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The concept of a satellite capable of multiwavelength observations of GRBs was discussed at the Santa Cruz meeting on GRBs in 1981. In 1986, the first realistic implementation of the HETE concept by an MIT-led International Team was proposed. This concept, which was adopted, emphasized accurate locations and multiwavelength coverage as the primary scientific goals for a sharply-focused small satellite mission which would ultimately solve the gamma-ray burst mystery.
In 1989, NASA approved funding for a low-cost "University Class" explorer satellite to search for GRBs. In 1992, the HETE-1 program was funded, and the design and construction of HETE-1 began.
The original spacecraft contractor for HETE-1 was AeroAstro, Inc., of Herndon, VA. AeroAstro was responsible for the spacecraft bus, including power, communications, attitude control, and computers.
The instrument complement for HETE-1 consisted of
- Four wide-field gamma-ray detectors, supplied by the CESR of Toulouse, France
- A wide-field coded-aperture X-ray imager, supplied by a collaboration of Los Alamos National Laboratory and the Institute of Chemistry and Physics (RIKEN) of Tokyo, Japan.
- Four wide-field near-UV CCD cameras, supplied by the Center for Space Research at the Massachusetts Institute of Techology.
The HETE-1 satellite was launched on November 4, 1996, along with the Argentine satellite SAC-B, on a Pegasus rocket from Wallops Island, VA. The Pegasus rocket achieved a good orbit, but the third stage failed to release the two satellites. As a result, SAC-B and HETE-1 were unable to function as designed and both died due to lack of solar power within a day of launch.
Due to the tragic fate of HETE-1 and the continuing timeliness of GRB science (see below), NASA agreed to a reflight of the HETE-1 satellite, using flight spare hardware from the first satellite. In July, 1997, funding for a second HETE satellite was granted, with a target launch date of late 1999 or early 2000.
Construction of the HETE-2 satellite began in mid-1997. The HETE-2 satellite bus was built entirely at MIT.
The results of observations of GRBs in early 1997 by BeppoSAX and ground-based telescopes made it clear that a) we should not expect copious UV or optical radiation from the GRB and b) the ratio of X-ray to gamma-ray flux from GRBs could be significantly higher that previously believed. As a result, the HETE team decided to remove the UV cameras from the spacecraft and replace two of them with a CCD-based coded-aperture imager sensitive to soft X-rays (the Soft X-ray Camera, or SXC). The other two cameras were replaced with optical CCD cameras, which serve as star trackers on HETE-2.
The experience of the BeppoSAX and RXTE satellites, the former in an equatorial orbit and the latter in a 28 degree orbit, made clear that the effect of background electrons and protons would have a profound effect on the observing efficiency and lifetime of HETE-2's X-ray instruments. In 1998, NASA agreed to flying HETE-2 in an equatorial orbit.
In January, 2000, the HETE-2 satellite was fully complete and being prepared for launch from Kwajalein Atoll in the Republic of the Marshall Islands. On January 14, 2000, NASA decided to postpone the launch of HETE-2, for reasons described in a press release reproduced here.
HETE-2 was successfully launched on October 9, 2000.
January 20, 2000: NASA Delays Launch of the HETE-2 Satellite
On Friday, January 14, 2000, a decision was made by NASA Headquarters to delay the High Energy Transient Explorer-2 (HETE-2) launch until mid-May.
When the decision was made, the fully-assembled and fully-tested HETE-2 satellite was in a launch ready state at Vandenberg Air Force Base in California. The HETE-2 spacecraft and its science instruments had met all of its original design, construction, and test requirements and had passed all of its NASA reviews. As of January 14, HETE-2 was completely integrated (fully checked out, with no anomalies) to its Pegasus launcher, the fairing was closed, and it was ready for ferrying to Kwajalein Atoll in the Republic of the Marshall Islands for a January 28 launch.
Several factors contributed to the decision to delay the launch. Although the required primary HETE-2 telemetry ground station at Kwajalein was completely launch ready, NASA was concerned that neither of the two HETE-2 backup stations (Cayenne, French Guiana; Singapore) were completely operational. Delays in the ITAR (State Department) export approval process meant that the Cayenne station would come on line just a week prior to the scheduled launch, while the Singapore station would possibly not be available until after January 28. The recent heightened concern for additional ground station availability was a consequence of NASA's desire that there be ample telemetry contact with HETE-2 in the critical early phases of the mission, so that any unforeseen satellite activation difficulties of the type encountered by a number of recent NASA-launched missions could be avoided or minimized.
Since the Kwajalein Missile Range (KMR) would only be reserved for HETE-2 from January 28 - February 8, NASA decided that the risk of not having all of the desired ground assets in place for a timely launch was significant. Thus, the decision was made by NASA to forego this first launch window at KMR in favor of a second one in the mid-May time frame. It was also decided that, given the extra time, the HETE-2 satellite would be returned to the East Coast for additional mission simulations and testing to further enhance the likelihood of mission success.
Obviously, the HETE-2 Team will cooperate in NASA's new plan that the satellite undergo additional testing to further ensure mission success. Due to HETE-2's limited budget (1/3 the cost of a SMEX) and constrained mass (125 kg for the spacecraft plus instruments), it relies heavily on "single string" designs for major systems. Thus, there are practical limits to the degree to which further testing can heighten reliability.
Testing that is planned will include additional shock and vibration tests, antenna tests, and 1000 hours of thermal vacuum cycling (ie 1.5 times longer than HETE-2's preshipment thermal vacuum testing, and 1/4 of the minimum mission life). Upon satisfactory conclusion of these additional tests, HETE-2 will be shipped back to Vandenberg for re-mating to its Pegasus launcher in mid-April, with an equatorial launch planned in mid-May from the Republic of the Marshall Islands.
Assuming a successful launch, HETE-2 will be checked out in orbit and could begin distributing accurate gamma-ray bursts localizations to the astronomical community as soon as July 1, 2000.