Page:HST Compton Chandra PaperModels.pdf/7



ASA’s Chandra X-ray Observatory (CXO) is the most sophisticated x-ray observatory ever built. It observes x-rays from high-energy regions of the universe, such as hot gas in the remnants of exploded stars. This observatory has three major parts: (1) the x-ray telescope, whose mirrors will focus x-rays from celestial objects; (2) science instruments, which record the x-rays so that x-ray images can be produced and analyzed; and (3) the spacecraft, which provides the environment necessary for the telescope and the instruments to work.

CXO will be boosted into an elliptical orbit by a built-in propulsion system. Two firings by an attached Inertial Upper Stage (lUS) rocket and three firings of its own onboard rocket motors after separating from the lUS will place the observatory into its working orbit. The onboard rocket motors, called the Integral Propulsion System, will also be used to move and aim the observatory. The orbit will take the spacecraft more than a third of the way to the Moon before returning to its closest approach to Earth of 10,000 kilometers. The time to complete an orbit will be 64 hours and 18 minutes.

The spacecraft will spend 85 percent of its orbit above the belts of charged particles that surround Earth. The radiation in these belts can overwhelm the observatory’s sensitive instruments. Uninterrupted observations as long as 55 hours will be possible, and the overall percentage of useful observing time will be much greater than for the low-Earth orbit of a few hundred kilometers used by most satellites.

CXO’s sensitivity will make it possible for more detailed studies of black holes, supernovae, and dark matter. It will also increase our understanding of the origin, evolution, and density of the universe.

The spacecraft system provides the support structure and environment necessary for the telescope and the science instruments to work as an observatory. For example, the sunshade door is one of most basic and important elements of the spacecraft system. This door remains closed until CXO has achieved pointing control in orbit. After being opened, it shadows the entrance of the telescope to allow it to point as close as 45 degrees to the Sun.

The thermal control system consists of a cooling radiator, insulators, heaters, and thermostats to control the temperatures of critical components of CXO. It is particularly important that the temperature near the x-ray mirrors be well controlled to keep the mirrors in focus. The temperature in many parts of the spacecraft is continually monitored and reported back to mission control.

The electrical power system generates electrical power from the solar arrays, stores it in three banks of batteries, and distributes it in a carefully regulated manner to the observatory. The solar arrays generate approximately 2 kilowatts of power for the heaters, science instruments, computers, transmitters, and so forth.

The communications, control, and data management system is the nerve center of the observatory. It keeps track of the position of the spacecraft in its orbit, monitors the spacecraft sensors, receives and processes commands from the ground for the operation of the observatory, and stores and processes the data from the instrument so that they can be transmitted to the ground. Typically, the data are transmitted to the ground during contacts with the NASA Deep Space Network about once every 8 hours.

The pointing control and aspect of determination system has gyros, an aspect camera, Earth and Sun sensors, and reaction wheels to monitor and control to very high accuracy where the telescope is pointing at any given moment. It is as if one could locate the bull’s eye on a target 1 kilometer away to the precision of 3 millimeters—about the size of a pinhead. This system can also place the observatory into various levels of inactive, quiet states, known as "safe modes" of operation, during emergencies.

The function of the science instruments is to record as accuratelyas possible the number, position, and energy of the incoming x-rays. This information can be used to make an x-ray image and study other properties of the source, such as its temperature.

The High Resolution Camera (HRC) will be one of two instruments used at the focus of CXO, where it will detect x-rays reflected from an assembly of eight mirrors. The unique capabilities of the HRC stem from the close match of its imaging capability to the focusing of the mirrors. When used with the CXO mirrors, the HRC will make images that reveal detail as small as one-half an arc second. This is equivalent to the ability to read a newspaper at a distance of 1 kilometer.

The primary components of the HRC are two Micro-Channel Plates. They each consist of a 10-centimeter-square cluster of 69 million tiny lead-oxide glass tubes that are about 10 microns in diameter (one-eighth the thickness of a human hair) and 1.2 millimeters long. The tubes have a special coating that causes electrons to be released when the tubes are struck by x-rays. These electrons are accelerated down the tube by a high