The Advanced Satellite for Cosmology and Astrophysics
(ASCA, formerly named ASTRO-D) was the fourth
cosmic X-ray astronomy mission by JAXA, and
the second for which the United States provided
part of the scientific payload.
The satellite was successfully launched on
20 February 1993.
The first eight months of the ASCA mission
were devoted to performance verification.
Having established the quality of performance
of all ASCA's instruments, the spacecraft
provided science observations for the remainder
of the mission.
In this phase the observing program was open
to astronomers based at Japanese and U.S.
institutions, as well as those located in
member states of the European Space Agency.
== X-ray astronomy mission ==
ASCA was the first X-ray astronomy mission
to combine imaging capability with a broad
pass band, good spectral resolution, and a
large effective area.
The mission also was the first satellite to
use CCDs for X-ray astronomy.
With these properties, the primary scientific
purpose of ASCA is the X-ray spectroscopy
of astrophysical plasmas, especially the analysis
of discrete features such as emission lines
and absorption edges.
ASCA carried four large-area X-ray telescopes.
At the focus of two of the telescopes is a
gas imaging spectrometer (GIS), while a solid-state
imaging spectrometer (SIS) is at the focus
of the other two.
The GIS is a gas-imaging scintillation proportional
counter and is based on the GSPC that flew
on the second Japanese X-ray astronomy mission
TENMA.
The two identical charge-coupled device (CCD)
cameras were provided for the two SISs by
a hardware team from MIT, Osaka University
and ISAS.
== Significant contributions ==
The ASCA was launched by ISAS (Institute of
Space and Astronautical Sciences), Japan.
The sensitivity of ASCA's instruments allowed
for the first detailed, broad-band spectra
of distant quasars to be derived.
In addition, ASCA's suite of instruments provided
the best opportunity at the time for identifying
the sources whose combined emission makes
up the cosmic X-ray background.It performed
over 3000 observations, and produced over
1000 publications in refereed journals so
far.
The ASCA archive contains significant amounts
of data for future analyses.
Furthermore, the mission is termed highly
successful when reflecting on what scientists
in many counties have accomplished using ASCA
data up to this time.
The U.S. contributed significantly to ASCA's
scientific payloads.
In return, 40% of ASCA observing time was
made available to U.S. scientists.
(ISAS also opened up 10% of the time to ESA
scientists as a good-will gesture.)
In addition, all ASCA data enter the public
domain after a suitable period (1 year for
U.S. data, 18 months for Japanese data) and
become available to scientists worldwide.
The design of ASCA was optimized for X-ray
spectroscopy; thus it complimented ROSAT (optimized
for X-ray imaging) and RXTE (optimized for
timing studies).
Finally, ASCA results cover almost the entire
range of objects, from nearby stars to the
most distant objects in the universe.
== Mission end ==
The mission operated successfully for over
7 years until attitude control was lost on
14 July 2000 during a geomagnetic storm, after
which no scientific observations were performed.
ASCA reentered the atmosphere on 2 March 2001
after more than 8 years in orbit.
The primary responsibility of the U.S. ASCA
GOF was to enable U.S. astronomers to make
the best use of the ASCA mission, in close
collaboration with the Japanese ASCA team
