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NEAR EMBRACES EROS

SKY & TELESCOPE'S NEWS BULLETIN
FEBRUARY 18, 2000

After a four-year journey, the Near Earth Asteroid Rendezvous spacecraft fired a quartet of small rockets at 15:33 Universal Time (10:33 a.m. EST) on February 14th and eased into a wide orbit around the minor planet 433 Eros, becoming the first spacecraft to orbit an asteroid.

In the following days, planetary scientists happily reported that Eros is not just a boring hunk of asteroidal rock. "We're on an adventure that will last for a year," quipped NEAR project scientist Andrew E. Cheng. By the time NEAR eased into a 325-by-370-km orbit around the asteroid, it had become obvious that the highly elongated, 33-by-13-km body is peppered with so many craters that its surface must be quite old. This suggests that it was not ejected toward Earth's general vicinity by a relatively recent collision within the asteroid belt, because the impact would likely have erased most of the existing craters. However, investigator Mark Robinson points out that a portion of Eros's midsection, near what has been dubbed the "saddle," has many fewer craters that may hint at a youthful "landslide" or other surface event. Many giant, mansion-size boulders also dot the surface, undoubtedly pieces of impact debris that may prove crucial in deciphering the body's internal composition.

Spectroscopists are eager to know more about the composition of Eros. This object falls into the S-type asteroid category, perhaps meaning that it consists of primitive rocky material largely unchanged from early in solar-system history, or that it may have melted to produce distinct rock and metal layering. Although it is much too early to know which idea is correct, NEAR has already recorded hints of internal structure that imply the latter history. Moreover, the size of the spacecraft's orbit, coupled with its period (about 20 days) have provided an accurate total mass and in turn a mean density of about 2.4 grams per cubic centimeter. This means that Eros is fairly solid, observes dynamicist Donald K. Yeomans, though "the jury is really still out" on whether the interior is porous and riddled with cavities.

Once the shape and mass of Eros are known more accurately, mission controllers will ease NEAR closer to it in a series of stages. Later this month a pulsed-laser altimeter will begin to measure altitudes on the surface. The orbit will be shrunk to 100 km high on April 1st, and soon thereafter NEAR should be circling just 50 km high -- close enough to use its magnetometer and X-ray/gamma-ray spectrometers.

The historic arrival was especially satisfying because the mission nearly met an untimely end in December 1998 during its initial attempt to rendezvous with Eros. At that time a malfunction during a crucial engine firing caused the spacecraft to shut down, nearly exhausting its fuel reserves in the process. Emergency efforts by the flight team not only recovered the spacecraft fully but allowed it a second chance to accomplish its goals. With a total mission cost of $212 million, NEAR is expected to study Eros for approximately one year.


Copyright 2000 Sky Publishing Corporation. S&T's Weekly News Bulletin and Sky at a Glance stargazing calendar are provided as a service to the astronomical community by the editors of SKY & TELESCOPE magazine. Widespread electronic distribution is encouraged as long as these paragraphs are included. But the text of the bulletin and calendar may not be published in any other form without permission from Sky Publishing (contact permissions@skypub.com or phone 617-864-7360). Updates of astronomical news, including active links to related Internet resources, are available via SKY & TELESCOPE's site on the World Wide Web at http://www.skypub.com/.

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