The ion thrusters that will help get ESA's BepiColombo spacecraft to Mercury have been successfully put through their paces in space for the first time. Testing of the thrusters that make up the most powerful solar electric propulsion system ever to be launched into space began on November 20, with two of the four used to carry out its first course maneuver last Sunday.
Launched on October 20 this year, the BepiColomo mission, with its two ESA/JAXA-built orbiters, is currently at the start of the seven-year voyage that will take it a total distance of 9 billion km (5.6 billion mi) in order to cover the 240 million km (149 million mi) that separates Earth from the planet Mercury. In the process it will execute a series of slingshot maneuvers with a total of nine flybys of Earth, Venus, and Mercury itself until it matches orbit with the smallest planet in the Solar System.
However, even these tricky orbital maneuvers won't be sufficient to provide BepiColombo with the necessary change in velocity. For the added boost, the Mercury Transfer Module (MTM) carrying the Mercury Planetary Orbiter and Mercury Magnetospheric Orbiter is rigged with four state-of-the-art ion thrusters developed by an industrial consortium led by British company QinetiQ.
Called the T6, this is the most powerful ion thruster system yet flown. Because it's still something of an untried commodity, mission control made the first tentative firing of a single thruster on November 20. Satisfied with the results, three hours later the thruster was ordered to fire at full throttle for five hours, when it generated 125 mN (millinewtons).
This is a tiny amount of force, but what the T6 thrusters lack in power, they make up for in duration. Each thruster is a 22 cm (8.7 in) diameter, 4.5 kW Kaufman-type that uses xenon gas atoms as a propellant. An electrically-charged grid powered by a solar panel ionizes and accelerates these gas atoms until they shoot out at a speed of over 50 km/s (112,000 mph). The clever bit is that, unlike chemical rockets, these thrusters can fire for days or even weeks at a time, building up tremendous speeds.
As part of BepiColombo's shakedown phase, the thrusters were put through a week of rigorous testing as each was fired in turn and the results monitored by telemetry. ESA says that these test firing had to occur while the probe is still close to Earth, so the spacecraft could be continually monitored by the space agency's network of ground tracking stations. When the thrusters go fully online later this month, the antenna will be pointed in the wrong direction and the onboard computers will have autonomous control.
All four of the thrusters have now been fired, causing a minor change in trajectory, but telemetry indicates that the first two firings performed within two percent of specifications. When in full operation, the thrusters will fire for a week at a time with an eight-hour pause to allow the spacecraft to make navigation fixes. If all goes well, BepiColombo is scheduled to arrive in Mercury orbit on December 5, 2025.
"To see the thrusters working for the first time in space was an exciting moment and a big relief," says Paolo Ferri, ESA's Head of Operations. "BepiColombo's seven year trip to Mercury will include 22 ion thrust arcs – and we absolutely need healthy and well performing thrusters for this long trip. Each thruster burn arc will last for extended periods of up to two months, providing the same acceleration from less fuel compared to traditional, high-energy chemical burns that last for minutes or hours."
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