The BepiColombo mission, a joint effort between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) to study Mercury, has taken its first major step toward its destination after nearly eight years in transit.
The mission's Mercury Transfer Module, which propelled the spacecraft for years, separated from BepiColombo's two science probes at 8 a.m. Eastern time on Thursday. ESA broadcast the separation live, and confirmation reached mission control at 9:53 a.m. Eastern time when the agency received a signal from the spacecraft. The mission team said the spacecraft was in good condition and operating as expected.
BepiColombo launched aboard an Ariane 5 rocket from the European Space Center near Kourou, French Guiana, in October 2018. It has since traveled more than 10 billion kilometers. The probe is named after Giuseppe "Bepi" Colombo, the Italian engineer and mathematician who studied Mercury's unusual rotation and proposed the trajectories that made NASA's first flybys of the planet possible in 1974.
Earth sits roughly 10 times closer to Mercury than it does to Jupiter, yet BepiColombo took longer to reach the small planet than missions such as Galileo and Juno took to reach Jupiter. Frank Budnik, the BepiColombo flight dynamics manager at ESA, said a spacecraft can travel faster to Mercury on a direct route, but the challenge is matching Mercury's own speed once there, and that transfer demands enormous amounts of energy.
A combination of electric propulsion and gravity assists from nine flybys of Earth, Venus and Mercury helped the spacecraft build up the speed it needed for the long journey. ESA's flight dynamics team said the discarded transfer module will remain in orbit around the sun for the foreseeable future and will not affect Mercury.
Ignacio Tanco, head of Solar System Internal Mission Operations at ESA, compared the separation to launching a new spacecraft around a different planet, adding that everything went according to plan but that this marks only the start of a complex process. Unlike other missions, he said, BepiColombo's arrival is not a single event, and about six months of intensive operations lie ahead.
The challenges of venturing toward the sun
Settling into an observation orbit around Mercury is even harder than the journey to get there. ESA's Mercury Planetary Orbiter (MPO) and JAXA's Mercury Magnetospheric Orbiter (MIO) will be captured by Mercury's gravity and enter orbit around the planet on November 21.
The two orbiters will then separate from each other between December 9 and 10, with the Mercury Planetary Orbiter reaching its orbital position on December 16 and the Mercury Magnetospheric Orbiter following on March 10. Science operations for both orbiters are due to officially begin in April.
Tanco said it will be the first time a mission has released two fully independent, fully operational spacecraft around a different planet. Santa Martinez, BepiColombo's mission manager at ESA, said the MPO will study the planet itself while the MIO will focus on the space environment surrounding Mercury.
Mercury sits about 58 million kilometers (36 million miles) from the sun on average and completes an orbit around it every 88 days, according to NASA. That proximity means surface temperatures on the planet can reach about 430 degrees Celsius (800 degrees Fahrenheit) during the day.
That closeness to the sun is the main reason BepiColombo is such a complex mission. Only two previous missions have flown near Mercury: NASA's Mariner 10 in 1974 and MESSENGER in 2011. Like BepiColombo, MESSENGER took nearly seven years to reach the planet.
Tanco said the spacecraft itself was built to operate in extremely harsh conditions, comparing it to running with a very hot pizza oven strapped to your back. Both orbiters carry solar panels for power and face a risk of overheating because of their proximity to the sun. The MIO will spin 15 times per minute, while the MPO will angle its solar panels nearly perpendicular to the sun to receive just the right amount of sunlight and avoid radiation damage.
Unraveling Mercury's mysteries
Placing two spacecraft in orbit around Mercury will offer unprecedented views of the planet and open a new chapter in scientists' understanding of the solar system, Martinez said. The orbiters will help map the planet at high resolution and answer questions raised by MESSENGER's detailed survey of Mercury.
Each orbiter carries a set of instruments to examine Mercury's structure and composition, including craters and evidence of ancient volcanic activity on its surface, as well as the planet's magnetic field and its exosphere, a thin cloud of gas that surrounds it, said Geraint Jones, BepiColombo project scientist at ESA.
One mystery scientists hope to solve involves the so-called hollows detected on Mercury by the Mariner 10 and MESSENGER probes. Jones said there are depressions on the surface that appear to indicate it is being eroded, and that it may be possible to observe changes in these regions over time. No such hollows have been found on any other planet in the solar system, and without wind or water to carve the thousands of puzzling depressions, scientists want to know what forces shaped them.
The satellites could also help settle questions about Mercury's density given its small size. Jones said that about 4.5 billion years ago, when the planets formed from debris around the sun, Mercury is believed to have begun forming as a modestly sized planet before being struck by another large planetesimal that stripped away its outer crust. That would mean the overall density of the material left behind was much higher, which he said explains the planet's high density today. Jones said learning more about Mercury's surface and interior could confirm that theory or produce new answers.
