I I ASTRID I AN ATTEMPT TO MAKE THE MICROSATELLITE A USEFUL TOOL FOR SPACE SCIENCE I Sven Grahn, Anna Rathsman Science Systems Division, Swedish Space Corporation, Solna, Sweden I ABSTRACT with the "small team afproach" first used in the FREJA project (Grahn ). ASTRID was launched The ASTRID microsatellite (27 kg) was built and into orbit from the Plesetsk cosmodrome in Rus­ launched to demonstrate that "good science" can sia on 24 January 1995. ASTRID was as a pig­ I be done with a microsatellite. It has deployable gyback passenger on a Kosmos-3M rocket from solar panels to provide plenty of power, a high the Polyot Design Bureau in Omsk. The satellite downlink data rate and a flexible structure design is in a circular orbit at about 1000 km altitude and I to provide a good field-of-view for the scientific 83° inclination. ASTRID. carries an neutral parti­ sensors. The satellite was developed and launched cle imager, an electron spectrometer and two UV in 15 months at a total cost of $1.4 million and imaging photometers. The neutral particle data set has provided pioneering imagery of energetic gathered during the mission is a first in space sci­ I neutral particles in the earth's magnetisphere. ence (Norberg et. all) I 1 A new role for microsatellites? 2 The development of ASTRID Very small satellites with a total mass below 50 2.1 The scientific instruments kg, i.e. microsatellites, have been used for tech­ I nology tests and amateur radio for almost two The Neutral particle imager, PIPPI (Prelude in decades. Because of their minute size they have Planetary Particle Imaging), is ASTRID's main not generally been viewed as useful for cutting­ instrument. The operation of PIPPI in orbit was edge space science research. the first time that a dedicated instrument meas­ I ured the neutral particle flux from the ring cur­ The low power level available to the payload has rent. The instrument consists of two cameras. The been one reason behind the lukewarm interest. SSD camera use solid state detectors which re­ Lack of DC power also limits transmitter ouput solve the energy of detected particles. The MCP I and downlink data rate. Microsatellites are often camera uses a technique whereby incoming neu­ "a box of electronics" with solar panels covering trals cause charged secondary particles to be the outside of the box which makes it difficult to emitted from a graphite target. The secondaries I locate scientific sensors for the best field-of-view. are then detected by a microchannel plate (MCP). Both cameras have deflection systems which can At the Swedish Space Corporation (SSC), we reject charged particles up to an energy of 140 think that microsatellites will be common in fu­ keV. The instrument aperture plane is perpendicu­ I ture space science primarily because the lack of lar to the spin plane, all directions are thus cov­ funds will lead to long intervals between larger ered in half a spin period or approximately 1.5s. projects. Microsatellites can fill the gaps between PIPPI also serves as the data processing unit for I these "major" flight opportunities. Microminia­ EMIL and MIO (see below). turization will help make such small satellites more and more capable. SSC and the Swedish PIPPI Features SSD MCP Institute of Space Physics carried out the Camera Camera I ASTRID project to address the perceived short­ Energy range [keY) 13-140 0.1-70 Energy resolution 8 levels - comings of the microsatellite and demonstrate Sampling time [ms] 31.25 31.25 that "good science" can be achieved with such a Number of apertures 14 31 I small space vehicle. • Angular resolution· 11x9 Geometric factor rcm~ ster] 0.035 0.08 The ASTRID microsatelIite was built quickly and Bit rate rkbps] 60.5 16 at low cost at the Swedish Space Corporation Mass 3.1 kg I Power 4.0W I I I The Electron Spectrometer, EMIL (Electron Sampling time 2 ms Measurements - In-situ and Lightweight) consists Geometric factor 2x10-4 em" ster of a swept-energy toroidal electrostatic analyzer Bit rate 8 kbps I and a microchannel plate (MCP) detector. The Mass 0.3 kg O.16W instrument measures the electron distribution at Power 62.5 ms or 125 ins resolution. 2.2 Designing for "good science" I No. of angular channels 6 sectors in spin plane. Energy range 50 eV - 40 keV, resolu- Detailed technical specifications for ASTRID are tion (LlE/EkO.1 0 listed on page 9 , the main features of the satellite I Energy steps 32 or 64 are shown in Figure 1 and the dimensions of Sampling time per step 2ms ASTRID are found in Figure 2. However, the re­ Geometric factorl sector 4x10'4 cm~ ster keV/keV Bit rate 24 kbps quirements of the science instruments satellite's determines the design of ASTRID in terms of its I Mass 0.9 kg Power 0.9W shape, structure, attitude and power generation. The UV imaging photometers, MIO (Miniature The size and shape of the satellite I Imaging Optics), are mounted in the satellite spin plane. One observes Lyman alpha-emission from ASTRID weighs 27 kg. In the launch configura­ the Earth's geocorona, the other observes auroral tion the dimensions of the satellite are approx. emissions. Each photometer consists of optics 0.45 x 0.45 x 0.29 m. We picked this size and I mounted in a stainless steel tube with a ceramic mass because it is roughly half of the maximum channel electron multiplier in the opposite end. permitted size and mass of a microsatellite that the Ariane rocket accommodates on its platform I MIO-l passband (Lyman-a) 121 nm (MgF2 + Oxy- for small auxiliary payloads, ASAP. The Swedish gen gas filter + KBr) Institute of Space Physics specified that the sci­ MIO-2 passband (Oxygen)125-160 nm entific sensors needed to scan the sky, so it was (CaF2+KBr) Focal width 255mm natural for us to decide that the satellite should I Field-ot-view 1 degree spin. I Antenna for recep­ Ni-Cd battery tion on 450 MHz and transmission on 400 /""---1 UV imaging photometer 1 MHz I /'"'~ __ I Electron-spectrometer Energetic neutral "System Unit" particle imager, ASTRID's main in­ Spin-up rocket I strument I I Solar panel I Thin-film solar cells I ~_---1 Spin axis 1Separation hook 1-----1 I Figure 1 Main features of the ASTRID microsatellite I I 2 I I I To achieve a stable spin the moment-of-inertia letting the on-board computer switch it "on" for ratio (axial/transverse) needed to be larger than one minute and "off' for four minutes. In case the I 1.0. An oblate shape makes it easier to achieve computer "crashes" the default state of the switch this ratio. This was the main factor in the choice is to turn the receiver "on". If a command is re­ of the dimensions of the satellite - especially the ceived the receiver stays "on" for five minutes. I height. We kept the transverse dimensions com­ patible with the Ariane ASAP. The small size and How to provide a good view for the scientific mass of ASTRID also helped to keep launch sensors costs low it is always easier to find room for a I small piggyback payload than a large. The scientific sensors that we mounted on ASTRID were developed for other projects, so it Electrical power for the experiments and the was impossible to redesign them during the I data link ASTRID project, which lasted only seventeen months from first idea to launch. It was easier to The easiest way to obtain a high output from the design the satellite to fit the sensors. A conven­ solar arrays is to keep them perpendicUlar to the tional design of the structure with honcycomb I direction of the Sun. Four solar panels deploy equipment platforms and longerons to connect the from the base of the satellite to a position roughly platforms is very flexible and allowed us to fit the perpendicular to the spin axis. Commands from instruments and still provide the required fields­ the ground keep the spin axis pointing towards of-view. Other satellite units were relatively easy I the Sun. This arrangement provides plenty of to "move around" on these platforms to accom­ power from the solar arrays (S45 W) to supply modate the sensors. Of course some units could the experiments and the radio transmitters - the not be placed anywhere. The batteries, for exam­ I largest consumers of power. The experiments and ple, have to be located on the shadowed platform their memory unit consume up to 9.3 Wand the in order to keep them cool. S-band transmitter consumes 16 W. It was necessary to make the top and bottom plat­ I The energy balance is also a problem for a micro­ forms 0.42 x 0.35 m , i.e. not perfectly square, to satellite. To save electrical energy we do not run accommodate the main instrument, the energetic the command receiver continuously. The receiver neutral particle imager, on one side face of the I is a spare part from the FREJA project and its satellite. In this way it could have a 3600 field-of­ substantial current drain of 106 rnA is reduced by view in a plane parallel to the spin axis. I Field-of-view of sensor I I I Volume for locating sensor head of the neutral particle I imager I ISpin axis Figure 2 The location of ASTRID's main instrument.