How do asteroids function and what are their characteristics?

In space exploration, NASA achieved a remarkable feat on February 12, 2001, when a spacecraft successfully landed on an asteroid for the first time ever.

After spending a year in orbit around asteroid 433 Eros, the NEAR (Near Earth Asteroid Rendezvous) spacecraft performed a controlled descent to land on the surface. But what exactly are asteroids, and what was the purpose of the NEAR Shoemaker mission?

The Discovery of Asteroids

In 1772, Johann Titus, a mathematician, and Johann Bode, an astronomer, uncovered a mathematical pattern that outlined the distances of known planets from the sun, though there seemed to be one planet missing.

Titus's sequence suggested a planet might orbit between Mars and Jupiter, at a distance of 2.8 AU (about 2.6 x 10 miles / 4.2 x 10 km) from the sun. AU, or astronomical unit, is based on the average distance from Earth to the sun, which is roughly 9.3 x 10 miles (1.5 x 10 km).

Astronomers began their search for the missing planet, and in 1801, Italian astronomer Giuseppi Piazzi discovered a faint body at this predicted location, naming it Ceres. However, it appeared dimmer than Mars or Jupiter, leading Piazzi to believe it was significantly smaller.

Subsequent discoveries of other small bodies in the same area led astronomers to name them asteroids, meaning 'star-like,' or minor planets.

Enhancing Space Rocks

In 2006, Ceres was upgraded from asteroid to dwarf planet due to its relatively large size. Nine years later, NASA's Dawn spacecraft landed on this dwarf planet, making Ceres the first of its kind to achieve this remarkable milestone.

What Are Asteroids?

Asteroids are small, rocky objects that revolve around the sun. Most asteroids are found in the primary asteroid belt, which lies between the orbits of Mars and Jupiter. This region spans distances from 2.1 AU (1.95 x 10 miles or 3.15 x 10 kilometers) to 3.2 AU (3.0 x 10 miles or 4.8 x 10 kilometers) from the sun.

There are over 1 million recognized asteroids [source: NASA]. These irregularly shaped bodies vary in size, with radii ranging from 33 feet (10 meters) to several hundred kilometers. Vesta, the largest asteroid, has a radius of 329 miles (530 kilometers). By observing changes in their brightness, scientists know that many asteroids complete a full rotation every three to 30 days.

A Guide to Asteroid Classifications

Asteroids are classified based on their composition.

Asteroids are believed to originate from two primary sources:

It is thought that asteroids are the remnants of planetesimals, the building blocks of the solar system, which formed in the region between Mars and Jupiter. Some of these planetesimals began to form planets, but Jupiter's massive gravitational pull shattered many of them. The others never progressed into planets, possibly due to unknown factors.

Jupiter’s gravitational force continues to influence the orbits of asteroids. Occasionally, it causes an asteroid to be pulled out of the asteroid belt and sent into other regions of the solar system.

These wandering asteroids can collide with planets, or in the case of Trojan asteroids, they can share an orbit with a larger planet. NASA tracks asteroids that intersect Earth’s orbit to protect the planet from potential impacts. In fact, NASA’s DART (Double Asteroid Redirection Test) mission successfully demonstrated our ability to deflect an asteroid and change its trajectory in 2022.

Project NEAR

Project NEAR marked the first mission to orbit a small body in the solar system. Launched in February 1996, NEAR passed by the asteroid Mathilde in June 1997, coming as close as 753 miles (1,212 km) to its surface. It then proceeded to orbit asteroid 433 Eros in February 2000.

Eros, one of the largest asteroids, was discovered by Gustav Witt and August Charlois in 1898. It has a potato-like shape, measuring 21 miles (33.8 km) long, 8 miles (12.9 km) wide, and 8 miles thick. Eros rotates every five hours and orbits the sun at a distance of approximately 1.5 AU (1.4 x 10 mi / 2.25 x 10 km). It is classified as an S-type asteroid.

NEAR spent nearly a year orbiting Eros, getting as close as 4 miles (6 km) and as far as 300 miles (500 km) from its surface. During this time, it conducted gravity measurements, captured images of the asteroid, and mapped its surface while also performing chemical analyses.

After a year in orbit, NEAR made history by landing on the surface of Eros in February 2001, becoming the first spacecraft to do so. It spent two weeks gathering and transmitting data until the severe cold, with temperatures reaching about minus 279 degrees Fahrenheit (minus 173 degrees Celsius), disabled its communication systems — [source: NASA].