January 1982:the Asterex mission

Europe's most consistently successful steps in space to date have been in the field of space science. Before the European Space Agency was formed in 1975, the European Space Research Organisation (ESRO, formed in 1964) brought together the scientists and industrial teams to design, build and use an impressive variety of Earth-orbiting science satellites. Twelve have been launched to date, covering studies of the ionosphere,Earth's geomagnetic field, Sun-Earth relations as a whole, and X-ray, Gamma-Ray and Ultraviolet astronomy.

Since the establishment of ESA much attention has (rightly) gone to applications projects - such as the Meteosat weather satellite and the Orbital Test Satellite communications satellite - but the science programme remains near the heart of the European space effort. Although now accounting for only some 15% of ESA’s budget it is the only activity in which all Members are obliged to participate. New projects are selected by the Science Programme Committee. Last year Hipparcos, an astrometry satellite (to measure the position and motion of 100,000 selected stars), was selected from a list of five candidates.

Four proposals were the subjects for assessment studies in the first half of 1981: Magellan (an astronomical satellite for ultraviolet spectrography), Kepler (a Mars geophysical orbiter for studying the Martian atmosphere, ionosphere and magnetic fields), Disco (Dual Spectral Irradiance and Solar Constant Orbiter, a solar observatory) and Asterex (an asteroid probe). Magellan and Kepler will now undergo a Phase A study and be put forward for selection in late 1982/early 1983, and Disco will be studied further. Asterex, even though it has lost out to the other proposals,will surely be flown someday, albeit in an altered form. The Americans have always been interested in their own version but it fell by the wayside when development work on electric thrusters was cancelled.

The Asteroids

Asterex would take a small unmanned probe on a reconnaissance mission to fly-by three or more asteroids. The asteroids are the minor planets which circle the Sun, most of them in a belt between the orbits of Mars and Jupiter. They mark the divide between the inner solid planets (Mercury,Venus, Earth and Mars) and the outer gaseous giants (Jupiter,Saturn, Uranus and Neptune). First discovered in 1801, some 2000 of these small rocky bodies have been discovered to date;it is thought there may be between 40,000 and 100,000 in total. They vary enormously in size and shape - only ten have diameters greater than 100 miles and many hundreds are only a few miles across. It is thought that if all of the asteroids were brought together they would form a body one-thousandth the size of Earth. Some asteroids have orbits highly inclined to the plane of the ecliptic (the plane in which most bodies of the Solar System orbit the Sun); some have very eccentric orbits - to quote an extreme, that of Icarus brings it well inside the orbit of Mercury at perihelion (closest approach to the Sun) but out to the main Asteroid Belt at aphelion (furthest distance from the Sun).

The main Belt forms a doughnut-shaped region some 100 million miles wide, with the asteroids in the centre orbiting the Sun at approximately 38,000 miles per hour. One should not picture the asteroids as “filling” the Belt: they are so spread out as to rarely come within half a million miles of each other. The Pioneer 10 and 11 and Voyager 1 and 2 spacecraft on their way to Jupiter and Saturn passed through the Belt unscathed. Asteroids have been mentioned as potential sources of raw materials for future space utilisation.

We know very little about the asteroids - little more than outlined above. Despite the great progress in Solar System exploration in the last two decades, they remain just sources of light. In the last ten years the study of asteroids has progressed from that of simple optical observation to include studies of their spectra. However, Earth-based study has nearly reached its limits: with such tiny and distant bodies there is little more to be discovered from Earth.

An asteroid reconnaisance mission will provide us with our first close view, our first chance to see what they look like, to find out more than just their orbital parameters or the hint we get from spectroscopy as to their composition. To date there have been surprises at many stages of our exploration of the Solar System - the giant Martian rift valleys for instance, or the rings of Jupiter and the active volcanos on its moon Io. Who knows what we might see in the Asteroid Belt?

Scientific Interest

The most important properties of asteroids to be studied are:

1. cratering: the mechanics of high velocity impact cratering are not well understood. Both gravity effects and the mechanical characteristics of the target material seem to have a dominant influence on crater form and structure. Comparative study of cratering on different asteroids will provide a unique means of testing the relative importance of these two effects (e.g. compare craters on asteroids of similarsurface gravity but different surface properties).

2. surface geology: any lava flows, groove patterns or unusual features? Variations in texture or colour?

3. mineralogy: surface mineralogy can be obtained by studying the spectra of light reflectances on asteroids at certain wavelengths. Knowledge of minerals will greatly help our understanding of the relationship between asteroids and meteorites.

4. shape, size and spin.

Asterex’s Scientific Instruments

Asterex has been kept as simple as possible and designed to carry three instruments:

1. an imaging camera (for optical images with a resolution of 30 metres at 1000 km distance)

2. an infrared spectrometer (to provide reflection spectra of each asteroid)

3. a radar altimeter (to measure distance between spacecraft and surface, and velocity of spacecraft relative to the asteroid).

Spacecraft design

Asterex weighs approximately 750 kg (1653 lb) and is 3-axis stabilised with a gyro-controlled platform with two degrees of freedom, carrying the scientific instruments and sensors. Power is generated by a deployable 2-wing solar array,together with a battery of limited capacity. A bi-liquid propellant system provides the required velocity increments,while a cold gas system provides attitude control using information supplied by on-board Sun sensors, a star-tracker and gyros. The craft is designed for a lifetime of three years, though the actual encounters will last only a little over one hour each. With radio signals taking up to 75 minutes for each roundtrip at those distances, actions during encounter will have to be fully automatic, findings being read back to Earth afterwards.

Flight Plan

Launch is by Ariane 4 (an uprated version of ESA’s present Ariane 1 launcher) into a parking orbit around the Earth. After vehicle checks and navigational alignment a fouth-stage engine fires the probe out of Earth orbit into a ballistic trajectory that brings it to the Asteroid Belt after some eight months of flight.

Much work remains to be done on plotting trajectories to pass as many asteroids of interest as possible, and also to ensure that there are options to cover the several years when launch might actually occur. In an example flight plan, launch is on 15 April 1987, with first encounter - with the asteroid Mtskhe - on 12 December. Lamber is flown past nine months afterwards (13 September 1988), and Ceres - the first asteroid to be discovered - a further year on (16 September 1989). After fourth encounter - with asteroid number 1974 VA on 23 February 1990 - Asterex returns to the vicinity of Earth’s orbit round the Sun at the start of its second circuit of the Sun. The fifth and final encounter occurs with the asteroid 1930 OL on 25 November 1990, over three years and seven months after launch.