A handful of spacecraft have used ion engines to reach their destinations, but none have been as powerful as the engines on the BepiColombo spacecraft. BepiColombo is a joint mission between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA.) It was launched on October 20, 2018, and has gone through weeks of in-flight commissioning. On Sunday it turned on its powerful ion thrusters for the first time.

“We put our trust in the thrusters and they have not let us down.” – Günther Hasinger, ESA Director of Science.

BepiColombo is a three-part spacecraft. It has two orbiters, the Mercury Planet Orbiter (MPO) built by the ESA, and the Mercury Magnetospheric Orbiter (MMO) built by JAXA. The third part is the Mercury Transfer Module (MTM), built by ESA. The MTM is the propulsion part of the spacecraft and contains the spacecraft’s four ion engines.

When BepiColombo turned on its engines on Sunday, it followed more than a week of testing. Each of the thrusters were fired individually and monitored and tested meticulously. It’s the first time the ion engines were fired in space.

“Electric propulsion technology is very novel and extremely delicate.” – Elsa Montagnon, Spacecraft Operations Manager for BepiColombo.

“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 testing of the ion engines was timed precisely to coincide with a window in the spacecraft’s mission. During this window, BepiColombo remained in continuous view of antennas on Earth, allowing the ground crew to stay in communications with the spacecraft during the crucial engine testing.

This communications window will only occur once during the mission, and it was the only chance to check the ion engines out in detail. Once mid-December arrives, and the engines are fired more routinely, the position of the spacecraft means that the antennas won’t be pointing at Earth, so mission operators won’t have the same bird’s eye view of the spacecraft.

“To see the thrusters working for the first time in space was an exciting moment and a big relief.” – Paolo Ferri, ESA’s Head of Operations.

The first engine was test-fired on November 20th, and after an initial period, was put into full throttle. It maintained maximum power for 5 hours, and then the process was repeated for each of the other three engines. During this whole procedure, the performance of the ion engines was monitored in precise detail. The Flight Dynamics team determined that the spacecraft was performing within 2% of its expected value during the first two firings. Analysis of the last two firings is ongoing, but if it’s the same, BepiColombo is well on its way.

“To see the thrusters working for the first time in space was an exciting moment and a big relief. 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,” explained Paolo Ferri, ESA’s Head of Operations. “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.”

The first routine electric propulsion thrust arc will begin in mid-December, steering BepiColombo on its interplanetary trajectory and optimising its orbit ahead of its swing-by of Earth in April 2020. After that, it will fly-by Venus twice, in 2020 and 2021. Then, from 2021 to 2025, it will perform six flybys of Mercury itself. In December 2025, the two orbiters will separate from the Mercury Transfer Module and be inserted into orbit around Mercury.

The ion engines on BepiColombo are four QinetiQ T6 ion thrusters. They operate singly or in pairs, to provide a maximum combined thrust of 290 mN (millinewtons), which makes it the most powerful ion engine in space. For comparison, NASA’s Dawn spacecraft used an Nstar ion engine that produced only 92 mN.

BepiColombo’s ion engines are not just the most powerful ever used on a spacecraft, they’re also a ground-breaking design. The QinetiQ T6 Gridded Ion Thrusters are also called a Solar Electric Propulsion System (SEPS). Because it’s powered by solar energy, it’s a super efficient system, and can reach maximum velocity with minimal fuel consumption.

Visually, there’s something calming about the blue glow of ion engines, especially compared to the violent ignition of a powerful rocket like SpaceX’s Falcon Heavy. It makes sense: rockets like the Falcon Heavy are built to power heavy payloads away from Earth’s strong gravity and into space, and they burn tons of fuel to do that. They lift off on a pillar of fire and accompanying noise. But ion engines aren’t built for that. They have a different purpose.

In an ion engine, nothing is combusted. They are silent. They create thrust by using electricity to propel positive ions in their fuel, in this case, xenon. The xenon is not combusted, indeed, it can’t be combusted. But ion engines have a trick up their sleeve. Though they generate only a small amount of thrust, it’s cumulative out in space. In BepiColombo’s case, thruster burns can last for two months, and during that time they accelerate continuously.

“We put our trust in the thrusters and they have not let us down. We are now on our way to Mercury with electro-mobility,” said Günther Hasinger, ESA Director of Science. “This brings us an important step closer to unlocking the secrets of the mysterious innermost planet and ultimately, the formation of our Solar System.”

Sources:

• ESA Press Release: BepiColombo Now Firing On All Cylinders
• ESA Science: Getting to Mercury
• Wikipedia Entry: BepiColombo
• ESA: BepiColombo Operations
• QinetiQ Press Release: Pioneering QinetiQ Solar Electric Propulsion System (SEPS) to power ESA’s BepiColombo mission to Mercury
• The Conversation: Falcon Heavy: SpaceX stages an amazing launch – but what about the environmental impact?
• Wikipedia Entry: Ion Thruster