March 9, 2000
Although the focus of the NEAR mission is on asteroids, NEAR has made an important contribution to the study of cosmic gamma ray bursts as part of the Interplanetary Gamma Ray Burst Network, known as the IPN. Gamma ray bursts are the most powerful explosions in the Universe - exceeding even the exploding stars called supernovae. First discovered in the late 1970's by defense early warning satellites, gamma ray bursts last for only tens of seconds, but during that brief time they are the brightest gamma ray emissions in the entire sky. The sources of gamma ray bursts are distributed uniformly around the sky, and for decades after their discovery scientists were unsure as to whether most of these sources were exotic objects within our own galaxy or much farther away, at "cosmological" distances (meaning that the sources are so far away that gamma rays, moving at the speed of light, require a significant fraction of the age of the universe to travel to Earth). The farther away the sources of the gamma ray bursts, the greater must be the energy release to explain the brightness of the bursts. If the bursts originated within our galaxy, they would not match the energy releases in supernovae, but as we have learned in recent years that the burst sources are at cosmological distances, they far outshine the supernovae.The IPN has played a key role in this advance, by providing precise localizations of the burst sources rapidly enough to allow identification of their optical and radio counterparts before they fade from view. If the host galaxy can be identified, its redshift can be measured from spectra to find the distance to the burst. The IPN localizes sources by timing the burst detections at spacecraft mutually separated by great distances. If a burst is detected by only two spacecraft, or in other words if there is only one baseline, then the burst source can be anywhere on a ring in the sky. With a third spacecraft in the network, the burst is localized to either of two points in the sky. Before NEAR, the IPN consisted only of a constellation of satellites in Earth orbit plus the Ulysses spacecraft at around 5 AU from the sun, so there was only one interplanetary baseline. NEAR at Eros provides the additional interplanetary baselines that allow the IPN to locate burst sources to within about 3 arc minutes - roughly, the angle subtended by a 1 meter plate at a distance of 1200 meters. This is fine enough to enable detection of optical and radio counterparts to burst sources.
Before NEAR became the third leg in the IPN tripod, a total of about 10 cosmological burst sources were discovered and identified. In only a few months of operation within IPN, NEAR has enabled four more source definitions that would otherwise have been impossible. Since the IPN with NEAR now resolves about 1 source per week (including those also localized by other missions), within this year IPN should double the total number of identified sources.
This exciting advance has been enabled by NEAR's gamma ray spectrometer, which was modified after launch (with a software patch) to be able to detect gamma ray bursts. The NEAR spacecraft engineering team also worked hard to refine the accuracy of spacecraft timing to within 100 milliseconds. The upshot is, as NEAR studies Eros over the next year, it is also contributing to the study of gamma ray burst sources. Scientists are still unsure as to how these bursts are produced, but one possibility is that they result from the merger of binary neutron stars - creating a compound object initially about the same size as Eros, but containing more mass than our entire solar system.
Andy Cheng
NEAR Project ScientistCorrection: On March 2, 2000 I should have said that the NLR recorded the first ever laser range returns from an asteroid. The first radar range returns from an asteroid were obtained from Arecibo, Puerto Rico to Eros in 1975.
