The SIRTF spacecraft
Posted: April 14, 2003

Illustration shows the sides of SIRTF. Credit: NASA/JPL/Caltech
Like all spacecraft, the design of the Space Infrared Telescope Facility is a delicate balance of form and function. The form is the spacecraft's dimensions and weight, dictated by the size and power of the launch vehicle, or rocket, lifting it into space. The function is the infrared-observing capabilities of the scientific cargo onboard the spacecraft.

The spacecraft is unique in that it is smaller and lighter than past missions involving cryogenically cooled telescopes. In past missions, a vacuum shell surrounded the telescope like a thermos bottle, actively chilling the observatory and science instruments. On the Space Infrared Telescope Facility, the vacuum shell surrounds only the instrument chamber and the liquid helium tank. Engineers refer to this configuration as a "warm launch architecture." It means that much less coolant is needed, allowing for the use of a relatively smaller launch vehicle. In addition, it will permit the spacecraft to collect science data for up to five years, twice the length of the longest previous infrared mission.

A new name for the Space Infrared Telescope Facility is expected to be announced about four months after launch, when the first images and science results will be released. The new name was selected after a worldwide contest with more than 7,000 entries.

The Space Infrared Telescope Facility is only one-third as long and one-eleventh the weight of its bigger sibling, the Hubble Space Telescope. But by incorporating the latest technological innovations into the new spacecraft's design, the mission architects have packed a remarkable amount of science capability into a relatively small package. Within the four-meter-tall (14.6-foot), 865-kilogram (1,907-pound) observatory is enclosed a cryogenically cooled telescope and three science instruments - a multiband imaging photometer, an infrared spectrometer and an infrared-detector array camera - as well as all the power, computer, communications and navigation equipment required to make the mission a success.

Illustration of SIRTF. Credit: NASA
The observatory is composed of three main sections. The tube-shaped cryogenic telescope assembly includes the telescope and scientific instruments, enclosed within a protective outer shell. At the rear of the telescope is an eight-sided spacecraft structure that houses all the computers, electronics, antennas and thrusters needed to keep the observatory operating and oriented correctly in space. The spacecraft's third main section is the solar panel array, which serves as the observatory's power plant and does double duty as a heat shield to shade the rest of the spacecraft.

Thermal protection
The ultra-sensitive instruments onboard require special shielding to protect them from the environmental hazards of deep space. Because thermal protection is very important for the mission, the telescope is cooled using extremely cold liquid helium. Over the course of the mission, this ultra-cold helium will slowly leak out into space. After five years, all of the helium will be lost. The cooler that engineers can keep the telescope with strategies other than the liquid helium, the slower the coolant will be consumed and the longer the mission can last.

The outer shell that encloses the telescope serves as both a dust cover and a heat shield. Shaped like a cylinder, the entire outer shell is composed of aluminum - a quarter inch layer in a honeycomb pattern is sandwiched between two sheets. The side of the outer shell that faces the Sun has a shiny silver coating to reflect any incoming sunlight not absorbed by the solar panels. The side facing away from the Sun has a black coating designed to radiate any residual heat from the solar panel and spacecraft. In engineering parlance, the outer shell provides "passive" thermal protection for the spacecraft.

The observatory is also protected by an "active" thermal control system that consists of heat pipes, thermally conductive adhesives, heaters and temperature sensors. Propane and ammonia flowing through pipes embedded in the spacecraft's exterior panels conduct heat away from the observatory. Various parts of the spacecraft that need to be heated in order to operate are equipped with controlled heaters but insulated to avoid heating the telescope. The spacecraft's solar panels are made out of special material to minimize heat flow to the telescope. The finishes on the solar panels themselves also help regulate panel temperature.

Command and data handling
The command and data handling system is the spacecraft's brain. It can operate the observatory either with commands stored in computer memory or via "real-time" commands radioed from Earth for immediate execution. In addition, it handles engineering and science data destined to be sent to Earth. The system's design is based on technology from NASA's Mars Surveyor and Stardust projects. At its heart is a RAD6000 processor, a radiation-hardened version of the PowerPC chip used on some models of Macintosh computers. A duplicate backup system is available to take over spacecraft control if needed.

Electrical power
The solar panel array provides the electrical power needed to operate the observatory for five years. The array consists of two solar panels, each with 392 solar cells. Each solar cell is 5.5 by 6.5 centimeters (2.2 by 2.6 inches). Together the cells can convert radiation from the Sun into a total of 427 watts of electrical power at the beginning of the mission and 386 watts towards the end. Unlike most spacecraft solar arrays that are deployed shortly after launch, this mission's solar array is fixed. To ensure that sunlight will hit the solar panels properly, the telescope cannot be pointed at targets more than 120 degrees away from the Sun.

The solar array also shades the telescope from direct exposure to the Sun. Half of the solar array's surface area is covered with solar cells. The other half is covered with flexible optical solar reflectors that reduce the overall solar panel temperature to about 57 C (134 F).

Attitude determination and control
The Space Infrared Telescope Facility uses a pointing control system to orient and maneuver in deep space (or, in engineering language, to determine and control the spacecraft's "attitude"). It can also execute small maneuvers such as turning, or by speeding itself up or slowing itself down, to point the telescope at science targets. The system also keeps the spacecraft pointed in the correct angle from the Sun, with the high-gain antenna pointed toward Earth.

The spacecraft has four modes used to point the telescope at observing targets. In the first mode, called "inertial pointing," the observatory essentially sits and stares at the same point in space without moving. This is useful for observing faint, distant objects.

In a second mode, called "incremental pointing," the spacecraft again sits and stares at the same point in space without moving. After capturing one image, however, the spacecraft moves slightly and fixes on a new point. This process may be repeated several times, so that a given target object appears in different parts of the image frame. This helps to guarantee that at least some of the pictures will be high-quality. It is useful for super-resolution images.

A third pointing scheme called the "scan map mode" is used only by one of the three science instruments, the multi-band imaging photometer. This instrument has a single moving part, a scan mirror. In this pointing mode, the observatory moves in one direction, while the photometer's scan mirror moves at the same speed, but in the opposite direction. This technique freezes a big portion of the sky for a period of time, which is useful for mapping large areas.

The fourth and final pointing mode is called the "tracking mode." It is useful for taking pictures of comets and other moving objects within our solar system. For this mode, information about the movement of solar system objects (called "ephemerides") is loaded into the observatory's onboard computer. Tracking is done autonomously without intervention by ground controllers.

The observatory can accurately move 180 degrees in 1,000 seconds (16 minutes, 40 seconds), 1 degree in 100 seconds or 1 arc-minute in 20 seconds (an arc-minute is 1/60th of one degree). A star tracker calculates the spacecraft's position by comparing observed stars to an onboard catalog of 87,000 stars. Other onboard sensors keep the telescope aligned correctly relative to the stars, stabilize the pointing system and keep the observatory positioned safely relative to the Sun.

The spacecraft moves itself by changing the momentum of four spinning devices similar to gyroscopes called reaction wheels. In addition, the spacecraft is equipped with six primary thrusters and six backups that use gaseous nitrogen as a cold propellant. These thrusters keep the amount of momentum stored in the reaction wheels within a specified range, and are used to "unload" excess momentum as it builds up. An onboard tank stores 15.6 kilograms (about 34.4 pounds) of nitrogen propellant.

The radio system onboard the observatory is designed to operate at a maximum distance from Earth of 96 million kilometers (about 60 million miles). The system uses a parabolic dish high-gain antenna, two receiving low-gain antennas and two transmitting low-gain antennas. The system can receive commands from Earth at speeds ranging from 7.8125 to 2,000 bits per second, and can send data to Earth at speeds from 40 to 2.2 million bits per second.

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