The GALEX spacecraft. Credit: NASA

The Galaxy Evolution Explorer would fit on a tabletop. It is a cylinder measuring about 1 meter (3 feet) in diameter and 2.5 meters (6.4 feet) high, weighing 277 kilograms (609 pounds). Its structure is composed primarily of aluminum.

The basic satellite design is new, but shares elements in common with several satellites now under development by Orbital Sciences Corp. Much of the flight software is derived from software developed for NASA's Far Ultraviolet Spectroscopic Explorer and for Orbview 4, a commercial imaging satellite.

Command and Data Handling
All of the satellite's computing functions are performed by the command and data handling subsystem. The heart of this subsystem is a Rad 6000 computer, a radiationhardened version of the PowerPC chip used on some models of Macintosh computers. The chip is also used on many other spacecraft such as the Mars Pathfinder lander. With 128 megabytes of random access memory and three megabytes of non-volatile memory, which allows the system to maintain data even without power, the computer runs Galaxy Evolution Explorer's flight software and controls various other parts of the satellite.

Among tasks managed by the computer are Sun avoidance; deployment of the solar arrays; precision determination and control of the satellite's orientation (or "attitude"); thermal management; automated fault detection and correction; communication with the telescope instrument; and acquisition, storage and transmission of science data.

A 24-gigabit solid-state recorder stores engineering data from the satellite and science instrument, and science data from the instrument.

An uplink card operates independently of the onboard computer and is responsible for receiving, validating and decoding commands from the ground. Commands are routed to three paths for execution: special commands, normal command traffic and stored commands. Special commands operate independently of the onboard computer, thereby bypassing the processing associated with normal command traffic. Special commands are typically used for reconfiguration and/or hard resets for the command and data handling subsystem. They can also be used to place the satellite in a "safe" mode, if this is commanded from the ground.

A downlink card is responsible for receiving telemetry data received from various spacecraft subsystems, the science instrument and the solid-state recorder, and preparing them for transmission to ground stations.

Electrical Power
This subsystem is responsible for generating, storing and distributing the satellite's electrical power. It is designed to provide substantial power margins using a minimal solar array area.

Power is generated by 2.9 square meters (31.2 square feet) of gallium arsenide solar cells mounted on two fixed wings. Total area of the arrays is 3 square meters (32.2 square feet).

Power is stored in a 15-amp-hour nickel-hydrogen battery. The battery can support spacecraft power needs for the 36 minutes of each orbit during which the Sun is eclipsed from the satellite.

The power electronics suite consists of three boxes: the battery sensor electronics, the power module, and the power and thermal control electronics. The battery sensor electronics box conditions battery temperature and pressure, and controls battery heaters. The power module converts the voltage from the solar arrays to levels required by various devices on the spacecraft. The power and thermal control electronics box provides overall control of the power subsystem.

Thermal Control
This subsystem maintains proper temperature of equipment on the satellite. It uses passive components, such as radiators, blankets and thermal paints, as well as redundant heaters controlled by flight software. Mechanical thermostats control survival heaters in the event that the satellite enters a safehold condition.

To control battery temperature, the battery cells are connected to a dedicated radiator by heat-conducting sleeves. A thermal blanket is used to protect the cells from the space environment.

Attitude Determination and Control
The Galaxy Evolution Explorer is a "three-axis-stabilized" spacecraft, meaning that it can be held in any orientation in relation to space. The system is fully autonomous, meaning that it relies on onboard systems to control the satellite orientation, or "attitude," with no intervention required from ground controllers.

Unlike some spacecraft, the Galaxy Evolution Explorer has no thrusters to adjust its orientation. Instead, it achieves this with four devices called reaction wheels, which use the momentum of spinning wheels to nudge the satellite in one direction or another. Occasionally the reaction wheels accumulate too much momentum, which requries the use of devices called torque bars -- somewhat like large electromagnets -- to push against Earth's magnetic field and cancel out some of the momentum in the wheels.

The torque bars are controlled by a device called a magnetometer that senses the orientation of Earth's magnetic field.

The satellite's orientation is sensed by a star tracker, while its rotation rates are sensed by gyroscopes. The star tracker is also used to correct slow drifts that occur in the gyroscopes' sensing circuits. Two Sun sensors provide a relatively coarse measure of the Sun's direction when the satellite is in safehold modes.

Telecommunications
The radio system operates in both the S-band and the X-band ranges of the microwave spectrum. The S-band transmitter sends basic spacecraft data, or "telemetry," to Earth, while two S-band receivers accept commands from the ground. The 5- watt S-band transmitter sends data at 2 megabits per second, and the receivers operate at 2,000 bits per second. The X-band transmitter, which sends science data, transmits at 24 megabits per second with an output power of 6 watts.