BepiColombo blasts off to Mercury

Only two missions have ever tried to explore the planet closest to our sun - Mercury. First in 1973 with the Mariner 10 mission, then there was MESSENGER, reaching the planet in 2015. On the 20th October, the European Space Agency and the Japan Aerospace Exploration Agency have launched a new spacecraft to Mercury. But why go back, and what does this new mission hope to learn? Izzie Clarke spoke to David Rothery from the Open University who is involved in the mission, along with Johannes Benkhof from the European Space Agency. 

David - Mercury is three times closer to the Sun than the Earth, and it’s the smallest planet in the solar system. It’s a very dense, rocky body which means its core of mainly iron, is much bigger compared to its overall size than in the case of the Earth. And it was thought be quite a dull, boring place because of this very thin rocky shell, but it turns out that the rocky shell is full of active ingredients. It’s a volatile rich planet. The whole surface has been made by a series of big volcanic eruptions, mostly lava flows, but in the more recent past the eruptions have been explosive, which is a surprise because to have an explosive eruption you’ve got to have gas involved in the magma. So we’re now mystified - did Mercury form there and somehow retain its volatiles? It’s very hard to understand how that could happen. Or did Mercury form further from the Sun and migrate inwards as part of the birth of the solar system?

Izzie - To answer these big questions, the European Space Agency and the Japan Aerospace Exploration Agency are sending a twinned spacecraft called BepiColombo to explore this hostile planet…

David - One of the aims of the mission is to understand the planet’s magnetic field, so seeing the same thing from two different perspectives is really going to reveal what’s going on in the magnetic field.

Izzie- Another part of the mission is to explore what Mercury’s surface actually looks like…

David - My role is, I’m leading the European Space Agency’s Surface and Composition working group for the planet, and that suits me as a geologist. I want to understand the surface history and what it’s made of. I’m also on the team for the British built instrument, which is the Mercury Imaging X-ray Spectrometer, or MIXS, led from Leicester University and that will determine the elemental abundances across the surface. We know there’s a lot of sulphur; we don’t know the distribution of the sulphur. We think it’s probably depleted where there have been volcanic explosions, but we don’t know for sure. That’s one thing we hope to find out.

Izzie - But travelling to Mercury, the closest planet to our Sun, is no easy task...

Johannes - Oh, I’m very excited. On the pass we had some problems, we had some hiccups because it’s a very challenging mission with a challenging design.

Izzie - That’s ESAs Johannes Benkhoff. He’s the project scientist of the BepiColombo mission…

Johannes - The problem is when you want to send a spacecraft to Mercury. It’s not the distance, it’s you need a lot of energy to go in the direction of the Sun. For that reason, we have built our mission on a solar electric propulsion system, which we use in order to bring our spacecraft to Mercury. And this solar electric propulsion engine is very energy efficient so that we have to use less propellants than when we would use a conventional propellant in order to bring our spacecraft there.

Izzie - Having left the Earth, BepiColombo will come back past our planet, which will slingshot BepiColombo around Venus twice and then complete half a dozen flybys of Mercury. On the final approach, the spacecraft will have slowed down enough to glide into the planet’s orbit. But why not just fly directly?

Johannes - You would need much more energy, or much more propellant to go there directly which we, unfortunately, cannot design a spacecraft for, because we have no rocket which will bring such a heavy spacecraft and fly it directly to Mercury.

Izzie - Once orbiting, the two spacecrafts will separate. The Japanese craft will stay in orbit to explore Mercury’s magnetic field. But the European craft will nudge a bit closer to get a good look at the planet’s surface. Another challenge is that the twinned spacecraft will need to tolerate Mercury’s fiery hot neighbour, our Sun…

Johannes - For the heat we had to develop a special insulation blanket in order to protect our spacecraft because on the outside we have the heat from the Sun, but also the infrared re-radiation from the planet itself which makes it very complicated to keep the inside of the spacecraft cold. So we have a special insulation blanket which is several centimetres thick and contains a ceramic textile outer layer, and then we have titanium and aluminium layers. And at the end we are able to have on the outside temperatures around 400/450 degrees, and in the inside we have almost room temperatures to operate our instruments.

Izzie - Johannes has been working on this project for the last 15 years, and now BepiColombo is finally launching. So how long until we get that all important data? Back to David Rothery...

David - It will take 7 and a bit years to get to Mercury. We’ll get there round about the end of 2025. We’ll be in a weakly captured orbit and have to optimise that, and it won’t be until the spring of 2026 that we start our science operations. The spacecraft has a design life of a year. If it survives in Mercury for one earth year, the mission will be a success. If it’s still healthy, we hope to go for a second earth year. NASA’s mission Messenger managed 4 years at Mercury, which would delight us if can do it.