ABOUT THE CHANDRA X-RAY OBSERVATORY
NASA's premier X-ray observatory was named the Chandra X-ray Observatory in
honor of the late Indian-American Nobel laureate, Subrahmanyan
Chandrasekhar. Known to the world as Chandra (which means "moon" or
"luminous" in Sanskrit), he was widely regarded as one of the foremost
astrophysicists of the twentieth century.
Chandra immigrated in 1937 from India to the United States, where he joined
the faculty of the University of Chicago, a position he remained at until
his death. He and his wife became American citizens in 1953.
Trained as a physicist at Presidency College, in Madras, India and at the
University of Cambridge, in England, he was one of the first scientists to
combine the disciplines of physics and astronomy. Early in his career he
demonstrated that there is an upper limit - now called the Chandrasekhar
limit - to the mass of a white dwarf star. A white dwarf is the last stage
in the evolution of a star such as the Sun. When the nuclear energy source
in the center of a star such as the Sun is exhausted, it collapses to form a
white dwarf. This discovery is basic to much of modern astrophysics, since
it shows that stars much more massive than the Sun must either explode or
form black holes.
Chandra was a popular teacher who guided over fifty students to their
Ph.D.s. His research explored nearly all branches of theoretical
astrophysics and he published ten books, each covering a different topic,
including one on the relationship between art and science. For 19 years, he
served as editor of the Astrophysical Journal and turned it into a
world-class publication. In 1983, Chandra was awarded the Nobel prize for
his theoretical studies of the physical processes important to the structure
and evolution of stars.
"Chandra probably thought longer and deeper about our universe than anyone
since Einstein," said Martin Rees, Great Britain's Astronomer Royal.
The Space Shuttle Columbia delivered Chandra to a low Earth orbit. Then, the
Inertial Upper Stage rocket boosted Chandra up to a higher altitude where a
built-in propulsion system took Chandra to its final orbit. This elliptical
orbit takes the spacecraft to an altitude of 133,000 km (82,646 mi) - more
than a third of the distance to the moon - before returning to its closest
approach to the Earth of 16,000 kilometers (9,942 mi). It takes
approximately 64 hours and 18 minutes to complete an orbit.
The Chandra spacecraft spends approximately 85% of its orbit above the Van
Allen belts, charged particles that surround the Earth. This makes
uninterrupted observations of as long as 55 hours possible, making the
overall percentage of useful observing time much greater than the low Earth
orbit of a few hundred kilometers used by most satellites.
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on
July 23, 1999. It was named in honor of Indian-American physicist
Subrahmanyan Chandrasekhar who is known for determining the mass limit for
white dwarf stars to become neutron stars. "Chandra" also means "moon" or
"luminous" in Sanskrit.
Chandra Observatory is the third of NASA's four Great Observatories. The
first was Hubble Space Telescope; second the Compton Gamma Ray Observatory,
launched in 1991; and last is the Spitzer Space Telescope. Prior to
successful launch, the Chandra Observatory was known as AXAF, the Advanced
X-ray Astrophysics Facility. AXAF was assembled and tested by TRW (now
Northrop Grumman Space Technology) in Redondo Beach, California. Chandra is
sensitive to X-ray sources 100 times fainter than any previous X-ray
telescope, due primarily to the high angular resolution of the Chandra
Since the Earth's atmosphere absorbs the vast majority of X-rays, they are
not detectable from Earth-based telescopes, requiring a space-based
telescope to make these observations.
Unlike optical telescopes which possess simple aluminized parabolic surfaces
(mirrors), X-ray telescopes generally use a Wolter telescope consisting of
nested cylindrical paraboloid and hyperboloid surfaces coated with iridium
or gold. X-ray photons would be absorbed by normal mirror surfaces, so
mirrors with a low grazing angle are necessary to reflect them. Chandra uses
four pairs of nested iridium mirrors, together with their support structure,
called the High Resolution Mirror Assembly (HRMA).
Chandra's highly elliptical orbit allows it to observe continuously for up
to 55 hours of its 65 hour orbital period. At its furthest orbital point
from earth, Chandra is one of the furthest from earth earth-orbiting
satellites. This orbit takes it beyond the geostationary satellites and
beyond the outer Van Allen belt. 
With an angular resolution of 0.5 arcsecond (2.4 Árad), Chandra possesses a
resolution over one thousand times better than that of the first orbiting
- Chandra flies 200 times higher than Hubble - more than 1/3 of the way to
- Chandra can observe X-rays from clouds of gas so vast that it takes light
five million years to go from one side to the other.
- During maneuvers from one target to the next, Chandra slews more slowly
than the minute hand on a clock.
- At 45 feet long, Chandra is the largest satellite the shuttle has ever
- Chandra's resolving power is equivalent to the ability to read a stop sign
at a distance of twelve miles.
- The electrical power required to operate the Chandra spacecraft and
instruments is 2 kilowatts, about the same power as a hair dryer.
- The light from some of the quasars observed by Chandra will have been
traveling through space for ten billion years.
- STS-93, the space mission that deployed Chandra, was the first NASA
shuttle mission commanded by a woman.
- Chandra can observe X-rays from particles up to the last second before
they fall into a black hole.
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This web page was last updated on:
28 July, 2011