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.