The Status and Future of the Third Interplanetary Network

The 3rd interplanetary network (IPN), which has been in operation since 1990, presently consists of 9 spacecraft: AGILE, RHESSI, Suzaku, and Swift, in low Earth orbit; INTEGRAL, in eccentric Earth orbit with apogee 0.5 light‐seconds; Wind, up to ∼7 light‐seconds from Earth; MESSENGER, en route to Mercury; and Mars Odyssey, in orbit around Mars. Ulysses and HETE have ceased operations, and the Fermi GBM is being incorporated into the network. The IPN operates as a full‐time, all‐sky monitor for transients down to a threshold of about 6 × 10^(−7) erg cm^(−2) or 1 photon cm^(−2) s^(−1). It detects about 275 cosmic gamma‐ray bursts per year. These events are generally not the same ones detected by narrower field of view imaging instruments such as Swift, INTEGRAL IBIS, and SuperAGILE; the localization accuracy is in the several arcminute and above range.