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Asteroids of the Solar System
NEAR-Shoemaker Lands on Asteroid Eros

NASA artist concept of NEAR at 433 Eros
Click to enlarge NASA concept of
NEAR at 433 Eros

The NEAR spacecraft was launched on Feb. 17, 1996, as the first in NASA's Discovery series of low-cost science missions. It became the first spacecraft to orbit an asteroid in February 2000 and then the first to land on an asteroid in February 2001. The asteroid was 433 Eros.

NEAR is short for Near Earth Asteroid Rendezvous. Near-Earth asteroids seem to have broken away from the main Asteroid Belt which orbits the Sun between Mars and Jupiter. They fly into the inner Solar System, within 121 million miles of the Sun. Aside from the Moon. they're our closest neighbors in the Solar System.

Gene Shoemaker
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On March 14, 2000, NASA renamed the spacecraft NEAR-Shoemaker as a tribute to the late Gene Shoemaker, a geologist who influenced decades of research on the role of asteroids in shaping planets. Shoemaker had died in a 1997 car accident while on an annual study of impact craters in the Australian outback. He had said he wanted to tap a rock hammer on the asteroid Eros.

NEAR-Shoemaker conducted the first long-term, close-up study of an asteroid. The NEAR mission was conceived to answer fundamental questions about the nature and origin of asteroids and comets close to Earth. Such "near Earth" objects contain clues about the formation of Earth and other planets.

Eros' previously pristine surface offered a look at conditions in space when Earth formed more than 4.5 billion years ago. NEAR's sensors and detection equipment collected information on Eros' mass, structure, geology, composition, and gravity.

The NEAR spacecraft
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The Delta-2 rocket used to launch NEAR from Cape Canaveral Air Station, Florida, in 1996 was the smallest rocket ever used for a planetary mission.

The NEAR spacecraft is about the size of a car. It's made of eight 18-square-ft. aluminum panels. The spacecraft is 9 ft. 2 in. long including its main antenna. Each of its four solar panels are 6 ft. long and 4 ft. wide. They surround the 5-ft.-diameter high-gain antenna on top of the spacecraft. Including instruments and fuel, NEAR weighed 1,775 lbs. at launch.

Today, NEAR-Shoemaker sits 160 million miles from Earth on the 21-mile-long asteroid known as 433 Eros.

Flight plan
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The lightweight spacecraft was sent to visit the asteroid Mathilda, an extremely dark object that reflects only 4 percent of the sunlight that hits it, and then rendezvous with 433 Eros, an asteroid tumbling in an odd orbit.

NEAR snapped several photos of Earth as it flew within 336 miles of our planet on January 23, 1998, enroute to Mathilda and Eros. As NEAR gathered momentum from Earth's gravity, it captured on film numerous captivating images of the planet, including a close-up of the Southern Hemisphere and a picture with the Earth and the Moon in the same frame.

The swing-by trajectory collected energy from Earth's gravity to create a slingshot effect which fired NEAR out toward its asteroid targets. If the small craft had been sent directly from Earth to the asteroids, it would have needed a much larger, more expensive rocket.

NEAR looped around Earth on its way out to Eros. A direct trip from Earth to Eros would have taken only a year, but to be able to use a smaller, more economical rocket, flight planners decided to bring the craft back to Earth for a swing around our planet. That gave NEAR the slingshot gravity boost. Mission planners and operators used the extra flight time to practice with the spacecraft's instruments.

NEAR-Shoemaker was supposed to reach Eros two years and 327 days after launch. Unfortunately, the first attempt to insert the spacecraft into orbit around Eros failed on Dec. 20, 1998. Mission planners had to add another year and 23 days to the trip.

Working along the way
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On June 27, 1997, NEAR undertook a high-speed close encounter with asteroid 253 Mathilde. The data it sent back gave scientists their first close-up look at a carbon rich type-C asteroid.

The visit was unusual in that NEAR had not been designed for flyby encounters. Rather, it was designed to be an orbiter around asteroid Eros.

The five year mission
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The trip out to Eros took about five years from launch from Earth to landing on the surface of Eros. When NEAR-Shoemaker encountered Eros, the spacecraft and asteroid were about 160 million miles from Earth. Scientifically speaking. That's 1.7 astronomical units (AU). One AU equals about 93 million miles, the average Sun-to-Earth distance.

When NEAR-Shoemaker touched down on Eros, it had traveled nearly 2.3 billion miles around the Sun. That includes the 417 million miles traveled during the spacecraft's year-long orbit around Eros.

At that point, Eros was about 197 million miles from Earth. That's slightly more than 2.11 AU. In fact, NEAR-Shoemaker actually was closer to the Sun, which was 133 million miles (1.43 AU) away from the asteroid.

From Feb. 14, 2000, to Feb. 12, 2001, the spacecraft circled Eros 230 times at various distances. NEAR-Shoemaker spent more than 100 of those days orbiting 31 miles from the center of Eros. Sometimes its orbit elongated out to about 200 miles. A low-altitude pass on Oct. 26, 2000, brought it to within 3 miles of the surface. After that, the probe headed back out to 125 miles for global observations. In December 2000 it descended to 22 miles and operated from no farther out for the rest of its mission.

NEAR-Shoemaker snapped 160,000 pictures while orbiting 433 Eros.

On Feb. 12, 2001, NEAR-Shoemaker used a set of five de-orbit and braking maneuvers to make an historic controlled descent and touch down on the surface of the asteroid. The 69 close-up images of the surface it transmitted during the final three miles of its descent were the highest resolution images ever obtained of an asteroid. The clear pictures from as close as 394 feet revealed features as small as a golf ball.

The controlled descent from its circular 22-mile-high orbit took about four and a half hours. The spacecraft touched down at a gentle 4 mph, just outside the asteroid's large saddle-shaped depression named Himeros. The tips of two solar panels probably touched down first. It was the first human probe to land on an asteroid.

Despite being built to be an orbiter, not to land, NEAR-Shoemaker continued communicating with its home team back on Earth at the Johns Hopkins University Applied Physics Lab in Maryland. Team members commanded the probe's gamma-ray spectrometer to gather data on the elemental composition of the asteroid's surface and and materials just below the surface.

After two mission extensions and two weeks of operating on Eros, NEAR-Shoemaker sent its last report to Earth on Feb. 28, 2001. NASA's large Deep Space Network (DSN) antennas received the last bits of NEAR data and the first close-up study of an asteroid came to an end. Altogether, the robot spacecraft had managed to gather ten times more data than originally planned.

Why Eros?
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Basically, because it's big and close. Asteroid 433 Eros is one of the largest near-Earth asteroids, with a mass thousands of times greater than similar asteroids. Its orbit provided a launch opportunity at the right time and it allowed use of a medium-class launch vehicle instead of a much larger and more expensive rocket.

The number 433 indicates Eros was the 433rd asteroid to have its orbit calculated by astronomers.

How big is the asteroid? Eros is about 21 miles long, 8 miles wide, and 8 miles thick. That compares with most near-Earth asteroids, which little more than a half-mile across.

Gravity. Gravity on Eros is weak, but strong enough to hold NEAR-Shoemaker in orbit, and to keep the spacecraft down on the surface.

NASA says a 200-lb. person on Earth would weigh about two ounces on the asteroid. A baseball pitched at 22 miles an hour would escape into space. A basketball player with a 36-inch vertical leap on Earth could jump a mile on Eros, and risk going into orbit.

The surface gravity on Eros varies a lot, since it is an elongated body shaped described as looking like a peanut or potato or shoe.

Temperature. NEAR-Shoemaker carried no instrument to measure temperature on Eros, but scientists estimated, from the distance the asteroid is away from the Sun, that the day temperature on Eros is 212 degrees Fahrenheit, while night is minus 238 F.

Rock-strewn rock
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Eros probably is more than 4 billion years old. It may be the remnant of a larger asteroid broken up by a collision with another asteroid. The asteroid rotates once every 5.25 hours.

Eros itself may have been struck by an object a billion years ago. The other body may have been a meteorite or small comet. While the collision dug out a crater 5 miles wide, the incoming meteorite or small comet shattered into rocks of all sizes. Some of which went straight up and back down, while most traveled up to two-thirds of the way around the rotating asteroid before coming to rest on the surface.

The entire surface of Eros appears to be blanketed with a fine material, some of which appears to have come from the asteroid's soil.

Lots of rocks. The thousands of NEAR-Shoemaker high-resolution images have been stitched into a global map of the asteroid's surface. In the photos, researchers counted 6,760 rocks larger than about 16 yards across wide strewn over the asteroid's 434 square miles.

Most of the larger rocks strewn across Eros seem to have been ejected from a single crater near one end of the asteroid in a meteorite collision a billion years ago. Some 44 percent were seen inside a crater the researchers named Shoemaker. Most of the rocks along the asteroid's equator seem to have been ejected from Shoemaker.

Enigma. It remains a mystery as to why the same thing didn't happen with two other large craters on Eros. Himeros is a saddle-shaped depression on the astroid's convex side, and Psyche is a crater on the concave side. Either their rocks subsequently have been buried, or they have eroded away, or they weren't made in the first place.

The Johns Hopkins University Applied Physics Laboratory built NEAR and manages the NEAR mission for NASA.

Learn more about asteroids
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NEAR: Asteroids: NASA Research:
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