artist concept of ISEE-3 approaching a comet

* Mission Overview

The International Sun-Earth Explorer (ISEE) program consisted of 3 satellites-- ISEE-1 and ISEE-3 were the principal US contribution to the International Magnetospheric Study, and ISEE-2 which was built and managed by ESA. ISEE-1 and -2 were launched on 22 October 1977 into almost coincident orbits. The orbital period was 57 hours, and their separation in the orbit was controlled by manuvuering ISEE-2. ISEE-3 was launched on 12 August 1978. It was inserted into a "halo" orbit about the libration point some 240 Earth radii upstream between the Earth and Sun. ISEE-3 was renamed ICE (International Cometary Explorer) when, after completing its original mission in 1982, it was gravitationally manuvuered to intercept the comet P/Giacobini-Zinner. On September 11, 1985, the veteran NASA spacecraft flew through the tail of the comet.
The ISEE-3/ICE bit rate was nominally 2048 b/s during the early part of the mission and 1024 b/s during the P/Giacobini-Zinner encounter. The bit rate then dropped to 512 b/s on 9/12/85, 256 b/s on 5/1/87, 128 b/s on 1/24/89, and finally 64 b/s on 12/27/91. An extended ICE mission was approved by NASA in 1991 for the continued investigation of coronal mass ejections, continued cosmic ray studies, and coordinated observations with Ulysses. As of January 1990, the satellite was in a 355 day heliocentric orbit with an aphelion of 1.03 AU, perihelion of 0.93 AU, and inclination 0.1 degree. It will return to the vicinity of the Earth-moon system in August 2014.

* Instrumentation

The University of California put an X-ray Spectrometer aboard ISEE-3. It was designed to study both solar flares and cosmic gamma-ray bursts over the energy range 5-228 keV. The detector provided full-time coverage, 3-pi sr field of view for E > 130 keV, time resolution of 0.25 ms, and absolute timing to within 1 ms. It was intended to be a part of a long baseline interferometry network of widely separated spacecraft. The efforts were aimed primarily at determining the origin of the bursts through precise directional information established by such a network. The experiment consisted of 2 cylindrical X-ray detectors: a Xenon filled proportional counter covering 5-14 keV, and a NaI(Tl) scintillator covering 12-1250 keV. The proportional counter was 1.27 cm in diameter and was filled with a mixture of 97% Xenon and 3% carbon dioxide. The central part of the counter body was made of 0.51 mm thick beryllium and served as the X-ray entrance window. The scintillator consisted of a 1.0 cm thick cylindrical shell of NaI(Tl) crystal surrounded on all sides by 0.3 cm thick plastic scintillator. The central region, 4.1 cm in diameter, was filled by a quartz light pipe. The whole assembly was enclosed (except for one end) in a 0.1 cm thick beryllium container. The energy channel resolution and timing resolution could be selected by commands sent to the spacecraft. The proportional counter could have up to 9 channels with 0.5 s resolution; the NaI scintillator could have up to 16 channels and 0.00025 s resolution. The gamma-ray burst mode was triggered by two simultaneous conditions: the count rates in certain PHA channels rose above a selected level and the spectral hardness of incident photons as determined by the ratio of 2 selected PHA channels exceeded a certain value.

Also aboard ISEE-3 was the Goddard Gamma-Ray Burst Spectrometer. This instrument represented the first successful flight of a high purity germanium detector on a satellite. It provided an order of magnitude improvement in the measurement of spectral properties of gamma-ray bursts than any previously flown detector.

The germanium detector was a high purity 4.02 x 2.9 cm right circular cylinder. The total germanium volume was 35 cu-cm. The detector was hermetically sealed in a Mg enclosure, which also provided the radiative surface for cooling. This structure reached a temperature of 130K roughly 3 days after launch and remained stable at that value for over a year. The detector system had a resolution of 10 keV at 570 keV and had 4096 energy channels. It operated in the range 200 keV - 3 MeV. The nominal time resolution was an 8 ms spectral data integration.

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