June 1982:the DISCO satellite

The January and March issues of Spaceflight carried reviews of the proposed Asterex and Kepler projects of the European Space Agency. Here, Andrew Thomson describes the proposed DISCO solar observatory.

ESA, of course, are studying other projects with a view to selecting one or more for further work. By the end of 1980, seven proposals had been received, four of which were selected by the Astronomy and Solar Working Groups for further study. These were Magellan (ultraviolet observatory), Kepler (Mars orbiter), Asterex (asteroid probe) and DISCO. The results of ESA’s own assessments of these missions vere presented to their Science Advisory Committee last June, with the result that Magellan and Kepler were selected for Phase A studies, Asterex was dropped for the time being and DISCO was recommended for further study with a revised scientific objective. DISCO was recommended for Phase A study after the reassessment was completed last November.

The Magellan, Kepler and DISCO proposals thus joined those of X-80 and ISO to go forward for possible selection at the end of this year. The 1,100 kg X-80 satellite is designed to study high-energy X-rays with five instruments over the energy range 0.5-200 keV. ISO (Infrared Space Observatory) consists of a 60 cm diameter telescope mirror cooled by liquid hydrogen and helium for making precise observations in the 1-150 pm infrared band.

Introduction

DISCO (Dual Spectral Irradiance and Solar Constant Orbiter) is designed to study:

1. The variability of the global oscillations of the Sun in visible light;

2. The variability of the total irradiance at Earth’s orbit (the "solar constant");

3. The variability of spectral irradiance in the ultraviolet, visible and infrared.

"Irradiance” is the received radiation from the Sun at the Earth's orbit (“illuminance” or "illumination” is the equivalent word for visible light). DISCO will measure both the total radiant output received across the whole electromagnetic spectrum (total irradiance). together with the radiant output in one or more specific parts of the spectrum (spectra) irradiance - DISCO has the capability to measure this in ultraviolet, visible light, and infrared). DISCO’s continuous observations over six years should allow progress to be made in two main areas:

Investigation ol the Solar Interior

Measurements of the global solar oscillations and irradiance, and their variations, will enable a concise investigation of the physical properties and dynamics of the Sun’s interior to be made. Clobal oscillations will provide information on the behaviour of the Sun from the surface down to the centre. Long-term monitoring of the total irradiance will provide essential information for understanding the dynamics of the 11-year solar cycle. With its lifetime of six years DISCO will not.be able to cover a complete cycle, but it will prove that long-term measurements are possible, leading the way to a much longer-term investigation in the future.

Relevance to Earth's Atmospheric Processes

Several authors have made claims of a connection between a variable Sun and the. Earth's climate. There are a host of ingenious ideas ranging from decreased solar radiation caused by black spots on the Sun. through cosmic ray flux changes, cosmic dust clouds, movement of the Earth’s magnetic held, or conjunctions of the planets, to solar ultraviolet effects on the ozone layer. To create some order in this chaos it is necessary to establish whether or not there is a variation in the solar radiation, and if there is. we need to find its magnitude, spectral distribution and period. DISCO is designed to do just thisPeriodic flights of balloon-, rocket- and Shuttle-borne instruments will produce frequent accurate recalibration of the spacecraft instruments.

The experiment payload and the various subsystems are housed inside a cylindrical structure 1.2 m in diameter and 0.8 m long. The spacecraft is spin stabilized with the main axis pointed to the Sun. There is a fixed (non-deployable) solar array on a circular platform of 2.4 m external diameter. Total cell area is 3 m 2,sufficient to meet the planned power require¬ ments of 265 watts (peak). Telecommunications use the stand ard S-band; in the operational phase only one ESA ground station is necessary. Data will be acquired from Villafranca, Spain or Odenwald, Germany for about 8 hours each day. Spacecraft and payload operations will be controlled from ESOC, Darmstadt.

The Payload

The model payload for DISCO includes: two pyrheliometers to measure the intensity of the Sun’s radiant energy; a far-UV spectrometer to observe radiation originating from the solar chromosphere and corona; photometers to measure spectral irradiance at several fixed wavelengths (one each for UV, visible and IR); and a high-resolution spectrometer to measure and resolve small amplitude pulsations. Most of these instruments already exist; some have already flown on rockets and balloons or will be placed aboard Spacelab. (One version of the high-resolution spectrometer has been operating on the ground, at the South Pole, and needs to be adapted to the satellite conditions).

The Orbit

DISCO will orbit near Lagrangian libration point 2 (or L2), constantly on the F,arth-Sun line, 1.5 million km from Earth. Here, the Sun is permanently visible from the spacecraft without eclipses, and the radial velocity with respect to the Sun is very low (a requirement of the high-resolution spectrometer). An orbit close to L1 has been recently flown by NASA’s International Sun-Earth Explorer 5 spacecraft.

I.aunch will be by Ariane 2 or 3, using the SYEDA dual launch facility (ample mass is available for a second passenger). After launch, DISCO will follow the “standard” Ariane procedure of injection into an intermediate geostationary transfer orbit. At perigee of this intermediate orbit, a solid boost motor inserts DISCO into a cruise orbit towards L1. To reduce outgassing contamination on the scientific instruments, the boost motor is ejected after burn out. The motor, propellant and attachment casing account for just over one-third of the total spacecraft weight at launch of approximately 900 kg. Transfer to L2 will take about 120 days, following which onboard hydrazine thrusters will be used for final orbit insertion.

Conclusions

ESA's space science projects presently include:

Exosat - X-ray observatory (1982);

Giotto - probe to Halley’s Comet (1985);

Space Telescope - ESA developing Faint Object Camera, solar arrays (1985);

Hipparchus - astrometry satellite: to measure position and motion of 100,000 selected stars (1986);

Solar Polar Mission - probe to fly over Sun’s unobserved polar regions (1986?).

Until recently many feared that following the great successes of the first decade of ESRO/ESA science satellites. European space science research would enter something of an eclipse because of constricted budgets and competition from applications projects. The projects before ESA in 1982 show that the enthusiasm and breadth of new ideas is still there.