Low-Budget NEAR Knocks the Socks off Scientists
(From the May/June 2001 issue of StarDate magazine)
After the car-sized NEAR spacecraft’s unprecedented February 12 landing on asteroid 433 Eros, scientists studied the asteroid’s composition using NEAR’s gamma-ray spectrometer (GRS), poised only four inches from the ground.
“What we’re looking for is information that will help us more precisely classify Eros and determine the relationship between the asteroid and meteorites that have fallen to Earth,” said Jack Trombka, GRS team leader at NASA’s Goddard Spaceflight Center.
GRS works like any spectrometer — by breaking up light, in this case high-energy gamma rays, into its component wavelengths. It’s a high-tech way of passing light through a prism to make a very detailed rainbow. Studying those details uniquely made by gamma rays on Eros’ surface can tell astronomers what the asteroid is made of.
The gamma rays being studied are created when cosmic rays (high-energy particles from distant supernova explosions) hit Eros. “Cosmic rays shatter atomic nuclei in the asteroid’s soil,” Trombka said. The shattered nuclei scatter subatomic particles called neutrons, which in turn hit other atoms. The excited atoms give off gamma rays that GRS picks up. “We can detect cosmic-ray excited oxygen, iron, and silicon, along with the naturally radioactive elements potassium, thorium, and uranium,” Trombka said.
GRS data could confirm that Eros is a planetesimal — a relic of the planet formation process during the early stages of our solar system. Orbital data have already confirmed that the material in Eros dates to the time when planets were forming, and that the asteroid is probably a piece broken off of a larger parent body. But unlike the planets in our solar system, Eros doesn’t have differentiated layers underneath its surface.
Eros data will also help scientists know where and when in our solar system’s evolution to file the information gained from studying meteorites that have fallen to Earth. “We’d really like to know their geologic context,” said Mark Robinson, NEAR Imaging Team member. “We’re making very good progress connecting Eros to ordinary chondrite meteorites,” he said.
Launched February 17, 1996, NEAR went into orbit around Eros on Valentine’s Day, 2000. The craft orbited Eros for a year at low altitude, taking pictures and laser measurements to build a precise model of the rocky body, and amassing 10 times more data than planned.
To get “bonus science” out of the spacecraft, mission planners decided to have NEAR take pictures of Eros’ surface during a controlled descent. Scientists are now puzzling over strange landscapes (fractured boulders, a football-field-sized crater filled with dust, and a mysterious area where Eros’ surface seems to have collapsed) seen by NEAR.
No one expected the spacecraft to survive impact. But remarkably, it was still ticking, and its low-gain antenna was pointed at Earth, enabling communication. That’s when scientists got a 10-day mission extension to perform the up-close gamma-ray experiments on the Manhattan-sized asteroid, now 200 million miles from Earth.
The Applied Physics Laboratory (APL) at The Johns Hopkins University managed the mission for NASA. NASA cut off contact with NEAR on March 1. Rebecca Johnson
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