Inter Planetary Network - The 21st Century: Staring Spacecraft

The 21st Century: Staring Spacecraft

New techniques and designs in high-energy astronomy spacecraft are challenging the traditional operation of the IPN. Because distant probes require sensitive ground antennas for communication, they introduce a time lag into GRB studies. Large ground antennas must split time between spacecraft, rather than listen continuously for GRB notifications. Typically, GRB coordinates determined by deep space probes are distributed many hours to a day or two after the GRB. This is very frustrating for studies of events which are measured in seconds.

A new generation of spacecraft are designed to produce GRB locations on board, then relay them to the ground within minutes or even seconds. These positions are based not on time correlation, but on X-ray telescopes, as on BeppoSAX but much faster. HETE-2, launched in 2000, stares at a large region of sky. Should a GRB trigger the gamma detectors, X-ray masks report sky coordinates to ground stations. Because HETE is in a low, consistent orbit, it can use many inexpensive ground stations. There is almost always a ground station in view of the spacecraft, which reduces latency to seconds.

The Swift spacecraft, launched in 2004, is similar in operation but much more powerful. When a GRB triggers the gamma detectors, generating a crude position, the spacecraft spins relatively rapidly to use its focusing X-ray and optical telescopes. These refine the GRB location to within arcminutes, and often within arcseconds. The fine position is reported to the ground in approximately an hour.

INTEGRAL is a successor to Compton. INTEGRAL can similarly determine a coarse position by comparing gamma counts from one side to another. It also possesses a gamma-ray telescope with an ability to determine positions to under a degree. INTEGRAL cannot pivot rapidly like the small HETE and Swift spacecraft. But should a burst happen to occur in its telescope field of view, its position and characteristics can be recorded with high precision.

RHESSI was launched in 2002 to perform solar studies. However, its gamma instrument could detect bright gamma sources from other regions of the sky, and produce coarse positions through differential detectors. Occasionally, a GRB would appear next to the Sun, and the RHESSI instrument would determine its properties without IPN assistance.

Note however, that all these spacecraft suffer from Earth blockage to varying degrees. Also, the more sophisticated the "staring" instrument, the lower the sky coverage. Randomly occurring GRBs are more likely to be missed, or detected at low resolution only. The use of non-directional deep space probes, such as MESSENGER and BepiColombo, will continue.