Smart 1: the Star Trek propulsion system

On 18 August 2003, the European Space Agency (ESA) announced that “Europe was to send a spacecraft to the moon”.

The unmanned craft would be powered by a revolutionary engine which has been called the Star Trek propulsion system. ESA’s Smart 1 spacecraft forms part of its “Small Missions for Advanced Research in Technology” (SMART) project, the purpose of which is to test new technologies that will eventually be used on bigger projects.

The European Space Agency (ESA) Smart 1 spacecraft was launched on 4 September 2003 from French Guiana. It carried a British‑built sensor to analyse the lunar surface and scientists hope it will answer questions about how the moon was created. The mission could also confirm the suspected existence of water beneath the lunar surface.

The key to the mission is a new development known as an ion engine. This “Star Trek propulsion system” is much smaller than other spacecraft engines and uses solar panels to charge electrically heavy gas atoms, which propel the craft forward as they are pushed away at high speed.

The ion engine begins very slowly, its thrust barely as strong as the force a postcard would produce as it falls through the air. But over long periods of time it can generate much more power and produce high speeds.

Scientists hope it could one day allow manned missions to faraway stars. Guiseppe Racca, the Smart 1 project manager at ESA, said:

“This engine opens up a whole new era of exploration.”

The one‑square‑metre craft will take 18 months to reach the moon and will then swoop to within 300km of the lunar surface, using its array of sensors and cameras to analyse the lunar surface.

Scientists hope the mission will finally end a row over where the moon came from. Analysing the lunar surface should allow them to tell if the moon is, as they suspect, the remnant of a massive collision between a young Earth and another planet.

If this theory is correct, the moon should contain less iron than Earth, something Smart 1’s D‑CIXS sensor developed at the Rutherford Appleton Laboratory in Hampshire can spot.

Bernard Foing, a scientist on the project at ESA, said:

“We’ll be able to make the first comprehensive inventory of chemical elements in the lunar surface. We’ll also carry a multi‑colour camera, so we will get some new views of the moon.

“As the moon is effectively the daughter of the Earth, we should also get some indications of the early conditions here.”

While conventional rocket engines use vast amounts of fuel, and can only run for short periods of time, ion engines use very little propellant. NASA has been running an experimental ion engine continuously for five years.

Although NASA has already launched a space probe using ion engines, the ESA project will test several advances in technology, and also be far more manoeuvrable than NASA’s craft.

Smart 1 should gradually accelerate from 0 to 70,000 mph. At its slowest, the craft travels at 0.2mm per second – slower than a snail – but over several years this increases to speeds of up to 70,000 mph.

The Smart 1 craft weighs about 367kg, less than a small family car. It is no more than a metre square at launch (the size of a washing machine) but extends to the length of three delivery vans. The initial push created by the ion engine feels no more powerful than having a postcard dropped onto your hand. Smart 1 will get to within 300km of the moon’s surface, far closer than previous orbiting probes.

Smart 1 was launched by an ESA Ariane 5 booster and was the smallest part of the payload of the Ariane 5 V162 mission. The main payload was INSAT‑3E, India’s largest telecom satellite to date, and e‑Bird, the first of Eutelsat’s craft, purpose built for high‑speed, two‑way, Internet access.

Two minutes after being released, Smart 1’s on‑board computer was activated and 21 minutes later its 14‑metre solar generators unfolded over a lengthy nine minutes. An hour later, ground controllers at ESA’s Satellite Operations Centre in Darmstadt got their hands on the baby. The new‑technology solar drive motor is due to function for the first time on 30 September, the lunar journey itself taking between 15 and 18 months. After so much interest in the launch, ESA hoped that public interest continued during the journey as “we” all ride up to the moon.

On 6 January 2004 the spacecraft reached its 176th orbit with all functions performing nominally. It had achieved its first mission target: to exit the most dangerous part of the radiation belts. The pericentre altitude (the closest distance of the spacecraft from the centre of the Earth) reached the prelaunch target of 20,000km on 7 January 2004.

Between 23 December 2003 and 2 January 2004, the thruster fired continuously for a record duration of more than 240 hours. Later in the week Smart 1 changed from a continuous thrust strategy to a more orbitally efficient thrust arcing.

By 6 January, the total cumulated thrust was more than 1,500 hours. It had consumed 24 kg of Xenon which provided a velocity increment of about 1070 ms‑1 (equivalent to 3,850km per hour, 2,406.25 mph). The electric propulsion engine’s performance, periodically monitored by means of the telemetry data transmitted by the spacecraft and by radio‑tracking by the ground stations, showed a small overperformance in thrust varying from 1.1 per cent to 1.5 per cent over the previous week.

At first there was a little degradation of the electrical power produced by the solar arrays, however this ceased and the power available has remained virtually constant since November 2003. The communication, data handling, on‑board software and thermal subsystems all performed well.

By 23 February 2004 Smart 1 had already reached its top speed and slowed down. The milestone occurred during the first orbit, about 10 hours after launch, when the craft hit about 22,400 mph (10 kilometres per second). After that it fluctuated dramatically between 3,800 and 19,900 mph (1.7 and 8.9 kilometres per second) depending on its distance from Earth.

As with any object in an elliptical, off‑centre orbit, Smart 1 travels fastest when it is close to Earth and slower on the outer reaches of its path.

The craft will spiral outward in ever‑widening ellipses around Earth for another two months. Ion thrust will then carry it to a special spot in space, about 37,300 miles (60,000 kilometres) from the moon, called Lagrangian point L1. Lagrangian points are locations where gravity from two objects balances out. A craft at a Lagrangian point is free to go either way with very little thrust.

“It is kind of a gateway for the entrance into moon orbit, almost free of charge,” Racca said.

Smart 1 will enter lunar orbit in March 2005, when it will look for clues to the moon’s origin and try to confirm that there is frozen water hidden at the lunar poles.