InterPlanetary Network

The InterPlanetary Network (IPN) is a group of spacecraft equipped with gamma ray burst (GRB) detectors. By timing the arrival of a burst at several spacecraft, its precise location can be found. The precision for determining the direction of a GRB in the sky is improved by increasing the spacing of the detectors, and also by more accurate timing of the reception. Typical spacecraft baselines of about one AU (astronomical unit) and time resolutions of tens of milliseconds can determine a burst location within several arcminutes, allowing follow-up observations with other telescopes.

Gamma rays are too energetic to be focused with mirrors. The rays penetrate mirror materials instead of reflecting. Because gamma rays cannot be focused into an image in the traditional sense, a unique location for a gamma ray source cannot be determined as it is done with less energetic light.

In addition, gamma ray bursts are brief flashes (often as little as 0.2 seconds) that occur randomly across the sky. Some forms of gamma ray telescope can generate an image, but they require longer integration times, and cover only a fraction of the sky.

Once three spacecraft detect a GRB, their timings are sent to the ground for correlation. A sky position is derived, and distributed to the astronomical community for follow-up observations with optical, radio, or spaceborne telescopes.

Note that, since any IPN must consist of several spacecraft, the boundaries between networks are defined differently by different commentators.

Spacecraft naturally join or leave service as their missions unfold, and some modern spacecraft are far more capable than prior IPN members.

The Vela group of satellites was originally designed to detect covert nuclear tests, possibly at the Moon's altitude. Thus, the Velas were placed in high orbits, so that a time delay would occur between spacecraft triggers. In addition, each satellite had multiple gamma-ray detectors across their structures; the detectors facing a blast would register a higher gamma count than the detectors facing away.

A gamma-ray burst was detected by the Vela group on June 3, 1969, and thus referred to as GRB 690603. The location was determined to be clearly outside of the satellites' orbit, and probably outside of the Solar system. After reviewing archived Vela data, a previous burst was determined to have occurred on July 2, 1967. Public reports of initial GRBs were not disclosed until the early 1970s.

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