M I S S I O N A S P E C T S OF G R A S P E S A ' S P R O S P E C T I V E

G A M M A - R A Y A S T R O N O M Y S A T E L L I T E

C. W I N K L E R , G. T O M A S C H E K

ESA/ESTEC, Noordwijk, The Netherlands

G. F. B I G N A M I , G. VILLA

1FCTR-CNR, Milano, Italy

A. J. D E A N , D. R A M S D E N

Phys. Lab., Univ. Southampton, U.K.

PH. D U R O U C H O U X

CEN-Saclay, Gif-sur- Yvette, France

K. H U R L E Y , G. V E D R E N N E

CESR, Toulouse, France

N. L U N D

DSRI, Lyngby, Denmark

B. M c B R E E N

University College, Dublin, lreland

V. S C H O N F E L D E R

MPI extraterr. Physik, Garching, F.R.G.

and

B. N. S W A N E N B U R G

ROL, Leiden, The Netherlands

(Received 1 June, 1988)

Abstract. Mission aspects of the project GRASP (Gamma-Ray Astronomy with Spectroscopy and Position- ing) are described as documented in the ESA assessment study at the end of 1986. The goals of this study addressed scientific objectives, technical solutions and feasibility of the mission. Two accommodation studies accompanied the assessment study. Their results show that GRASP can be accommodated on two existing space platforms, EURECA-B and ROBUS, respectively.

1. Overview and Background

The investigation of mission aspects of the GRASP assessment study, i.e., analysis, definition, trade-off and selection of spacecraft and orbit were determined by optimisa- tion of scientific output in connection with realistic and, last but not least, cost-effective missions. For these reasons, the assessment study was accompanied by two industrial studies with the aim to accommodate GRASP onto different space platforms, EURECA-B and ROBUS.

Space Science Reviews 49 (1988) 173-177. �9 1988 by Kluwer Academic Publishers.

174 C. WINKLER ET AL.

EURECA-B is a retrievable platform, based on a launch with the NASA Shuttle and will be derived from the platform EURECA-A which is devoted to microgravity research. The carrier will have enhanced capabilities with the aim to make it more suitable for astrophysics from space. Major enhancements are data rate and storage, mission duration and pointing. EURECA-A is in phase C/D, EURECA-B in phase B.

ROBUS is a platform configured for an ARIANE launch. This platform is derived from and based on the German X-ray satellite ROSAT. ROBUS is ready to start phase B, C/D.

The GRASP/EURECA accommodation study lasted 6 months ending February 1987, the GRASP/ROBUS study lasted 6 weeks, ending December 1986.

2. GRASP Payload Requirements

The principal payload requirements are: Accommodation of GRASP consisting of detector, mask, electronics, cooler, and instrument star camera.

Total mass: 1000 kg Distance mask--detector (focal length): Diameter of mask: Targetting accuracy: Pointing stability: Attitude reconstitution: Mission duration: Coverage of celestial sphere: Minimise effects of: Data rate (continuously): Event timing (end-to-end):

Total power: 285-305 W, 350-600 cm, 130-140 cm, 30 arc min, 1 arc min hr- 1, 5 a r C s e c ,

up to 2 years, within one year, radiation belt and SAA, 60-70 kbps, 0.1 ms.

3. GRASP/EURECA-B Study Results

The investigation was focused on two items: feasibility of GRASP accommodation within the EURECA-B system performance requirements and 'fine-tuning' of EURECA-B design specifications according to GRASP requirements.

Several mechanical configurations are feasible: a 'turntable' deploying the mask (410 cm focal length) after EURECA-B has been released from the Shuttle (Figure 1); alternative deployment of the mask using the Remote Manipulator System (RMS) of the Shuttle; and mask and detector fixed on the payload deck (focal length 290 cm). The latter configuration is not preferred by the Science Team due to the short focal length. However, all configurations imply an asymmetric, time-dependent carrier- induced background on the detector plane. Therefore, the use of a rotating mask is of crucial importance. The original proposal intended to deploy the mask using a zoom mechanism up to 6 m vertically with respect to the payload deck. This configuration is not feasible within the scope of the study. The main reasons are resonances with the

MISSION ASPECTS OF GRASP 175

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solar array structure; required stiffening of booms and wires and an additional mounting plate both leading to critical mass budgets; orbit decay characteristics and safety requirements (jettison mechanism).

Analysis of the EURECA-B subsystem capabilities shows that GRASP requirements of mass, power, event timing, and GSE hard- and software can be fulfilled without

176 c. WINKLER ET AL.

modifications. The areas of pointing, data handling, data storage and communication need further study and major development effort on the carrier. The thermal control subsystem needs close cooperation with the experimenters because of the active cooling of the Germanium detectors using 4 Stirling cycle coolers.

GRASP/EURECA-B will be launched by the Shuttle into a low Earth circular orbit. The parameters are: height: ~ 500 km, inclination: 28 ~. 5. Data transmission is foreseen using two or three ground stations or a data relay satellite and one ground station. The mission duration will be between 12 months and 21 months depending on Shuttle

Fig. 2. GRASP/ROBUS configurations and ARIANE 4 launch accommodations (courtesy: DORNIER).

MISSION ASPECTS OF GRASP 177

availability for retrieval of the platform. The enhanced EURECA-B design allows pointing of the telescope in any direction of the sky (including the Sun).

In conclusion EURECA-B is suited to accommodate GRASP by using an available platform (EURECA-A) providing in-orbit check-out procedures in Shuttle vicinity with recovery option and near-real time link in case that a relay satellite is available.

4. GRASP/ROBUS Study Results

The primary objectives of this study were: reuse of the ROBUS carrier which is based on the existing ROSAT design together with its supporting hard- and software test facilities; provide an European autonomous mission, based on ARIANE 4 double launch; aim at minimum development and cost programme; use ESA ground facilities.

Two configurations are feasible: 350 cm and 470 cm focal length (Figure 2). No mask deployment mechanism is required, the baseline version (350 cm) fits in either the lower or upper ARIANE half Spelda (Figure 2) providing good flexibility for the passenger spacecraft. The GRASP/ROBUS spacecraft consists of two elements (platform and telescope) with well defined interfaces and responsibilities.

Analysis of the ROBUS subsystem capabilities shows that the GRASP requirements of mass, pointing, event timing, data handling and storage, GSE hard- and software can be fulfilled without major modifications. Sufficient power for the payload during Earth eclipse will be provided by increasing the area of the solar arrays by 5 ~o. Capabilities of up- and downlink communication are sufficient, compatibility with ground stations needs further study. Like EURECA thermal control can meet GRASP requirements, further study requires joint effort with experimenters.

GRASP/ROBUS will be launched by ARIANE 4 into a low Earth circular orbit (height: 500 km, inclination: 5 .o 2). Ground contact is provided every orbit thus requiring the use of only one ground station. The mission duration is up to 3.5 years (depending on orbit decay characteristics, solar activity, etc.). The telescope can be pointed in any direction perpendicular to the normal vector of the solar array which is to be pointed + 15 ~ to Sun direction for power reasons. The actual accessible region for observations is, therefore, a zonal region on the sphere with 30 ~ width oriented perpendicular to the Sun vector. Any target on the sphere (except the Sun) is accessible within six months.

In conclusion, GRASP can be accommodated by ROBUS without major modifi- cations of the carrier because of the ideal similarity between GRASP and the X-ray satellite ROSAT. Major system, subsystem and GSE components are reusable. No development risk (i.e., no mask deployment) exists for the effective telescope con- figuration. Furthermore, the platform mass (811 kg) is very low and provides a favoura- ble gamma-ray background. GRASP on ROBUS is an European autonomous mission with flight opportunity in the early 90's assuming an ARIANE 4 double launch.

Reference

GRASP Assessment Study: 1986, ESA SCI(86) 4, December.