BepiColombo Launches on Long Journey to Mercury

BepiColombo Launches on Long Journey to Mercury

Earth is going back to Mercury.

BepiColombo, a joint mission of the European and Japanese space agencies, set off from a launchpad in French Guiana aboard an Ariane 5 rocket on a humid and mostly clear Friday night. Should the spacecraft travel as planned in the hours and days to come, it will set off on a course that makes it only the third spacecraft to visit the solar system’s innermost planet.

The mission actually contains two spacecraft that will share a ride to Mercury, but then separate to different orbits to make different observations. It will be a long wait for the main phase of the mission — the spacecraft do not begin to orbit Mercury, the solar system’s innermost and smallest planet, until December 2025.

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It is easy to get to Mercury quickly. The hard part is stopping.

Flying toward the sun is like running down a steep hill. Near the bottom of the hill, it is hard to slow down, which is essentially what BepiColombo needs to do before it can swing into orbit around Mercury, instead of just whizzing by.

The first spacecraft to go to Mercury, NASA’s Mariner 10 in 1974, made the trip in less than five months. But that was only a short flyby, passing within 450 miles of the surface. It made two additional flybys but never entered orbit.

A rocket engine could act as a brake, but that would require far more fuel than BepiColombo would be able to carry.

Instead, the spacecraft takes advantage of a strategy often advised if the brakes of your car fail — sideswipe some obstacles to slow down. In terms of orbital mechanics, that means sideswiping planets. BepiColombo will use a series of flybys of Earth, Venus and Mercury to slow down enough to be captured by Mercury’s gravity.

The four innermost planets — Mercury, Venus, Earth and Mars — are what scientists call terrestrial planets. Each has an iron core and an outer shell of rockier material. Mercury is the smallest of the bunch. It is barely bigger than Earth’s moon, but contains a disproportionately large iron core.

”Why Mercury?” Joana Oliveira, a scientist at the European Space Agency, said during a news conference in September. “That is the first question we should ask ourselves. Mercury is one little piece of the puzzle that helps understand the evolution of our solar system.”

Planetary scientists do not understand how Mercury’s oddball makeup came to be, which means they do not fully understand how the planets formed in the solar system.

Additionally, studies of how the solar wind blows into and around Mercury could provide clues about the possibilities of life around other stars.

Scientists have started to study Earth-size planets that orbit close to small, dim stars known as red dwarfs in recent years. Data from BepiColombo could help indicate whether these distant earths could retain an atmosphere or whether any air would be stripped away by the strong stellar winds. The numerous planets around the red dwarf Trappist-1, for instance, are much closer to that star, but in our solar system, Mercury provides the closest analog for study.

“This topic will be the key step in future science, the habitability at the exoplanets,” said Go Murakami, the project scientist for Japan’s portion of BepiColombo.

In addition to Mariner 10, NASA’s Messenger spacecraft launched in 2004. Like BepiColombo, it traveled for 6.5 years on a circuitous path before arriving in orbit in 2011. It spent four years in orbit before the fuel for its thrusters ran out and it crashed into the planet.

Mariner 10’s photographs showed a heavily cratered surface resembling Earth’s moon. Messenger revealed more details about wider swaths of the surface, including a long-dead volcano larger than Delaware, cracks in the outer crust as the planet cools and shrinks and a tenuous atmosphere kicked up by the intense bombardment of the surface by energetic particles from the sun.

The data from Messenger also ruled out some hypotheses about why Mercury is so rich in iron. While some scientists had suggested the planet experienced extended heating during and after its formation, Mercury still contains lighter, more volatile elements that would have boiled away in that scenario.

The Japanese orbiter, named Mio, will focus on measuring the interaction between Mercury’s weak magnetic field and the stream of charged particles emitted by the sun.

The suite of instruments aboard the European Mercury Planetary Orbiter, is similar to what NASA’s Messenger carried. “We have a bit of overlapping, but many of the instruments have higher resolution,” said Johannes Benkhoff, the project scientist for BepiColombo at the European Space Agency.

Because both orbiters carry magnetometers, they will be able to record what is happening in two places simultaneously, providing a more global picture of Mercury’s magnetic field.

“There will be two spacecraft instead of one,” said Sean Solomon, who served as principal investigator of the Messenger mission, in an email. “They will be in orbits with approximately comparable coverage of the northern and southern hemispheres, and they collectively carry many more instruments than Messenger did.”

The mission is named after Giuseppe Colombo, an Italian scientist and mathematician who came up with the sideswipe-the-planets strategy for Mariner 10. His nickname was Bepi.

A number of spacecraft have launched this year, or have approached their destinations after a journey that lasted years. Here are some highlights:

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