The ESA-JAXA BepiColombo mission is on its way to Mercury, but instead of coasting to the smallest planet in the Solar System, it will be helped along by four state-of-the-art ion thrusters developed by an industrial consortium led by British company QinetiQ. The new solar-electric propulsion system, in conjunction with a series of nine planetary flybys, will allow the unmanned spacecraft to reach its destination against the pull of the Sun's gravity.

After a successful launch from the European Spaceport in Kourou, French Guiana, the BepiColombo mission to Mercury is currently undergoing system checks before deploying its instruments and going to hibernation for the long five-year journey. However, it won't be a quiet sleep.

Unlike traveling to the outer planets, getting to Mercury poses its own peculiar problems. To reach Mars, for example, a spacecraft needs rockets to boost its velocity, so it can go into an orbit farther from the Sun. It's a bit like pushing a wagon uphill. As you do so, you're actually pumping energy into the wagon as it is moved away from the center of the Earth. But in going to one of the inner planets, you're hurtling downhill and hoping there's something soft to crash into.

This suggests that sending a probe to one of the inner planets should be as easy as dropping a stone down a well, but it turns out to be anything but. True, a relatively small rocket could push a spacecraft toward Mercury, but as it traveled, it would accelerate, shoot past its target and slingshot around the Sun going much faster than before.

Therefore, to reach Mercury, BepiColombo has to lose velocity. This could be done using a rocket, but the amount of energy needed to send BepiColombo to Mercury, despite the much shorter distance, is equivalent to that of sending it to Pluto. Therefore, to make the necessary change in velocity, the probe is on a trajectory that will cause it to slingshot past the Earth, Venus, and Mercury a total of nine times. However, instead of speeding it up, these will be angled to slow the spacecraft down.

Unfortunately, even these slingshot maneuvers aren't enough to send BepiColombo to Mercury in a reasonably short time. So, the Transfer Module that carries the ESA and JAXA orbiters is equipped with a Solar Electric Propulsion System (SEPS) made up of four QinetiQ T6 Gridded Ion Thrusters – the first ion engines installed on an inner planet exploration mission.

Based on the T5 ion thruster on ESA's Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite that orbited the Earth for four years, the T6s are 22 cm (8.7 in) diameter, 4.5 kW Kaufman-type ion thrusters that use xenon gas as a propellant. Each thruster has an electrically-charged grid powered by a solar panel that ionizes and accelerates the xenon atoms until they shoot out at a speed of over 50 km/s (112,000 mph).

The thrust isn't much – only about 145 mN – but each has a specific impulse of around 4,000 seconds. Specific impulse is a measure of a rocket's efficiency and what these numbers mean is that though the thrust of the T6 is about that exerted by the weight of a sheet of writing paper, the ion engine can fire for weeks or even months at a time. The result is that tiny thrust adds up to a giant change in velocity.

This not only means that BepiColombo can reach Mercury, but that the launch vehicle could be smaller, the spacecraft lighter (because the propellant is up to 20 times less massive), and the craft can be controlled with much greater precision. In addition, the T6 is extremely robust to deal with launch pressures and vibrations and can handle the temperatures extremes of deep space.

According to QinetiQ, only two of the thrusters are needed, but two more are carried as spares. The SEPS also includes the Solar Electric Propulsion Harness (SEPH), the Solar Electric Propulsion Pipework (SEPP), Power Processing Units (PPU), and the Flow Control Units (FCU).

"It's a source of great pride that QinetiQ is playing such a pivotal role in this voyage to investigate the secrets of Mercury," says Peter Randall, Systems Engineer Electric Propulsion, from QinetiQ. "The use of solar electric propulsion has provided ESA with an extremely efficient and robust engine system, and our rigorous testing of the T6 ion thrusters we've delivered will ensure it achieves its mission objectives. QinetiQ has more than 50 years' experience of researching and testing electric propulsion thrusters – and this couldn't have resulted in a more thrilling ESA cornerstone science project."

Source: QinetiQ

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