How Asteroids Work

asteroid headed towards earth
There are more than 20,000 known asteroids.
Stocktrek Images/Getty Images

Credit NASA with another milestone in space exploration: On February 12, 2001, a spacecraft landed on the surface of an asteroid for the first time in history.

After a year spent orbiting the asteroid 433 Eros, the Near Earth Asteroid Rendezvous (NEAR) spacecraft made a controlled descent to the surface. But what exactly is an asteroid? And what was the NEAR Shoemaker mission about?


In 1772, a mathematician named Johann Titus and an astronomer named Johann Bode discovered a mathematical sequence in the distances of the planets from the sun -- this sequence predicted the possibility of a planet orbiting in between Mars and Jupiter at 2.8 AU (2.6x108 mi / 4.2x108 km) from the sun. So astronomers began to search for this possible planet, and in 1801, an Italian astronomer named Giuseppi Piazzi found a faint body at that distance that he named Ceres. However, Ceres was fainter than Mars or Jupiter, so Piazzi concluded that it was much smaller. Other small bodies were later found in this same vicinity. These objects were named asteroids (meaning star-like) or minor planets.

Asteroids are small, rocky bodies that orbit the sun in between the orbits of Mars and Jupiter, which is anywhere from 2.1 AU (1.95x108 mi / 3.15x108 km) to 3.2 AU (3.0x108 mi / 4.8x108 km) from the sun. There are more than 20,000 known asteroids. They are irregularly shaped and vary in size from a radius of 1 km (0.62 mi) to several hundred kilometers (Ceres is the largest, with a radius of 284 miles / 457 km). By measuring fluctuations in their brightness, we know that many asteroids rotate in periods of three to 30 days.­


Asteroid Classification

Eros asteroid
The asteroid Eros is 21 miles long and 8 miles thick -- a mini-planet! Explore the mission that landed on this asteroid after orbiting it for a year.
Photo courtesy NASA

Beyond size, shape and rotation, we know relatively little about these objects. Estimating their mass is difficult because they are not large enough to perturb the gravity of Mars or Jupiter, but Ceres is thought to be about 2.6 billion trillion pounds (1.2 x 1021 kg). Their densities are about 2 to 4 g/cm3, which is typical of rocky bodies. By examining the spectra of light reflected from these objects, we can classify asteroids as follows:

  • C - Dark, probably carbon-containing (carbonaceous)
  • S - Twice as bright as C, probably made of stony iron
  • M - Similar to iron meteorites
  • P and D - Low brightness, reddish

Asteroids appear to be of two different origins:­


  • Primitive, essentially unchanged pieces of the early solar system (C)
  • Smashed remnants of differentiated pieces of the solar system

We think that asteroids are the remainders of planetismals, early pieces of the solar system, that formed between Mars and Jupiter. Some of the planetismals began to form into planets, but were smashed apart by Jupiter's immense gravity. Others did not begin to form planets (for unknown reasons).

Many questions remain about asteroids, because we have never been able to study them closely. Until now.


Project NEAR

NEAR rocket launch
NEAR's launch aboard a Delta rocket
Photo courtesy NASA/JHUAPL

Project NEAR was the first spacecraft to orbit a small body of the solar system. It was launched five years ago, in February 1996. NEAR flew by the asteroid Mathilde in June 1997, coming to within 753 miles (1,212 km) of the surface. It continued on its journey to eventually orbit the asteroid 433 Eros in February 2000.

Eros is one of the largest asteroids, discovered by Gustav Witt and August Charlois in 1898. Eros is potato-shaped and is 21 miles (33 km) long, 8 miles (13 km) wide and 8 miles thick. It rotates every five hours and orbits the sun at about 1.5 AU (1.4x108 mi / 2.25x108 km). Eros is an S-type asteroid.


NEAR orbited Eros for almost a year, passing as close as 4 miles (6 km) and as far as 300 miles (500 km) from the surface. During this time, it measured the asteroid's gravity, photographed the asteroid and mapped and made chemical measurements of the surface.

The NEAR spacecraft is equipped with solar panels to provide electrical power. A rocket engine and thrusters allow it to maneuver into various orbits.

NEAR Spacecraft Instruments
NEAR's instruments
Photo courtesy NASA/JHUAPL

NEAR has the following instruments:

  • Magnetometer - Measures magnetic field to determine whether Eros is made of iron
  • X-ray/Gamma-ray spectrometer - Measures the chemical elements on the surface by the characteristic spectra of the radiation given off
  • Near-infrared spectrometer - Measures the spectrum of sunlight reflected from the surface to determine the minerals present
  • Laser altimeter - Uses laser-beam reflections to measure the topography of the asteroid
  • Multi-spectral imager - Uses many wavelengths of light to determine the rock types and landforms of the asteroid
  • Radio science experiment - Measures tiny changes in NEAR's radio frequency as Eros pulls on it in orbit to determine the mass and density of Eros

After a year in orbit, NEAR landed on the surface of Eros.


NEAR Lands on Eros

NEAR landing on Eros
The path of NEAR's descent from orbit
Photo courtesy NASA/JHUAPL

After a year orbiting Eros, NEAR was about out of fuel. It was only designed to orbit the asteroid and eventually crash onto the surface. Because all of its scientific objectives had been accomplished, NEAR's scientists decided to try to land the spacecraft rather than let it crash (because NEAR was never designed to land, it was not equipped with landing legs). The landing procedure would allow scientists to test complex maneuvers for a spacecraft, as well as get close-up pictures of the surface. These pictures would allow scientists to see objects as small as 4 inches (10 cm) in diameter.

Scientists ordered NEAR to slow from its circular orbit and execute a series of braking turns as it approached the surface. The landing site was in the saddle-shaped middle of the asteroid.


NEAR Landing Site on Eros
NEAR's landing site (yellow)
Photo courtesy NASA/JHUAPL

NEAR approached the surface and sent back pictures of Eros taken from ranges of 1,650 feet (500 m) down to 396 feet (120 m).

Eros Asteroid Surface
The surface of Eros, from 3,795 feet (1,150 m)
Photo courtesy NASA/JHUAPL
Surface of the Eros Asteroid
NEAR's last picture of the surface of Eros, from 396 feet (120 m)
Photo courtesy NASA/JHUAPL

The temperature on the asteroid varies from 212 degrees Fahrenheit (100 degrees Celsius) during the day to -238 F (-150 C) at night. The gravity is weak, with an escape velocity of a mere 22 mph (Earth's escape velocity is 25,000 mph), but it could hold NEAR, which survived the landing and could still radio information back to Earth.

For more information on asteroids and NEAR, see the links on the next page.


Asteroid FAQ

What is the difference between an asteroid and a comet?
Asteroids are rocky, airless leftovers from the formation of planets. Comets are made of mostly ice and dust that formed during the birth of the solar system.
What asteroid will hit Earth in 2029?
An asteroid called Apophis is expected to fly by Earth on April 13, 2029. It will pass within 19,000 miles of Earth's surface but is not expected to impact the planet.
What is the next asteroid to hit Earth?
Apophis was thought to be the next asteroid to hit Earth (in 2029), but scientists have ruled that possibility out.
How big is the asteroid that's coming in 2029?
The Apophis asteroid stretches about 1,100 feet (340 meters) across.
Is an asteroid coming to Earth?
Every year, NASA says that a car-sized asteroid hits the Earth's atmosphere. Luckily, it creates a fireball and burns up before reaching the surface.