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HETE-2 Spacecraft

The spacecraft structure of HETE-2 is comprised of two parts: the bottom half (closest to the solar panels) is mostly spacecraft hardware (power, communications, attitude control), and the upper half (furthest from the solar panels) is where the science instruments sit.

The mechanical structure of HETE is formed by the spacecraft hardware itself. As seen from the bottom of HETE, the mechanical structure consists of

1. the Marmon ring, which connects to
2. the baseplate, on top of which sit
3. two battery boxes, on top of which sits
4. the electronics box (spacecraft computer), to which is mounted
5. the WXM electronics and mask support structure.

All other pieces of spacecraft or science hardware are mounted to this structure.

• two magnetometers, one on the spacecraft body and one on one of the solar panels
• a set of twelve coarse, medium, and fine sun sensors, which tell where the sun in in their fields-of-view with precision befitting their names
• the optical camera system, which delivers spacecraft drift rate information during orbit night.

Control of spacecraft attitude is done with

• three orthogonal torque coils, which use electrical currents in the coils to generate IxB forces around the Earth's magnetic field
• a momentum wheel, which nominally spins at roughly 1800 RPM and which can be torqued to change the angular momentum of the spacecraft.

The ACS software consists of a sophisticated set of algorithms which take the current spacecraft orientation and spin and adjusts the torque coil currents and wheel momentum to create a stable orientation.

• an S-band radio, transmitting and receiving at frequencies near 2 GHz, which is used for uplink and downlink of spacecraft data,
• 5 dual patch transmit-receive S-band antennas, four of which are oriented on the tips of the solar panels and one on the "bottom" (sun-facing side) of the spacecraft,
• a VHF transmitter, which is used to transmit (only) important scientific and spacecraft-related housekeeping to the ground
• a "whip" antenna for the VHF, mounted to one of the solar panels.

• four solar panels, made of honeycomb aluminum with silicon substrate, each supplying 42W.
• power box with power point tracker,~90% efficient
• 6 battery packs, each made up of a string of 23 1.2V NiCd cells, and each with 1.5 A-hrs capacity

The power box has a sophisticated design which allows it to charge the batteries ("net positive charging") regardless of orientation or panel deployment state, when the spacecraft is in "safe-hold mode": the safe-hold power is 2W, while the minimum charging power is 7W.

1. spacecraft processing
2. SXC
3. optical cameras
4. WXM and Fregate

The "links" feature of the transputer allows for quick and efficient communications between processors. The DSPs serve as the interface to the instruments.