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How Asteroids Work

Project NEAR
NEAR's launch aboard a Delta rocket
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's 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.