Probe Headed for Mercury Fires Most Powerful Ion Drive Ever Built

The BepiColombo Mercury probe is a joint mission between the European Space Agency (ESA) and Japanese Aerospace Exploration Agency (JAXA). The probe is intended to study Mercury’s magnetic field, magnetosphere, interior, and surface structures. It consists of two vehicles — the Mercury Planetary Orbiter (MPO) and the Mercury Magnetosphere Orbiter (MMO). No word yet on how many simultaneous logins the MMO will support, or whether it will support PvP. Repeated complaints to the ESA about this missing information have resulted in our emails being blocked. (This did not actually happen -Ed). It also packs the most powerful set of ion engines we’ve ever built into a spacecraft. This week, scientists successfully tested those thrusters in space for the first time, in preparation for a mid-December burn that will put the spacecraft on its Mercury trajectory.

At first glance, the actual performance of “the most powerful ion drive ever built” may seem modest. The Mercury Transfer Module (aka the engine and support framework for the first two probes) uses four QinetiQ T6 ion thrusters capable of providing a maximum combined thrust of 290mN. One Newton is the amount of force required to give a 1kg mass an acceleration of one meter per second per second. 290mN attached to the bottom of a spacecraft may not sound like much, but this is the genius of ion drive propulsion. Unlike chemical rockets, which provide vastly more thrust over much smaller periods of time, ion thrusters provide a small amount of thrust for a very long period of time. They’re incapable of functioning in atmospheres or of lifting a spacecraft off the ground, but once in space they can provide sustained thrust with a fraction of the fuel required for a chemical rocket.

“Electric propulsion technology is very novel and extremely delicate,” explains Elsa Montagnon, Spacecraft Operations Manager for BepiColombo. “This means BepiColombo’s four thrusters had to be thoroughly checked following the launch, by slowly turning each on, one by one, and closely monitoring their functioning and effect on the spacecraft.”

The video below shows the orbital insertion pattern and gravity assists, including the six flybys of Mercury required to properly brake the spacecraft for orbital insertion.

Either approach can work to achieve Mercury orbit, but Mercury is actually one of the more difficult planets in the solar system to achieve a stable orbit around in the first place. Mercury is moving around the sun at an orbital velocity of 47.87km/s, compared with Earth’s 29.78km/s. Neptune, for the curious, moves at 5.43km/s, and the planets’ orbital velocity decreases as the distance from the sun increases. Much as Douglas Adams once wrote that the secret to flying was “to throw yourself at the ground and miss,” the secret to achieving orbit around Mercury is to throw yourself at the Sun in just the right fashion. Miss it properly, and you wind up in Mercury orbit. Miss it improperly, and your probe will find itself in an unprecedented position to make a very different sort of observation for a much smaller period of time. Whether one uses ion thrusters or chemical rockets, it takes years to properly insert into Mercury orbit.

BepiColombo will expand on the work done by NASA’s Messenger craft and perform its own detailed analysis of various aspects of Mercury’s current structure and future evolution. The ESA’s landing page contains an extensive discussion and comparison between the older NASA mission and BepiColombo’s planned scientific endeavors.

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