The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-classspace telescope launched aboard the Space ShuttleColumbia during STS-93 by NASA on July 23, 1999. Chandra was sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, enabled by the high angular resolution of its mirrors. Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes; therefore space-based telescopes are required to make these observations. Chandra is an Earth satellite in a 64-hour orbit, and its mission is ongoing as of 2024.
In response to a decrease in NASA funding in 2024 by the US Congress,
Chandra is threatened with an early cancellation despite having more
than a decade of operation left. The cancellation has been referred to
as a potential "extinction-level" event for X-ray astronomy
in the US. A group of astronomers have put together a public outreach
project to try to get enough American citizens to persuade the US
Congress to provide enough funding to avoid early termination of the
observatory.
History
In 1976, the Chandra X-ray Observatory (called AXAF at the time) was proposed to NASA by Riccardo Giacconi and Harvey Tananbaum. Preliminary work began the following year at Marshall Space Flight Center (MSFC) and the Smithsonian Astrophysical Observatory (SAO), where the telescope is now operated for NASA at the Chandra X-ray Center in the Center for Astrophysics | Harvard & Smithsonian. In the meantime, in 1978, NASA launched the first imaging X-ray telescope, Einstein
(HEAO-2), into orbit. Work continued on the AXAF project throughout the
1980s and 1990s. In 1992, to reduce costs, the spacecraft was
redesigned. Four of the twelve planned mirrors were eliminated, as were
two of the six scientific instruments. AXAF's planned orbit was changed
to an elliptical one, reaching one third of the way to the Moon's at its
farthest point. This eliminated the possibility of improvement or
repair by the Space Shuttle but put the observatory above the Earth's radiation belts for most of its orbit. AXAF was assembled and tested by TRW (now Northrop Grumman Aerospace Systems) in Redondo Beach, California.
AXAF was renamed Chandra as part of a contest held by NASA in 1998, which drew more than 6,000 submissions worldwide.
The contest winners, Jatila van der Veen and Tyrel Johnson (then a high
school teacher and high school student, respectively), suggested the
name in honor of Nobel Prize–winning Indian-AmericanastrophysicistSubrahmanyan Chandrasekhar. He is known for his work in determining the maximum mass of white dwarf stars, leading to greater understanding of high energy astronomical phenomena such as neutron stars and black holes. Fittingly, the name Chandra means "moon" in Sanskrit.
Originally scheduled to be launched in December 1998, the spacecraft was delayed several months, eventually being launched on July 23, 1999, at 04:31 UTC by Space ShuttleColumbia during STS-93. Chandra was deployed by Cady Coleman from Columbia
at 11:47 UTC. The Inertial Upper Stage's first stage motor ignited at
12:48 UTC, and after burning for 125 seconds and separating, the second
stage ignited at 12:51 UTC and burned for 117 seconds. At 22,753 kilograms (50,162 lb), it was the heaviest payload ever launched by the shuttle, a consequence of the two-stage Inertial Upper Stage booster rocket system needed to transport the spacecraft to its high orbit.
Chandra has been returning data since the month after it launched. It is operated by the SAO at the Chandra X-ray Center in Cambridge, Massachusetts, with assistance from MIT and Northrop Grumman
Space Technology. The ACIS CCDs suffered particle damage during early
radiation belt passages. To prevent further damage, the instrument is
now removed from the telescope's focal plane during passages.
Although Chandra was initially given an expected lifetime of 5
years, on September 4, 2001, NASA extended its lifetime to 10 years
"based on the observatory's outstanding results."
Physically Chandra could last much longer. A 2004 study performed at
the Chandra X-ray Center indicated that the observatory could last at
least 15 years. It is active as of 2024 and has an upcoming schedule of observations published by the Chandra X-ray Center.
On October 10, 2018, Chandra entered safe mode operations, due to
a gyroscope glitch. NASA reported that all science instruments were
safe.Within days, the 3-second error in data from one gyro was understood,
and plans were made to return Chandra to full service. The gyroscope
that experienced the glitch was placed in reserve and is otherwise
healthy.
In March 2024, Congress decided to reduce funding for NASA and its missions.
This may lead to the premature end of this mission.
In June 2024, Senators urged NASA to reconsider the cuts to Chandra.
Example discoveries
Crew of STS-93 with a scale model
The data gathered by Chandra has greatly advanced the field of X-ray astronomy. Here are some examples of discoveries supported by observations from Chandra:
In the Crab Nebula, another supernova remnant, Chandra showed a never-before-seen ring around the central pulsar and jets that had only been partially seen by earlier telescopes.
Chandra data suggested that RX J1856.5-3754 and 3C58, previously thought to be pulsars, might be even denser objects: quark stars. These results are still debated.
2006 Chandra found strong evidence that dark matter exists by observing super cluster collision.
2006 X-ray emitting loops, rings and filaments discovered around a super massive black hole within Messier 87 imply the presence of pressure waves, shock waves and sound waves. The evolution of Messier 87 may have been dramatically affected.
Observations of the Bullet cluster put limits on the cross-section of the self-interaction of dark matter.
Jupiter's x-rays coming from poles, not auroral ring.
A large halo of hot gas was found surrounding the Milky Way.
Extremely dense and luminous dwarf galaxy M60-UCD1 observed.
On January 5, 2015, NASA reported that CXO observed an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*, the supermassive black hole in the center of the Milky Way galaxy. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers.
In September 2016, it was announced that Chandra had detected X-ray emissions from Pluto, the first detection of X-rays from a Kuiper belt object. Chandra had made the observations in 2014 and 2015, supporting the New Horizons spacecraft for its July 2015 encounter.
In September 2020, Chandra reportedly may have made an observation of an exoplanet in the Whirlpool Galaxy, which would be the first planet discovered beyond the Milky Way.
In April 2021, NASA announced findings from the observatory in a
tweet saying "Uranus gives off X-rays, astronomers find". The discovery
would have "intriguing implications for understanding Uranus" if it is
confirmed that the X-rays originate from the planet and are not emitted
by the Sun.
Technical description
Assembly of the telescopeThe main mirror of AXAF (Chandra)HRC flight unit of Chandra
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 mirrors, together with their support structure, called the High Resolution Mirror Assembly
(HRMA); the mirror substrate is 2 cm-thick glass, with the reflecting
surface a 33 nm iridium coating, and the diameters are 65 cm, 87 cm,
99 cm and 123 cm.
The thick substrate and particularly careful polishing allowed a very
precise optical surface, which is responsible for Chandra's unmatched
resolution: between 80% and 95% of the incoming X-ray energy is focused
into a one-arcsecond
circle. However, the thickness of the substrate limits the proportion
of the aperture which is filled, leading to the low collecting area
compared to XMM-Newton.
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 most
distant 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 μrad), Chandra possesses a resolution over 1000 times better than that of the first orbiting X-ray telescope.
CXO uses mechanical gyroscopes, which are sensors that help determine what direction the telescope is pointed. Other navigation and orientation systems on board CXO include an aspect camera, Earth and Sun sensors, and reaction wheels. It also has two sets of thrusters, one for movement and another for offloading momentum.
Instruments
The Science Instrument Module (SIM) holds the two focal plane instruments, the Advanced CCD Imaging Spectrometer (ACIS) and the High Resolution Camera (HRC), moving whichever is called for into position during an observation.
ACIS consists of 10 CCD chips and provides images as well as spectral information of the object observed. It operates in the photon energy range of 0.2–10 keV. The HRC has two micro-channel plate components and images over the range of 0.1–10 keV. It also has a time resolution of 16 microseconds. Both of these instruments can be used on their own or in conjunction with one of the observatory's two transmission gratings.
