The SIRTF spacecraft
FROM NASA PRESS KIT
Posted: April 14, 2003
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 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 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 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.
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