ASTRONOMY (Herschel)

SMART – 1: Ion Propulsion & a Mission to the Moon

Talk chaired by Richard Phillips

Barry Kellett

D-CXIS Project Scientist, Rutherford Appleton Laboratory

1 October 2004

Small Missions for Advanced Research in Technology are faster, smarter, better. They are limited to a cost of around $100 million – a small cost for a space programme. This limits their total weight to about 500 kg so they cannot carry large quantities of fuel for maneuvering. The first SMART mission was a trial of a new method of propulsion that used very little fuel but had several advantages over the conventional space vehicle. If it was successful the method would be applied to a probe (Bepi Colombo) to go to Mercury, a difficult planet to get to. To make the trail useful it was decided to send it to orbit the Moon to investigate its composition.

SMART -1 weighed 370 kg, of which 19 kg was the payload of scientific instruments. It cost 110 million euro (£77m, $120m) and was launched on Ariane 5 on 27 September 2003, hitching a lift with two or three much larger commercial satellites.

Ion Propulsion was chosen to propel it, based on the Hall Effect. This can make the spacecraft travel at ten times the speed it can reach with a chemical engine – eventually, but not with a high initial acceleration. SMART-1 will reach the Moon using a sixth of the weight of fuel used by a conventional chemical rocket – only 86 kg of xenon gas. This gas is introduced into the space between two concentric tubes, closed at one end, that are surrounded by coils producing a magnetic field. At the closed end of this space there is a circular anode through which the xenon is introduced. Just outside the open end of the tubes is the cathode and the current between anode and cathode ionises the gas. The magnetic field causes it to accelerate down the tube to 30 km/second and the ions leave the tube resulting in a reactive force that propels the spacecraft – but the force is only 70 milliNewtons, about 7 grams, the weight of a sheet of A4 paper! Hence, the lack of acceleration.

The Hall Effect is the result of an electric current and a magnetic field reacting and producing a force at right angles to both fields (the Right-Hand Rule applies: the thumb, first and second fingers all at right angles to each other show the directions of the two fields and the force).

The electric power, 1500 w, to charge the gas and to run instruments and equipment is entirely provided by solar cells in two arrays each 10m (30ft) long, which have to be rotated so that they always point at the Sun.

This limits the use of ion propulsion to voyages inside the orbit of Mars; outside that the amount of sunlight is inadequate, but for a trip to Mercury or asteroids it is ideal. Indeed, ion propulsion will provide a quicker trip to Mercury (5 years) than the competing chemical rocket (7 years) launched by the Americans.

After launch the craft circled the Earth in an expanding orbit as the speed built up. To save fuel the engine only fired away from the perigee of the orbit. When the orbit reached a critical distance from the Earth the Moon's gravity exceeded that of the Earth and 'captured' the craft; reaching this point took 11 months, reaching the required orbit round the Moon will take only 3 months more. The initial Moon orbit will have to be reduced by slowing the craft by firing the motor to get the craft to the required height of 300 - 4000 km above the Moon's surface. On each orbit of the Earth or the moon the solar panels have to switched twice to keep them facing the Sun.

The craft carries seven experiments and will make 10 investigations run by scientists from five countries, Czech Republic, Finland, Germany, Italy and UK plus the European Space Authority, ESA. Dr Kellett is involved with the D-CIXS Demo(-nstration) Compact Imaging X-Ray Spectrometer, the UK investigation. The Moon absorbs X-rays from the Sun and (rarely) emits a new X-ray, which can tell us about the surface of the Moon; iron, sodium, magnesium, aluminium and silicon can be detected The Apollo missions visited a limited area of the surface that turns out to be untypical - it is high in iron; most of the surface is not, and , of course, the far side of the Moon from the Earth was not visited and is very different to the near side. For one thing there is the biggest hole, the largest impact crater, in the solar system – 1300 miles across and 14 miles deep.

The D-CIXS spectrometer is tiny and effective. It is a one-pixel (!) CCD with a collimator consisting of a grid of holes 167 micron square with 1 micron walls that enable three pictures, centre, left and right to be taken simultaneously. It will scan a square area 8 degrees on a slide and measure the iron-magnesium abundance ratio. This ratio will distinguish between various possible origins of the Moon and its formation. Over many months the spectrometer will scan successive lines until the whole surface has been covered. It has already produced some interesting results from the Sun and the Earth's atmosphere, where it detected the 1% argon content.

By 16 August 2004 SMART-1 had completed 320 orbits of the Earth, the engine had been fired 263 times for a total of nearly 3000 hours. SMART-1 will be captured by the Moon between 16 and 18 November 2004 and the science operations will start about February 2005. Progress can be followed on http://sci.esa.int/smart-1 web page.

Donald Lovell