NEAR Shoemaker

FIELDS OF STUDY: Sub-planet Astronomy; Space Technology

ABSTRACT: In 1996 the NEAR Shoemaker spacecraft was sent to analyze the asteroid 433 Eros. Although an error caused the spacecraft to fly by, a year later NEAR Shoemaker successfully entered an orbit around Eros and sent a great deal of information back to Earth. At the end of the mission, the spacecraft made a soft landing on the asteroid’s surface and sent images taken during its descent. Thanks to the NEAR mission, scientists know that asteroids are solid, not loose rubble, and gained a better understanding of their makeup.

The NEAR Shoemaker Mission

The Near-Earth Asteroid Rendezvous Shoemaker (NEAR Shoemaker) mission began on February 17, 1996, with the launch of a spacecraft of the same name. This was the first launch of NASA’s Discovery program. It was also the first mission designed to orbit an asteroid, with the goal of increasing understanding of asteroids’ makeup. The spacecraft entered orbit around the asteroid 433 Eros on February 14, 2000, where it remained for a year. Then, on February 12, 2001, the mission team, led by Robert Farquhar (b. 1932), landed NEAR on the surface of the asteroid—a first in space exploration.

Over the course of the mission, NEAR provided 160,000 images and a great deal of new information about asteroids. In fact, it transmitted ten times as much data as expected. Scientists the world over benefitted from the new evidence. The findings were published in prominent journals such as Nature and Science. The accomplishments earned the team the Trophy for Current Achievement from the Smithsonian National Air and Space Museum in 2001. The mission also generated new questions, which NASA hoped would be answered in future Discovery missions.

First Mission to an Asteroid

Asteroids were thought to be leftovers from the origins of the universe. Researchers believed that the makeup of space objects could tell them more about how it began. A small Delta II rocket launched NEAR into space toward its destination, an asteroid named 433 Eros some 2.38 astronomical units (AU) or 355 million kilometers (220.5 million miles), from Earth. The mission’s objective was for the spacecraft to orbit Eros for a year and measure its shape, density, and physical and geological properties. It was also to record the asteroid’s gravity and magnetic fields, map its topography, and analyze minerals and other elements on the surface.

For much of the long voyage, the spacecraft’s instruments were in a state of minimum activity, as it had few tasks to perform. However, on its way to 433 Eros, NEAR Shoemaker flew close to another asteroid, 253 Mathilde, and captured images of 60 percent of its surface. From these images, scientists concluded that Mathilde was a heavily cratered, completely black C-type asteroid.

As NEAR Shoemaker finally approached Eros in December 1998, the mission operators were unable to get it close enough to allow it to be caught by Eros’s gravity. A software problem caused them to abort the attempt to place it in orbit. For more than twenty-four hours, they lost contact with NEAR. The spacecraft was scheduled to enter orbit around Eros on January 10, 1999. After it flew past the asteroid, it had to be reprogrammed and turned around to make another pass at Eros. It was more than a year before the spacecraft finally made it into orbit around the asteroid.

Once in place, NEAR did the job for which it was sent. Between February 14, 2000, and early February 2001, the spacecraft orbited the asteroid. It collected a variety of data, including thousands of images. In the meantime, the mission team received permission to attempt to land the spacecraft on Eros. On February 12, 2001, in a controlled descent, NEAR became the first spacecraft ever to touch down on an asteroid. Its cameras captured high-resolution images from as close as 120 meters (394 feet) during the descent. Its instruments sent data back to Earth until February 28, when the mission was completed. All told, NEAR had traveled 3.7 billion kilometers (2.3 billion miles).

After NEAR Shoemaker had stood on Eros for nearly two years, the mission team made an effort to communicate with the spacecraft one more time. Its solar panels had been exposed to sun for several months, and in spite of the frigid temperatures on the asteroid, the operations team hoped NEAR Shoemaker would respond. They gathered on December 10, 2002, when the asteroid would be passing within about 0.9 AU of Earth. Eros had been more than twice as far away when NEAR Shoemaker first landed on it. The scientists listened for a signal but heard nothing. They sent commands to one of the computers on the spacecraft telling it to transmit data to show that it was still functioning. When there was no reply, they tried a second computer. Still no return signal came. Finally, after twelve hours, the team called off the experiment. The NEAR Shoemaker mission was over.

The NEAR Shoemaker Spacecraft

The NEAR Shoemaker spacecraft was designed and built by the Johns Hopkins University Applied Physics Laboratory, which also managed the mission. The octagonal spacecraft was made of aluminum panels and, when finished, weighed about 800 kilograms (1,775 pounds). It was fitted with four solar panels, each measuring about 1.8 meters by 1.2 meters (6 feet by 4 feet), and an antenna that was attached to the top. The solar panels could generate up to 1,800 watts of power, depending upon the distance from the sun. A rechargeable battery stored this power. The spacecraft was named in honor of Eugene Shoemaker (1928–97), a well-known astronomer and geologist. Shoemaker was a leader in astrogeology at the United States Geological Survey and NASA. His background in geology led to a strong interest in impact craters and the asteroids believed to have caused them.

NEAR Shoemaker carried with it many sophisticated instruments, such as a multispectral camera, which records various wavelengths of light, such as ultraviolet or infrared. The images helped identify elements and compounds by the way they reflected or absorbed light. The spacecraft also was fitted with an x-ray/gamma-ray spectrometer, a laser range finder, and other measuring and recording devices.

As it orbited Eros, the spacecraft transmitted several films, including one showing a full rotation of the asteroid. The researchers could see each end of the battered asteroid, its boulder patches, and a crater over five kilometers (three miles) wide. NASA’s decision to allow NEAR to attempt a landing on the asteroid delivered even closer pictures and more evidence of its makeup.

433 Eros

433 Eros is a near-Earth asteroid (NEA) measuring about 33 kilometers (20.5 miles) across its widest part. It is considered near Earth because it is within 0.3 AU of Earth’s orbit. Its shape has been compared to a potato, a shoe, and a peanut. Eros is an S-type asteroid, meaning it is made of stone and metals. The number appearing before its name indicates its place among known asteroids in the order in which their orbits of were calculated.

Before NEAR Shoemaker allowed a close-up view of Eros, scientists believed asteroids were made of iron or space rubble. The data collected by the spacecraft instead suggested that the asteroid was rock similar to Earth’s crust. It also supported the idea that asteroids could be made up of materials from the beginning of the universe. Eros is believed to have broken away from a larger space body. Its surface is pocked with more than 100,000 craters. Eros also has more than a million boulders the size of houses, patches containing smaller boulders and rocks, and dry ponds covered by powdery dust. These also suggest a history of collisions. Understanding the nature of asteroids such as Eros may one day help scientists if an asteroid ever needs to be deflected away from Earth.

PRINCIPAL TERMS

• C-type asteroid: one of three broad classes of asteroids. C-type asteroids are mainly composed of black carbon.
• Eros: a very large S-type asteroid, the first NEA discovered, and the first asteroid to be landed on by a spacecraft.
• NASA: abbreviation for the National Aeronautics and Space Administration; the US government agency responsible for developing aviation and space technologies and conducting both manned and unmanned space missions to research Earth, its solar system and galaxy, and the universe.
• near-Earth asteroid (NEA): a small, irregularly shaped celestial body with an orbit that brings it close to Earth. NEAs travel within 0.3 astronomical units (AU) of Earth’s orbit and within 1.3 AU of the sun. An astronomical unit is equal to about 149.6 million kilometers (about 93 million miles).
• S-type asteroid: one of three broad classes of asteroids. S-type asteroids are mainly composed of silicate (stone) and metals such as iron and nickel.

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