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William.A.Steigerwald					      
Feb. 14, 2000

William.A.Steigerwald.1@gsfc.nasa.gov
(Phone: 301-286-0039)

RELEASE NO: 00-22

NASA GODDARD COURTS EROS ON VALENTINE'S DAY

As the Near Earth Asteroid Rendezvous (NEAR) spacecraft settles into orbit around asteroid 433 Eros on Feb. 14, scientists at NASA's Goddard Space Flight Center (Greenbelt, Md.) are preparing to take full advantage of its suite of instruments to participate in the first detailed study of an asteroid.

Goddard scientists lead two NEAR science teams and believe their research will help the NEAR team in its quest to unravel key mysteries surrounding asteroids, including how they formed and their relationship to meteorites. 433 Eros, named for the Greek god of love, belongs to a class of asteroids that approach Earth's orbit. NEAR researchers therefore also hope to learn how strongly these asteroids are held together, so that the best course of action can be taken if ever one seems likely to impact the Earth.

The NEAR magnetometer, built at Goddard, will let scientists peek inside the asteroid and try to divine its history. The measurements will allow scientists to do this by determining if Eros has a magnetic field and, if so, measuring its strength and shape.

"The magnetometer instrument is one of the very few methods we have for determining what happened on the inside of an asteroid or planet, so we can to understand how it evolved," said Dr. Mario Acuna of Goddard, NEAR Magnetometer Science Team Leader. "If we find a strong magnetic field surrounding the entire asteroid, it implies certain things about the history and origin of 433 Eros. Such magnetic fields can only be created if there was a dynamo in the asteroid's parent body. This implies a core of molten metal, which through convection and rotation, generates a magnetic field. Since Eros is too small to have a molten core, it would have been part of a larger body that did posses a molten core and magnetic field. The magnetic field would have frozen into Eros as the parent body cooled, and then Eros broke off from its parent somehow. This will tell us that asteroids with strong magnetic fields are fragments of larger bodies, rather than aggregations of smaller objects."

Finding a weak magnetic field would make the history of Eros more difficult to fathom. According to Acuna, "Weak magnetic fields can be generated in many ways, and a wider range of phenomena and events will have to be included as possibilities in Eros' history or its interaction with the solar wind." Acuna doubts a magnetic field will be detected immediately, because NEAR's initial orbit will be too remote. He expects to begin taking measurements in late April or mid-May, as NEAR gradually spirals closer to Eros and the orbit is only about 31 miles (50 kilometers) from the surface.

"With the NEAR X-ray Gamma-Ray Spectrometer (XGRS) instrument, we can map the distribution of various elements at or near the surface of Eros, and by comparing their ratios, we can find out if Eros is a primordial relic from the formation of the solar system, or if it has been through thermal change," said Dr. Jacob Trombka of Goddard, XGRS Science Team Leader.

"Eros is a Rosetta stone that can return us to our cosmological roots by determining what happened during the solar system's formation, and the XGRS is a key to translating its language - that of geochemistry," said Dr. Pamela Clark of Goddard. "When a forming planet reaches a certain size, its heat and gravity become sufficient to cause differentiation of its elements - the planet becomes molten, and heavy elements, like nickel and iron, tend to sink to the center, while light elements, like silicon and carbon, rise to the surface. If we find Eros is composed mostly of light elements, this indicates that it is a fragment from near the surface of a larger body. If it is rich in heavy elements, it is likely from the center of a larger object. If instead it is undifferentiated, like the chondrite meteorites found on Earth, it probably never was exposed to much heat. This implies that it is a very primitive, unprocessed object, probably an aggregate of smaller objects from the early solar system nebula."

X-rays from the Sun strike Eros' surface and cause elements to fluoresce (to be stimulated, then glow) in specific X-ray "colors," invisible to the human eye but detectable by XGRS. This is similar to the way a black light uses ultraviolet light to make pop-art posters glow in various garish colors. Each element on Eros glows in unique X-ray colors, allowing XGRS to build a map of the element distribution at and just beneath the surface of Eros. XGRS will also use the cosmic radiation from space to map the elemental composition slightly deeper beneath the surface of Eros (to about four inches) via invisible gamma radiation. Like an atomic billiards game, rapidly moving particles from space crash into atoms within Eros, causing them to emit unique gamma-ray "colors," depending on their elemental classification. In addition, certain elements are naturally radioactive, and will reveal their presence to XGRS with a gamma-ray signature from their own internal decay.

XGRS, together with the other NEAR instruments, will tell us the composition, density and mass of asteroids like Eros, said Trombka.

Like the NEAR magnetometer, the XGRS has to get close to Eros before taking measurements; they are also scheduled to begin in late April or early May. XGRS research will pay off in other ways as well, from investigating mysterious blasts at the edge of the observable Universe to fighting crime here on Earth.

Gamma-ray bursts (GRBs) are the most powerful explosions known, equal in fury to hundreds of exploding stars, or supernovae. These remote explosions occur randomly and typically last just a few seconds before fading. The trick is to locate them quickly and accurately so that they can be studied by other telescopes sensitive to the different kinds of light emitted by the GRB fireball as it expands and cools.

"The NEAR XGRS instrument has been configured to identify GRBs," said Trombka. "It is electronically linked with GRB detectors on other spacecraft in Earth orbit, and a detector on the Ulysses solar mission, to form a space GRB detection network 130 million miles across. When all spacecraft are transmitting, we can accurately identify GRB locations from the differences in arrival times of the gamma-rays at spacecraft in the network. The data is relayed instantaneously through the Gamma-ray burst Coordinates Network at Goddard to telescopes all over the world. We have already autonomously identified three GRBs with the space network to date."

"The NEAR XGRS team also is working with the National Institute of Justice, applying XGRS technology to identify drive-by shooters from the gunpowder residue on their hands, which fluoresces in X-rays," added Trombka.

Goddard scientists will use the NEAR Laser Rangefinder (NLR) to make three-dimensional maps of the cratered surface of Eros to understand how cosmic collisions influence the shape of low-gravity objects like asteroids. "By measuring the three-dimensional properties of the best preserved and most recent impact events that have occurred on Eros, we can decipher how collisions influence the landscape of asteroids and how low gravity affects the shape of craters," said Dr. James Garvin of Goddard, Participating Scientist on the NLR science team. "Understanding the impact cratering process under low gravity is essential to understanding how planets and other solar system objects formed, because they originated from smaller, low-gravity objects that grew via collision with other solar system material."

The NLR fires extremely short-duration bursts of laser radiation from the NEAR spacecraft to the surface of Eros, which is later received at the spacecraft and timed to billionths of a second. By knowing where the spacecraft is located, where it is pointed, and how long it takes for the laser echo to be received, distance to the surface can be measured precisely, and a three-dimensional profile can be built up as NEAR progresses in its orbit. The NLR team expects to start mapping the asteroid in March, with higher resolution observations during spring and summer. More information and images are available on the Internet at:

http://pao.gsfc.nasa.gov/gsfc/spacesci/solarexp/near.htm

Kisha Wright
NASA/Goddard Space Flight Center
Public Affairs Assistant/Code 130
Email:  Kwright@pop100.gsfc.nasa.gov
Office:  301-286-4084  Fax:  301-286-1707

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