Mission Overview

--by Yasuo Tanaka, Hajime Inoue (ISAS) & Steve Holt (GSFC)

Overview

ASCA (originally Astro-D) is the fourth in the series of X-ray astronomy missions developed at ISAS (the Institute for Space and Aeronautical Sciences of the Japanese Ministry of Education). It was launched on February 20, 1993, and put into an approximately circular orbit with a perigee and apogee of 520 km and 620 km, respectively. Astro-D was renamed ASCA. The Japanese characters for ASCA literally mean a "flying bird", but, more appropriately, it is the name of an ancient era of Japan when the country was modernized and culture flourished. ASCA is also an acronym for Advanced Satellite for Cosmology and Astrophysics. A schematic view of ASCA in orbit is shown in Figure 1. The spacecraft weighs 417 kg, and is 4.7 meters long along the telescope axis and 3.5 meters wide across the solar paddles.

Each of Japan's four missions has been more ambitious than its predecessor. Hakucho (Astro-A) was similar in design to UHURU, the first US X-ray astronomy mission. Tenma (Astro-B) substantially increased the level of technical sophistication of the Japanese X-ray astronomy program, with the first satellite utilization of GSPCs (Gas Scintillating Proportional Counters). Ginga (Astro-C) was the first truly international mission in the series, with detectors from the United Kingdom, a monitoring instrument from the USA, and an international guest investigator program.

ASCA has a significant US involvement. The three X-ray instrument components are four conical-foil X-ray telescopes (XRT) from the Goddard Space Flight Center and Nagoya University, two X-ray CCD cameras (Solid-state Imaging Spectrometers, SIS) from MIT and ISAS and two Gas-scintillator Imaging Spectrometers (GIS) provided by the University of Tokyo, and ISAS. The overall Project Management, the spacecraft, the launch vehicle, and the mission operations are provided by ISAS. Data analysis software packages are written by GSFC, ISAS, MIT, Penn State University, Riken Laboratory and the University of Tokyo. The NASA Deep Space Network provides additional down link coverage.

The first eight months (approximately) of the ASCA mission were devoted to on-orbit calibration and performance verification, and to the only proprietary data rights to which the mission developers will be entitled. After the proprietary time for these data has expired, they will be archived with the remainder of the ASCA data and available to the science community for archival research. The first Guest Observer season is underway, and you should be preparing your proposals now for the second!

X-ray Observatory

ASCA is an X-ray observatory capable of imaging and spectroscopy with high-throughput optics. The elements composing the observatory are schematically shown in Figure 2. Four identical telescopes (XRT) of a 3.5 meter focal length are mounted on the top plate of an extendible optical bench (EOB), while the focal plane instruments, two CCD cameras (SIS) and two Imaging Gas Scintillation Proportional Counters (GIS) are placed on the spacecraft base plate. The details of XRT, SIS, and GIS are given in separate papers by P.Serlemitsos et al., G.Ricker et al., and K.Makishima et al., respectively.

Attitude Control System (ACS)

The spacecraft is three-axis stabilized. Its pointing accuracy is approximately 30 arcsec, with a stability better than 10 arcsec. The attitude control system is composed of a four-gyro (plus one back-up gyro) inertial reference system, four reaction wheels, two star trackers, three magnetic torquers for unloading angular momentum, and the control electronics. Spacecraft orientation is limited in such a way that the direction of the solar paddles is within 30 degrees from the sun. This orientation limits the observable sky to a belt within which the sun angle is between 60 degrees and 120 degrees. The spacecraft is given a bias angular momentum that always is directed to the sun so as to secure enough power when the spacecraft goes automatically into a safe-hold mode (spinning around the sun vector) if an abnormality in the attitude control occurs. The ACS can register six pointing directions sent by command. Maneuvers are performed by selecting one from these six. The slew rate is approximately 0.2 degree/sec.

Data Processing

Data from the X-ray detectors are processed by the on-board data processing unit (DP), and telemetered in real-time as well as stored in the on-board data recorder (BDR). Three different bit rate modes are available:

High bit rate: 32,768 bits/sec
Medium bit rate: 8,192 bits/sec
Low bit rate: 1,024 bits/sec

The data recorder has a capacity of 134 Mbits and can cover 68 minutes, 273 minutes, and 2,185 minutes for high, medium, and low bit rate modes, respectively. Playback of the data recorder takes 512 sec with a read out speed of 262 Kbits/sec.

Operation

ASCA operations are managed at ISAS and conducted mainly by the Japanese scientists and graduate students from various groups. The system diagram of the ASCA operation is shown in Figure 3. Several duty scientists are in charge of scheduling observations, programming and executing commands, tracking and quick look, monitoring the health of the spacecraft and instruments, first-order processing of the data, etc. Direct contact with the satellite is made 5 times from Kagoshima Station of ISAS and additional 5 to 8 times from the DSN stations of NASA (Goldstone, Madrid, and Canberra) out of 15 orbits a day. Each contact lasts about 10 min.

All the data received from ASCA are once collected at ISAS. These data are edited to the first reduction files (FRF), put together with the attitude and orbital data, and sent to the Japanese groups as well as to the U.S. ASCA Guest Observer Facility at the NASA Goddard Space Flight Center.

Initial Operations

Initial Sequence

The initial sequence of the satellite went normally on schedule. At the time of injection in to orbit, the spacecraft was spin-stabilized at a rate of about 130 rpm along the telescope axis (z-axis). The spin rate was brought down to about 8 rpm right after orbit injection by the action of the yo-yo despinner, and reduced further down to about 1 rpm over the following days. Then, the angular momentum was transfered to the y-axis (normal to the solar paddles) by turning on the four reaction wheels, and the solar paddles were deployed on February 25. After a maneuver for pointing the solar paddles to the sun, the spacecraft was fixed in the inertial frame. The star trackers (STT) were turned on, and we began verification of the ACS.

The EOB was extended and the sun shade was opened on March 2, when the complete configuration of the ASCA satellite was established. Among the focal plane detectors, the GISs were turned on first. The two high-voltage supplies were turned on on March 12 and 13, respectively, and the voltages were raised step by step. The first X-ray light on the GISs was an observation of the X-ray binary EXO 0748-676 on March 17.

The thermo-electric coolers of the SISs were switched on on March 17, which cooled the CCDs down to -62 C as designed. The aperture doors of the SISs, which had sealed the vaccuum container of the CCDs, were opened on March 26 (S-0) and March 29 (S-1). The first X-ray light on the CCDs was obtained during an observation of the black hole candidate Cyg X-1.

PV observations

In parallel to fine tuning of the ACS, we first performed observations for the calibration of the XRT, GIS, and SIS, preceeding the performance verification (PV) observations. On March 30, the ASCA Team was notified of the appearance of SN1993J in M81, and an observation was carried out on April 5. X-rays from the SN were successfully detected (IAU circular 5753) almost simultaneously with ROSAT. This detection, only ten days after outburst, was the earliest that any SN has been detected in X-rays.

The PV observations started on April 19. In the PV phase, about 150 targets were scheduled to be observed, and as of this writing, most of the observation already have been completed. The targets for the PV observations were chosen from various classes as follows:

Type	 		Targets
Stars			20
X-ray binaries	25
supernova remnants	23
galaxies		22
cluster of galaxies	23
active galactic nuclei26
X-ray background	10

Guest Observer Program

The guest observer program started in October, 1993. However, as announced, there will be an overlap with a small number of calibration and left-out PV observations until December. For the guest observer program, ASCA observing time is open to competitive proposals. The observing time, excluding a reserve of 5% as "observatory time," is apportioned as follows:

60% for Japanese investigators
15% for the U.S. investigators
25% for joint Japan/U.S. collaborative investigations

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