Mini-Throughput: The Long and Winding Road to Mercury

Mercury in true color, as photographed by MESSENGER

Been a busy week since on call for the spacecraft. So sorry about the delay.

Despite being one of the closest planets to our own, Mercury is one of the least explored. Only two missions have visited the speedy little innermost planet. Mariner 10 did a flyby in the 1970’s. And the MESSENGER mission spent a few years in orbit before crashing into it 2015 (on purpose).

The reason is not because Mercury is uninteresting. It has a relatively large iron core and is slightly denser than the Earth, which is a bit unexpected given our theories of how the Solar System formed. It shows signs of ancient volcanic activity and a weak magnetic field. The surface temperature ranges from -280 F at night to 800 F during the day. There’s an even an outside shot that it may have had the beginnings of life at some stage before it became the dead world it is now.

No, the reason is because Mercury is a rather difficult planet to reach. It is closer to the Sun, which means any probe is effectively going downhill as it approaches. It’s also moving faster than Earth, so you need to gain a significant amount of velocity to catch it. But because it’s small, it’s gravity can’t just grab a passing spacecraft. If you just blasted straight for it, you’d go sailing past into the Sun. In fact, a successful Mercury rendezvous requires more energy than leaving the Solar System completely.

So what our missions have to do is sneak up on it, using the gravity of the inner planets to slowly match Mercury’s orbit. MESSENGER was launched in 2004. It did a flyby of Earth, two flybys of Venus and three flybys of Mercury itself to adjust its trajectory enough to achieve orbit in 2011.

MESSENGER’s long and winding road to Mercury.

Well, last week, the new ESA/JAXA BepiColombo mission hit another marker buoy on its trip to the fast planet.

Hello again Mercury!

Here's a first look from today's #MercuryFlyby capturing such an amazing array of the planet's rich geological features 🤩https://t.co/hC6TfMmCy1#ExploreFarther pic.twitter.com/7WkYiGU0Kp

— Bepi (@ESA_Bepi) June 23, 2022

That’s a stunning picture of the planet, showing incredible detail in the craters. This is actually Bepi’s second of six Mercury flybys that will drop it into a nice stable orbit in December of 2025. This is in fact the closest Bepi will get to the planet — only 200 km out. Once there, it will separate into two spacecraft: the Mercury Planet Orbiter and the Mercury Magnetospheric Orbiter. These will move into stable elliptical orbits that will allow them to study the planet in tandem. From these orbits, they will do everything from studying Mercury’s tiny atmosphere to figuring out why its composition is so odd to confirming aspects of the Theory of Relativity.

Three-and-a-half years (or 14 Mercury years) is a long time to wait for the mission to really get going. But Mercury will be worth it. Our knowledge of the planet is going to explode over the next 5-10 years. And with it, our knowledge of the Solar System, the planets and ultimately our own home. In the meantime … enjoy the view.