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BepiColombo in low Earth orbit

Mercury spacecraft mission will be first to use electrical ion thrusters

Image credit: esa

Europe is poised to send its first spacecraft to Mercury later this month, equipped with electrical ion thrusters which have not previously been used in space missions.

The spacecraft, named BepiColombo, will take about seven years to reach Mercury, which is the closest planet to the sun and can reach temperatures of 427°C in the daytime.

When it finally reaches its destination in 2025 it will deploy two probes: the Mercury Planetary Orbiter (MPO), which is built in Britain, and the Japanese Mercury Magnetospheric Orbiter (MMO), which focuses on the electromagnetic environment surrounding the planet.

BepiColombo is powered by four T6 ion engines supplied by British defence and technology company QinetiQ which are fitted to the craft’s power unit, the Mercury Transfer Module (MTM).

They work by ‘ionising’ inert xenon gas - knocking an electron off the gas atoms to give them a positive charge.

The resulting ‘plasma’ is attracted by electrostatic forces to a grid with an opposite negative charge and fired out of the thruster at 90,000mph.

BepiColombo ministack on the move

Image credit: esa

Although the force produced is tiny, it can be maintained with high efficiency over a long period of time.

During BepiColombo’s seven-year trip to Mercury its ion thrusters will be operating for 4.5 years.

Two engines will fire at any one time producing 290 millinewtons of thrust, the equivalent of about an ounce of force.

Unusually, the spacecraft will use this energy not to speed up, but to put the brakes on as it ‘falls’ towards the sun.

It will achieve this both by firing the ion thrusters in the direction of travel and by means of a complex series of fly-bys past the Earth, Venus and Mercury.

Ion drives have been used before to power Earth-orbiting satellites and deep space missions to asteroids.

However, BepiColombo will be the first interplanetary mission to rely on the technology.

The spacecraft will also feature an instrument called the Mercury Imaging X-ray Spectrometer (Mixs), which was developed and built at the University of Leicester.

The 13kg metre-long device consists of two X-ray telescopes: one designed to capture images of the surface and the other to analyse its composition.

The planet is bombarded by high-energy sub-atomic particles from the sun, causing the surface to ‘fluoresce’ and emit X-rays.

Written into the rays are the ‘fingerprints’ of atoms and analysing them provides information about what the planet is made of.

To protect Mixs from the searing 350°C heat, the instrument’s finely crafted lenses are coated in aluminium and the frame holding the optics in place is covered by a thin layer of 22 carat gold.

Mixs scientist Professor Emma Bunce said there were “interesting quirks” about Mercury that scientists still did not fully understand.

Nasa’s 2011 Messenger mission to Mercury supplied some answers but “also raised more questions”.

One of the planet’s peculiarities was its unusually large iron core, which takes up much more room than the iron hearts at the centre of Earth, Mars and Venus.

Prof Bunce said: “It suggests that something dramatic happened early in Mercury’s evolution, like a massive impact with another body that stripped away most of the mantle.

“However, one thing Messenger taught us is that there are many volatile substances on Mercury’s surface. This is a puzzle because the heat of a major impact should have burned these volatiles away.

“One of the things we’re looking at is a much more detailed study of volatile substances like sodium, potassium and chlorine.”

“We want to understand how the solar system works and how it was formed. Mercury is a place of extremes; a small planet and the planet closest to the sun. Knowing how it has survived will help us piece together a bigger picture of the solar system and our place in it.”

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