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Next stop: The stars

NASA's next Great Observatory, the Chandra X-ray Observatory, moves closer to launch

Feb. 12, 1999: The Chandra X-ray Observatory is a giant step closer to the cosmos this week after being installed in a clean room for final tests and other work before being orbited later this year.

"Wow. It's such a nice milestone, it's such a great milestone," said Dr. Martin Weisskopf, the Chandra project scientist at NASA's Marshall Space Flight Center. Weisskopf explained that the observatory now faces two activities before launch: replacement of some circuit boards, and extensive integration and testing that will attach Chandra to a solid rocket motor and then install the assembly in Space Shuttle Columbia. Launch is scheduled for this summer.

The Chandra X-ray Observatory is the third of NASA's Great Observatories for Space Astrophysics. The first two are the Hubble Space Telescope and the Compton Gamma Ray Observatory. On Feb. 4, Chandra was flown from Redondo Beach, Calif, where it was built by TRW, to Kennedy Space Center, Fla., for final preparations in the Vertical Processing Facility.

Right: The Chandra observatory is prepared for tests in a thermal-vacuum chamber that simulates space conditions at TRW. The engineer at the bottom left corner shows the size of the observatory. (TRW photo)

Chandra will observe the universe in a portion of the spectrum - high-energy X-rays - between the windows opened by Hubble and Compton.

This will give scientists a new view of some of the most violent and energetic activities in the universe. The red-green-blue we know in visible light carries an energy of about 2 electron-volts (2 eV). Chandra will see activities with energies ranging from 50 to 10,000 eV (up to 10 keV).

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Weisskopf has been the project scientist for Chandra since 1977, a year before its predecessor, the second High Energy Astronomy Observatory (HEAO-2) was launched. Until recently, Chandra was known as the Advanced X-ray Astrophysics Facility (AXAF). When AXAF was started in 1977, NASA/Marshall was preparing to launch the first of HEAO spacecraft to give scientists their best views of the universe in X-rays, gamma-rays, and cosmic radiation.

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The HEAO-2 mission, also called the Einstein Observatory, carried what was then the largest focusing X-ray telescope. HEAO-2's telescope was actually a 4-in-1 apparatus, four telescopes nested within each other. Each comprised a primary mirror and a secondary mirror to focus X-rays and magnify the image.

Below, right: A current view of Chandra in the Vertical Processing Facility at NASA's Kennedy Space Center.

Because X-rays carry enough energy to penetrate most materials, the mirrors are shaped more like shallow tubes than the more familiar dish-shaped mirrors in visible light telescopes. At a shallow angle, the X-rays just graze and reflect, just as sunlight glares off a window if the angle is right.

As advanced as HEAO-2 was in the 1970s, NASA was planning something bigger.

"AXAF had already come into being as the follow-on to HEAO-2, since we recognized full well that it would take some time to develop the next generation," Weisskopf said.

Design and development has been a challenge because of the goals that astrophysicists set for the observatory now known as Chandra. It was renamed in 1998 in honor of noted Indian-American astrophysicist Subrahmanyan Chandrasekhar. Chandra's telescope comprises four nested mirror arrays that have three times as much collecting area as HEAO-2. The telescope will see a small portion of sky with details eight times finer than HEAO-2, and catch events at double the energy (10 vs 5 keV) as HEAO-2.

While Chandra's calibration tests, here at NASA/Marshall in December 1996-May 1997, showed that telescope met or exceeded specifications, the ultimate test will be in looking at the stars.

But that won't come right after launch. First, Columbia (left) will deploy Chandra atop an Inertial Upper Stage (a two-stage solid rocket). The IUS and Chandra's own thrusters will boost the spacecraft to an orbit 10,000 to 140,000 km (6,200 x 86,800 mi) above the Earth's radiation belts, a potential source of noise for Chandra's detectors. Then, the spacecraft will be turned on slowly. The delay is necessary for safety. Spacecraft normally are given a couple of days to adjust to the space environment and let the last traces of air trickle out and thus avoid electrical arcing when systems are powered up and tested.

"First X-ray light will be around three weeks after launch," Weisskopf said.

Science with Chandra

The astrophysics community is eagerly lining up to use Chandra to answer thousands of questions about the universe. Observations by scientists at NASA's Marshall Space Flight Center include:

How hot is the Crab? In early summer of 1054, long before the first Independence Day celebration in the United States, the people of Japan and China witnessed an amazing display of fireworks in the summer sky. The Crab Nebula (left), as the display came to be known, was an exploding new supernova so bright it was visible in the daytime sky for nearly a month. Soon after, it faded to a level where it would not be rediscovered until newly invented telescopes spotted it in the 18th century. Dr. Martin Weisskopf, Chandra's project scientist, will take the Crab's temperature.
Left: A detailed Hubble Space Telescope image of the Crab nebula's heart is inset on a wider false-color visible-light image of the nebula. Chandra will reveal even finer details of the neutron star that powers the Crab's emissions.
Looking for pulsars living in the fast lane.The discovery last year of the first magnetar - a highly magnetized star - put the spotlight on a small class of stars called Anomalous X-ray Pulsars, or AXPs. While the magnetar discovery involved another small class of cosmic sources of high-energy radiation, the Soft Gamma Repeaters (SGR), the magnetar theory holds that these objects may become AXPs before they fade from the scene altogether. Dr. Jan van Paradijs will use Chandra observations to shed more light on AXPs.
Why did the supernova "morph"? Most objects in the sky can be pigeonholed into a few of the hundreds of categories that classify stars, galaxies, and other bodies. Every now and then, you get one that changes its colors - literally - and seems to beg for closer examination. Dr. Doug Swartz will target one cosmic chameleon for closer examination with Chandra.

Web Links
The Chandra X-ray Observatory Center at Harvard University is the official home page for Chandra.
Kennedy Space Center carries the latest information on Chandra's launch preparations.
Exploring the X-ray Universe, the NASA/Marshall Chandra web site.
Countdown to Discovery. the Chandra newsroom.
X-ray astronomy at NASA/Marshall.
Chandra project science page has technical details and links to related web sites.
The Chandra Science Center at Harvard University will control Chandra operations.
X-ray calibration web site has plenty of techncial information about the telescope's high-resolution mirror assembly.
TRW Space & Electronics Group built Chandra.

While it's in the dark, Chandra will see its own nightlight, a set of radioactive buttons inside the contamination door. These emit X-rays and thus serve as calibration sources so the science team can recheck the performance of the instruments.

Once Chandra is powered up and the door opens, X-rays will be focused onto the science instruments. Like any new telescope, Chandra will have to be checked out and fine-tuned. The process will take another six weeks, a small price to pay to ensure first-class science from the world's greatest X-ray observatory.

After that, discovery.

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Dr. John M. Horack , Director of Science Communications
Author: Dave Dooling
Curator: Linda Porter
NASA Official: Ron Koczor