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A complex mission to Mercury that began its epic spacevoyage six years ago has encountered critical thrust problems that controllers say could History Archivesimperil plans to study the closest planet to the sun. 

BepiColombo, a joint European and Japanese mission, is on its way to reach Mercury on Sept. 5 for the first of three crucial flybys intended to put the robotic spacecraft on the correct course around the planet next year. In October 2025, its two science probes — one operated by the European Space Agency and the other by the Japan Aerospace Exploration Agency— would separate from a module to then investigate the planet's surface and magnetic field.

But whether those science operations can still happen as planned is uncertain. During a maneuver on April 26, the electric propulsion module, which runs on solar energy, didn't provide enough power to the spacecraft's thrusters, according to ESA. About 11 days later, engineers had restored the spacecraft's thrust almost to its previous level, but still 10 percent lower. 

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"A team of experts is restlessly working on understanding the root cause of the problem and further impact on the remainder of the trajectory," Camille Bello, a spokesperson for ESA, told Mashable. 

SEE ALSO: How a NASA nuclear rocket engine could unleash the solar system Mercury is perhaps the most understudied of the rocky worlds in the solar system. Credit: NASA / JHU Applied Physics Lab / Carnegie Inst. Washington

To adjust for the spacecraft's lower thrust level, the team has extended the duration of the propulsion arc so the craft can get back on track with the planned flybysbetween September and January 2025. As to what led to the thrusting issue, engineers are still trying to figure that out, scrutinizing every last bit of available data.

"We know that we are dealing with an issue with the availability of electrical power from the Mercury Transfer Module," Bello said. "The electric propulsion thrusters themselves are fine." 

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Mercuryis perhaps the most understudied of the rocky worlds in the solar system. Hot and harder to reach than even Saturn, it hasn't enjoyed the level of exploration that other planets around the sun have received. Only two previous spacecraft, both NASAmissions, have flown to the Swift Planet, nicknamed for its fast orbit around the sun.

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BepiColombo, launched in October 2018on an Ariane 5 rocket from a French spaceport in South America, seeks to study the polar craters filled with ice, the planet's magnetic field, and the enigmatic "hollows" on the surface. Mercury's outer shell is covered in ancient lava flows, pitted by space rocks for the past 4 billion years or so.

By expanding knowledge of Mercury's composition, atmosphere, and magnetism, scientists can better understand how rocky, Earth-like planets came to be.

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But this isn't the first rough patch for BepiColombo. Last year, mission controllers performed a significant course correctionto compensate for earlier thruster outages. Without it, BepiColombo might have veered about 15,000 miles off track and onto the wrong side of the planet, according to ESA.

The mission's many years of sequential flybys are necessary because of how difficult it is to get to Mercury's orbit. Credit: ESA

The mission's many years of sequential flybysare necessary because of just how difficult it is to get to Mercury. To enter orbit around the planet, the spacecraft needs to be flying slow enough to be reeled in by Mercury's gravity. Too fast and it will skip right past. The trouble is, as the spacecraft gets closer to the sun, it picks up speed like a bicycle downhill. 

Slowing down in the vacuum of space is no easy feat. The careful choreography of swinging around planets is a way for the spacecraft to burn off energywithout carrying excessive amounts of fuel that would otherwise render the spacecraft too heavyto launch in the first place. 

If mission control can sufficiently counter the spacecraft's power problem, science operations could begin in the spring of 2026.