February 13, 2000 -- NASA's Near Earth Asteroid Rendezvous (NEAR) probe is now less than 1000 kilometers from 433 Eros. If all goes well, NEAR will become the first spacecraft in history to orbit an asteroid when it reaches the 21 mile long space rock on Monday morning at 10:33 EST (7:33 PST).

On February 8, a 23-second engine burn accelerated NEAR from 18 mph to 22 mph relative to Eros, and shifted its target point by 0.6 degrees. The burn was the second of two adjustments since February 3 when another maneuver slowed the spacecraft's approach speed from 43 mph to 18 mph. These were the last scheduled engine firings before Monday's orbit insertion.

Above: On the right are pictures of asteroid Eros in February 6, 2000. On the left are pictures from an earlier NEAR flyby in December, 1998. All the photos were snapped at a distance of about 4200 miles (6800 km). The differences in Eros's appearance at the two times result from seasonal variations in solar illumination. Last year NEAR approached from the north when the asteroid was experiencing southern hemisphere summer and thus the north pole was in shadow. Nearly 14 months later, Eros is experiencing northern summer. The spacecraft is again over northern latitudes, but because of the difference in illumination it views a mostly sunlit, gibbous Eros.
 

 

Although most attention is focused on Monday morning when NEAR goes into orbit, critical science observations will begin about 11 hours earlier when the spacecraft passes directly between the Sun and the asteroid. In this "high noon" geometry, NEAR will see a part of the asteroid's surface with no shadows. That's perfect for the spacecraft's Near-Infrared Spectrometer (NIS), which measures the infrared spectrum of reflected sunlight. NIS data will provide the main evidence for the distribution and abundance of surface minerals like olivine and pyroxine. (Olivine is a mineral that is abundant in Earth's mantle; pyroxine is commonly found in lava flows.) The spectrometer can't detect areas of Eros that are in shadow, so the high noon data -- also called "zero phase" data, by scientists -- are very important.
  When NEAR arrives at Eros on February 14, the asteroid will be experiencing northern summer. At this time of the year on Eros (one year on Eros equals 1.76 Earth-years) the Sun never sets over the north pole. The south pole of the asteroid, where it is winter, is dark. NEAR can't accumulate infrared spectra of the southern regions until the seasons change. That will happen later this year and NEAR is scheduled to make zero phase observations of the southern side of Eros in October, 2000.

Left: Diagram shows the difference between "High Noon" (or "zero phase") and NEAR's in orbit orientation. At zero phase, NEAR lies directly between the Sun and asteroid Eros. While in orbit, the Sun, Eros, and NEAR form a 90 degree angle.

Just prior to Monday's orbit insertion is the only time during the entire Eros rendezvous that NEAR will be able to collect zero phase observations over the northern part of the asteroid. Once NEAR is in orbit, the angle between the spacecraft, the asteroid and the Sun will usually be near 90 degrees, which is a better geometry for the spacecraft's primary camera and other instruments.

Mission scientists expect the infrared observations just before orbit insertion to yield the highest quality IR spectra of Eros's northern regions. Similar data for the southern hemisphere will be obtained on one day in October, 2000. Together with measurements from the X-ray/Gamma-ray Spectrometer and color imagery from the MSI, these infrared data will reveal the connection between asteroids and meteorites and possibly clarify how asteroids were formed billions of years ago.

Because the spacecraft enters orbit just a few hours after the critical NIS measurements, NEAR has a lot to do in a short time. The sequence of commands has been loaded into the spacecraft in advance and will be executed autonomously by on board computers. The entire command script (except for the final rocket engine firing) was tested successfully on the spacecraft in January by the NEAR team. Hopefully, everything will go as smoothly on Monday as it did last month.

For more information about the NEAR mission, see the recent SpaceScience.com headline "Eros or Bust" and the NEAR Mission Home Page from Johns Hopkins University.

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.

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Eros or Bust - February 8, 2000. NASA Science News. NASA's Near-Earth Asteroid Rendezvous Mission is nearing 433 Eros. It is scheduled to go into orbit around the space rock on Valentines Day, 2000. University

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

NEAR Science Update, Jan. 24, 2000 - much of the material for this story was drawn from this source.

NASA Press Release - 8 Feb. 2000