The joint mission BepiColombo — coordinated between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) — is scheduled to arrive at Mercury this November, probing the planet's surface and core. Launched in October 2018, BepiColombo is the third mission sent to Mercury since NASA's Mariner 10 (1973-1975) and MESSENGER (2004-2015). ESA's Mercury Transfer Module (MTM) carries two separate orbiters — ESA's own Mercury Planetary Orbiter (MPO) and JAXA's Mercury Magnetospheric Orbiter (MIO) — that will separate upon arrival, focusing on planetary composition and magnetosphere, respectively.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're going to talk about BepiColombo.
What's BepiColombo, you ask?
It's a mission to Mercury that was launched eight years ago and is preparing to arrive at Mercury. It's had some bumps along the way, but the science instruments are healthy and should begin operation soon. The mission is shared between the European Space Agency ESA and the Japan Aerospace Exploration Agency JAXA.
It's really two spacecraft strapped to a third, which was the engine. But rather than being a normal rocket engine that burns fuel in brief high thrust maneuvers, BepiColombo used an ion engine called the Solar Electric Propulsion System. An ion engine uses electricity to make and accelerate ions using Newton's second law of action-reaction. You shoot tiny light atoms out the back at huge velocities and it provides thrust. In BepiColombo's case, that thrust was only about as much as the weight of a piece of paper, but it did so continuously along the voyage, 15 times more efficiently than standard rocket engines.
The electricity required for the ion engine was provided by solar panels and in 2024, there was an issue that reduced the electricity available to create the ion thrust. But the ingenious engineers modified BepiColombo's trajectory and the spacecraft will successfully arrive at Mercury just 11 months after it was scheduled. And BepiColombo's recent event was the jettisoning of that ion engine, the Mercury Transfer Module. For remaining two components, ESA's Mercury Planet Orbiter or MPO and JAXA's Mercury Magnetospheric Orbiter or Mio will now rely on standard propellant for their work, orbiting Mercury. They will enter that orbit in November and then science operations will begin in early 2027.
So what sort of science is BepiColombo going to do?
Well let's review a little bit of what we know about Mercury. Firstly, of course, it's the closest planet to our sun and while not the hottest planet, it does reach over 440 degrees Fahrenheit in the daytime. Mercury's shades of gray in color and heavily cratered really resembling our own moon. It's the smallest planet in our solar system and only 900 miles in diameter larger than our moon and 1,200 miles in diameter smaller than Mars, the next smallest planet.
And while it looks boring and old, Mercury actually has some very interesting features. It's heavily cratered but it's not saturated with craters, like the lighter regions of our moon known as highlands, and that means its surface is younger. During Mercury's first half a billion years, it experienced lots and lots of lava flows so the entire surface is very close to 4 billion years old, without much deviation. Being smaller and the closest planet to the sun, it's also tidally locked to the sun. Not like our moon where one side always faces the earth but in a 3:2 resonance where it spins 3 times for every 2 years.
It also has water, not liquid or vapor as its high temperature would suggest but actually frozen into the dirt within craters that sunlight can't directly reach. Amazingly, this was discovered in the 1990s when we began bouncing radar off of Mercury. Isn't that amazing?! And it was confirmed by the last Mercury mission called Messenger in the 2010s.
Mercury also has the largest core fraction and that's what really interests BepiColombo.
While the Earth's core makes up roughly half of our radius, Mercury's core extends to 85% of its radius, making it the largest core fraction in the solar system. And like our core, it also has solid and liquid parts but probably with some different layering. Since our liquid flowing iron core generates Earth's magnetic field and Mercury has a larger core fraction, one might suspect Mercury to have a larger magnetic field, but it doesn't. Its magnetic field is only 1% that of our Earth and that's primarily what JAXA's Mio will attempt to understand along with the magnetic field's interaction with the solar wind. ESA's MPO will work similar to Messenger, mapping the surface and using tiny variations in Mercury's gravity to probe its internal structure, but MPO will do a better job as its orbit will be closer to Mercury, more circular and over Mercury's poles. That will give it an excellent view of Mercury's entire surface.
Once science operations begin in 2027, we can look forward to BepiColombo making Mercury a little more exciting.