disappearances in addition to celestial positions. These historic comet annals have proven to be a valuable resource for later astronomers.

We now know that comets are leftovers from the dawn of the solar system around 4.6 billion years ago, and consist mostly of ice coated with dark organic material. They have been referred to as dirty snowballs. They may yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system.

Each comet has a tiny frozen part, called a nucleus, often no bigger than a few kilometers across. The nucleus contains icy chunks and frozen gases with bits of embedded rock and dust. The nucleus may have a small rocky core.

As theorized by astronomer Gerard Kuiper in 1951, a disc-like belt of icy bodies exists just beyond Neptune, where a population of dark comets orbits the sun in the realm of Pluto. These icy objects, occasionally pushed by gravity into orbits bringing them closer to the sun, become the so-called short-period comets. They take less than 200 years to orbit the sun, and in many cases their appearance is predictable because they have passed by before.

Less predictable are long-period comets, many of which arrive from a region called the Oort Cloud about 100,000 astronomical units (AU) (that is, 100,000 times the distance between Earth and the sun) from the sun. These Oort Cloud comets can take as long as 30 million years to complete one trip around the sun.

A comet warms up as it nears the sun and develops an atmosphere, or coma. The sun’s heat causes ices on the nucleus surface to change to gases so that the coma gets larger. The coma may be hundreds of thousands of kilometers in diameter. The pressure of sunlight and high-speed solar particles (solar wind) blows the coma materials away from the sun, forming a long, and sometimes bright, tail. Comets actually have two tails — a dust tail and a plasma (ionized gas) tail.

Most comets travel a safe distance from the sun — comet Halley comes no closer than 89 million km (55 million miles). However, some comets, called sun-grazers, crash straight into the sun or get so close that they break up and evaporate.

Scientists have long wanted to study comets in some detail, tantalized by the few 1986 images of comet Halley’s nucleus from the Giotto mission. NASA’s Deep Space 1 spacecraft flew by comet Borrelly in 2001 and photographed its nucleus, which is about 8 km (5 miles) long.

NASA’s Stardust mission successfully flew within 236 km (147 miles) of the nucleus of Comet Wild 2 in January 2004, collecting cometary particles and interstellar dust for a sample return to Earth in 2006. The photographs taken during this close flyby of a comet nucleus show jets of dust and a rugged, textured surface. Analysis of the Stardust samples suggests that comets may be more complex than originally thought. Minerals that formed near the sun or other stars were found in the samples, suggesting that materials from the inner regions of the solar system traveled to the outer regions where comets formed.

Another NASA mission, called Deep Impact, consisted of a flyby spacecraft and an impactor. In July 2005, the impactor was released into the path of the nucleus of comet Tempel 1 in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet’s surface. En route to impact, the impactor camera imaged the comet in increasing detail. Two cameras and a spectrometer on the flyby spacecraft recorded the dramatic excavation that revealed the interior composition and structure of the nucleus.

The Deep Impact spacecraft and the Stardust spacecraft are healthy and have been retargeted. Deep Impact’s mission, EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), comprises two projects: the Deep Impact Extended Investigation (DIXI) will encounter comet Hartley 2 in 2010 and the Extrasolar Planet Observation and Characterization (EPOCh) investigation will search for Earth-size planets around other stars. NASA returns to comet Tempel 1 in 2011, when the Stardust New Exploration of Tempel 1 (NExT) mission will observe changes since Deep Impact’s 2005 encounter.