On 3 September, The European Space Agency (ESA) reported on a special kind of space-wreck. It has found the exact location where SMART-1 crashed into the Moon. ESA launched its SMART-1 mission on 27 September 2003. The satellite reached its lunar orbit on 15 November 2004. It was the first of a new kind of ESA mission: Small Missions for Advanced Research in Technology (SMART).

Artist’s view of the SMART-1 spacecraft on its way to the Moon

Carrying a veritable arsenal of miniaturised instruments, it was the first European spacecraft to travel to, and orbit the moon. One of its missions was to investigate the theory that the Moon was formed as a result of a smaller planet colliding with Earth, “4.5 thousand million years” ago. To that end, it carried six experiments, including three remote sensing instruments to be used during the mission’s nominal six months in orbit. It was hoped that the instruments would contribute to key scientific questions. Particularly theories about the origins and evolution of the Moon. SMART-1 was the first craft to perform a comprehensive inventory of key chemical elements in the surface of the Moon, specifically searching for cold traps at the lunar poles and mapping potential lunar resources.

True to its mission name, it was also a testbed for new technology, being the second spacecraft ever to use ion propulsion as its primary propulsion system. It was a demonstration of ESA’s new electrical ion propulsion system. Taken into Standard Geostationary Transfert Orbit (GTO), SMART-1 then made its way to lunar orbit powered by its new experimental propulsion.

Artist’s impression of SMART-1 ion engine

Unable to compete with the ‘whoosh’ of a chemical rocket, electrical ion engines none the less offer several advantages. They are much more fuel efficient, for starters. They deliver about ten times as much thrust per kilo of propellant when compared to conventional rocket engines. What they lack in immediacy, they make up for in longevity. Using electricity from the satellite’s solar panels, they can operate for months, even years. As ESA puts it in ‘the magic of ion engines’:

for as long as the sun shines and the small supply of propellant lasts.

So how does an electrical ion engine work? Simply put: by running an electric current across a magnetic field. This creates an electric field, sideways to the current. This, in turn, is used to accelerate the charged atoms (also known as ‘ions’; hence the name) of the propellant. In the case of SMART-1, xenon.

Xenon is used because it is chemically inert, and because it is a gassy element with atoms about 131 times heavier than hydrogen atoms. When running the electric current produced by the solar panels (1350 watts) across the xenon’s magnetic field, a thurst of 0.07 Newton is generated. That is about what a postcard would weigh. This seems like nothing – but in outer space, it is enough to launch SMART-1 out of the Solar System. In theory, at least.

Though incredibly gentle, the lack of friction in space, and the fact that this thrust can be sustained for months, means that this postcard of power was enough to bring SMART-1 to the moon, accelerating it at 0.2 millimeters per second. Fighting against Earth’s gravitational field, it put itself in orbit around the moon. Eventually, though frugal, SMART-1 ran out of fuel and was inextricably attracted by the Moon. So on 3 September 2006, it crashed, after successfully fulfilling its mission and remaining active beyond expectations. For more information on electrical ion propulsion, check out this link, in which one of ESA’s engineers explains the workings, challenges and advantages of this ‘electrifying’ type of propulsion.

After the crash, SMART-1 was lost for over a decade. Despite observing a bright flash and calculating the position where SMART-1 ought to have crashed, there was no confirmation of the crash site for over a decade. This changed last year, when ESA was finally able to procure images of SMART-1’s crash site:

SMART-1 crash site, as photographed by NASA’s Lunar Reconnaissance Orbiter (LRO)

Rest in space.