A significant step toward revealing the mysteries of gamma-ray bursts was taken this week by The Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md., when NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft sent back unexpected data showing a major gamma-ray burst. APL manages the NEAR mission for NASA.
The observation came after researchers reconfigured the gamma-ray spectrometer to make more frequent data returns as NEAR travels to a rendezvous with the asteroid Eros in February 1999. If they are as distant as new evidence suggests, gamma-ray bursts are the most violent explosions known, emitting in one second as much energy as the sun will emit in its lifetime.
The gamma-ray spectrometer was not originally planned to begin its work until the spacecraft reached Eros. But while en route a simple software change was added that gave a new astrophysics capability to this planetary spectrometer, which resulted in detection of a gamma-ray burst on Sept. 15, that lasted for about 10 seconds. Since then six more bursts have been detected. Several of the bursts have been confirmed by the European Space Agency/NASA Ulysses spacecraft, now in a polar orbit around the sun and by two detectors on NASA's Wind spacecraft near the Earth. These three spacecraft, along with other Earth-orbiting spacecraft, form a 3-dimensional interplanetary network for observing gamma-ray bursts that has not been possible since the loss of the Mars Observer in 1993.
"Seeing this burst validates that the NEAR detector can be a true working partner in the interplanetary network for gamma-ray burst detection," says APL's lead gamma-ray instrument engineer John Goldsten, who was the first to see the gamma-ray burst data.
Jacob Trombka, NASA's Science Team Leader for the gamma-ray instrument, says, "NEAR's instrument is more sensitive than we believed it would be. It's seeing bursts that other spacecraft aren't seeing." The success of the instrument is the result of a good design, he says. "Originally we didn't have time to include a burst mode on the instrument, but the system was so well designed that we were able to upload such a system a few weeks ago."
Gamma-ray bursts remain one of the great mysteries of astrophysics since their discovery more than 30 years ago. They tend to be randomly distributed over the sky and occur with a frequency of about one per day for the most sensitive detectors. If they are of cosmological origin, they represent the most powerful events that are known in the universe. The debate as to their local or cosmological origin will most likely be resolved by locating sources of gamma-ray bursts and then identifying them with optical and radio telescopes. NASA's Hubble Space Telescope made the first observation of a fuzzy object associated with a burst that was detected last Feb. 28 by the Italian BeppoSAX satellite.
The sources of gamma-ray bursts can be located in the sky by timing the arrival of the gamma rays at three well-separated spacecraft. Since 1993, the spacecraft instrumented to observe such bursts have been the Ulysses spacecraft plus several spacecraft near the Earth: The BeppoSAX and Wind as well as NASA's Compton Gamma-Ray Observatory (CGRO) and Rossi X-ray Timing Explorer. The near-Earth spacecraft are too close to each other to allow a unique determination of the location of the bursts.
The addition of the Near Earth Asteroid Rendezvous Spacecraft to the interplanetary network will provide 3-dimensional triangulation of source locations and should greatly increase the probability of associating a gamma-ray source with a particular source from optical and radio telescopes.
The new capability on NEAR will expand the network and enable it to obtain the locations of moderate and stronger bursts, which occur at least several times per month, to a position in the sky accurate to about a minute of arc (about a thirtieth of the size of the moon).
NEAR, the first mission of NASA's Discovery Program for "faster, better, cheaper" planetary exploration, will be the first spacecraft to orbit an asteroid. On June 27, NEAR sent back spectacular images of 253 Mathilde as it flew past the asteroid. In February 1999, NEAR will reach Eros and begin the first long-term, close-up look at an asteroid's surface composition and physical properties.
NEAR was designed and built by The Johns Hopkins University Applied Physics Laboratory.
For more information contact Helen Worth, JHU/APL Office of Public Affairs. Phone: (301) 953-5113; e-mail: Helen.Worth@jhuapl.edu; or fax: (301) 953-6123; or Donald Savage, NASA Headquarters Office of Space Science. Phone: (202) 358-1547; e-mail: firstname.lastname@example.org; or fax: (202) 358-3093.
Information on the NEAR mission can be obtained on APL's NEAR homepage