Feb 7, 2000

Eros or Bust

NEAR image the Earth and Moon
February 8, 2000 -- If all goes well on Valentines Day, 2000, NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft will go into orbit around asteroid 433 Eros. Thruster firings on February 3 slowed NEAR to a leisurely 18 mph and it is now less than 3000 miles from the asteroid.

When NEAR enters orbit around Eros on February 14, it will become the first spacecraft to circle an asteroid. NEAR will use its instruments to scrutinize the potato-shaped space rock, which is about twice the size of Manhattan Island, for an entire year.

Above: In 1998 NASA's NEAR spacecraft captured this mosaic image of the Earth and Moon after it flew by Earth in a gravity assist maneuver that sent it hurtling toward asteroid Eros. It will arrive at the asteroid on February 14, 2000.

Most asteroids are concentrated in a vast doughnut-shaped ring between the orbits of Mars and Jupiter. These diminutive space rocks orbit the Sun like planets, but they have no atmosphere and very little gravity. It's likely that asteroids are leftover pieces of a planet that tried to form 4.6 billion years ago when the solar system was young, but couldn't because of nearby Jupiter. The giant planet's powerful gravity prevented the 'planetesimals' from accreting into a planetary body. If you put all the asteroids together, they would form a body about 1500 km (580 miles) in diameter, roughly half the size of Earth's moon.

Not all asteroids are far away in the asteroid belt. Some, called Near Earth Asteroids (or NEAs), have orbits that bring them very close to Earth. 433 Eros is one of these. NEAs are thought to be fragments ejected from the main asteroid belt by asteroid-asteroid collisions or by gravitational perturbations from Jupiter. Some NEAs might also be the nuclei of dead, short period comets.

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As any dinosaur can tell you, it's important to keep an eye on Near Earth Asteroids. Many NEAs have struck Earth and its moon in the past. One widely accepted theory blames the impact 65 million years ago of an asteroid or comet at least 6 miles (10 kilometers) in diameter for mass extinctions among many life forms, including the dinosaurs. Other theories suggest that the chemical building blocks of life and much of Earth’s water arrived on asteroids or comets that bombarded the planet in its youth.

Left: This diagram shows the orbits of Earth, Mars, the asteroid belt, and Jupiter. The asteroid belt lies between Mars and Jupiter.

On June 30, 1908, a small asteroid 330 feet (100 meters) in diameter exploded over the remote region of Tunguska in Siberia, devastating more than half a million acres of forest. One of the most recent close calls occurred on March 23, 1989, when an asteroid 0.25 mile (0.4 kilometer) wide came within 400,000 miles (640,000 kilometers) of Earth. Surprised scientists estimated that Earth and the asteroid - weighing 50 million tons and traveling at 46,000 mph (74,000 kilometers per hour) -had passed the same point in space just six hours apart!

Approximately 800 NEAs have been found to date, probably only a small percentage of their total population. The largest presently known is 1036 Ganymed, with an approximate diameter of 25.5 miles (41 kilometers). Estimates suggest that at least 700 NEAs may be large enough - 0.6 mile (1 kilometer) or more in diameter - to threaten civilization if they were to strike the Earth.


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Although we've known about asteroids for nearly 200 years, many of their basic properties remain shrouded in mystery. What exactly are they made of? What is their relationship to meteorites found on Earth? How were asteroids formed? If you wanted to destroy one before it hit the Earth, how large should the missile be? Some of these questions are not solely academic, and scientists need a close-up look at an asteroid to answer them.

The target of the NEAR mission, 433 Eros, was the first near-Earth asteroid to be discovered and is the second largest known. It is one of only three known NEAs with diameters of more than 6 miles (10 kilometers). With dimensions 21 by 8 by 8 miles (33 by 13 by 13 kilometers), Eros is one of the most elongated asteroids.

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Right: On February 4, NEAR's multispectral imaging camera captured this sequence from a distance of 4620 miles (7700 km). The images of Eros were acquired every 15 degrees of rotation for one Eros "day", which is 5.27 hours long. Eros's overall shape has been compared to a boat, a shoe, a peanut, and a banana with a bite taken out of it. This sequence of images is the first to show the major geographic features of the northern and equatorial latitudes of Eros. (click for more info from Johns Hopkins Applied Physics Laboratory)

Into Orbit

On Feb. 14, at 10:33 a.m. EST, when NEAR is 207 miles (333 kilometers) from the center of Eros, it will fire its hydrazine engines to slow it enough to be captured by the asteroid's weak gravitational pull. Confirmation of a successful orbit is expected to come at about 11:30 a.m. EST to waiting team members in the Mission Operations Center on the Applied Physics Laboratory campus of Johns Hopkins University.

That these critical events will happen on Valentine's day is appropriate because Eros is named for the Greek god of love.

During the first few weeks after achieving orbit the spacecraft will slowly descend toward the asteroid. Because it is irregularly shaped and rotating, this early stage of the mission can be very tricky.

"No one has ever orbited a small body in space,"says Dr. Robert Farquhar, NEAR mission director. "The orbital stability is rather tenuous, and as we travel around Eros our navigation maneuvers must be perfect to keep us from crashing into it."

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Using a multispectral imager, laser rangefinder, and the onboard radio science experiment, mission scientists and engineers will collect enough information about Eros's shape, mass and gravity field to allow the spacecraft to come closer.

Left: A diagram of the NEAR spacecraft, showing locations of the magnetometer, multi-spectral imager, infrared spectrometer, the laser rangefinder, the X-ray spectrometer, and the Gamma-ray spectrometer.

"Soon after we go into orbit we should know the asteroid's mass and therefore its density to within 5 percent," says Dr. Andrew Cheng, mission scientist.

The onboard magnetometer will determine the strength of the asteroid's magnetic field -- if there is one.

"This will give the scientific community the first definitive measurement of an asteroid's magnetism, which contains clues to its thermal and geologic history," Dr. Cheng says. "The results of these measurements and others that we will take over the next year will help us to determine the origin of the asteroid and give us an unprecedented understanding of asteroids in general."

For the first two months NEAR will slowly descend to within 31 miles (50 kilometers) from Eros. During this low-orbit phase scientists will use the x-ray/gamma-ray spectrometer to measure the abundance of various elements that make up Eros. X-rays from the sun striking the asteroid can produce significant count rates of fluorescence X-rays from surface elements such as magnesium, aluminum, and silicon. The elements sulfur, calcium, titanium, and iron are also present in asteroids, but count rates will be lower and data will take longer to accumulate. Similarly, cosmic ray protons (and energetic particles associated with solar flares) can interact with the asteroid surface to produce gamma rays characteristic of the nuclear energy levels of a given element. Gamma rays also can be spontaneously emitted by naturally occurring radioactive elements such as potassium, uranium, and thorium.

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Right: A distant image of Eros is shown for comparison with a montage of main belt asteroids. The picture of Mathilde was obtained by NEAR during a flyby on June 27, 1997. The pictures of Gaspra and Ida were captured by the Galileo spacecraft during flybys in 1991 and 1993, respectively. Brief flybys are not sufficient to answer many outstanding questions about asteroids. The NEAR mission will remedy that with a year-long study of Eros.

In late August the spacecraft will begin to climb from 31 to 311 miles (50 to 500 kilometers) above the center of Eros. During this ascent the spacecraft's camera will continue to snap pictures of the asteroid's surface that will be compiled into a complete map. In December NEAR will descend, possibly to less than a mile, from the surface of the asteroid. From that vantage point the near- infrared spectrometer can collect extremely high resolution data of the asteroid's surface, making it possible to distinguish the composition of rocks as small as a grapefruit. Final events of the mission, which will end in February 2001, will be determined sometime this summer.

NEAR was launched Feb. 17, 1996, from Cape Canaveral Air Station, FL. Its original rendezvous date of Jan. 10, 1999, was postponed when a firing of the spacecraft's bipropellant engine, designed to put the spacecraft on target for the rendezvous, exceeded preset acceleration limits and caused the spacecraft to retreat into safe mode. But valuable information about the asteroid was collected by a hastily programmed flyby of Eros on Dec. 23, 1998. Early images can be found on the Internet at:

The NEAR (Near Earth Asteroid Rendezvous) mission, a NASA Discovery Program being conducted by the Johns Hopkins University Applied Physics Laboratory, Laurel, MD, is the first mission to orbit an asteroid.Web Links

Near Earth Asteroid Rendezvous mission - NEAR home page from Johns Hopkins University

NASA Press Release - 8 Feb. 2000