Mercury-Bound Satellite Slingshots Around Earth Today


It’s been a few years since we’ve talked about BepiColombo, the joint ESA / JAXA spacecraft headed for Mercury. When last we checked in with the probe, scientists were testing its ion drive. BepiColombo features the most powerful ion engine ever built. On Friday morning, the probe will use Earth as a gravitational slingshot to both accelerate and alter its course towards Mercury.

I’m going to be honest. I thought this was mildly interesting, but not much more, until I actually saw the camera footage from BepiColombo itself. Watch our planet spin as BepiColombo is on approach to us.

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It’s one thing to see a photo of Earth from space. It’s another to have footage of our own home spinning in front of the camera. The footage is from April 9 and covers 2-2.5 hours. I’ve seen photos like “Earthrise” and “Pale Blue Dot” before, of course, but somehow this black and white footage of the Earth spinning in space drives home the tremendous possibility of what we could achieve and a sense of just how small we are in the cosmic void. To see it in motion at such distance is quite neat. I don’t know if other footage of Earth slingshot maneuvers exist, but this is the first I’ve seen.

The video below shows the trajectory BepiColombo is taking to Mercury; I’ve set the video to start at the original launch of the mission. Evidently the April 13 date for our rendezvous was approximate, as the spacecraft will hit its closest point of approach to Earth at about 6:25 AM Central European time.

The reason BepiColombo has to take such a roundabout path to Mercury is that Mercury is rather hard to hit. There are two reasons for this: First, Mercury is moving quite fast, at 47.87km/s. The Earth, for comparison, moves at 29.78km/s. So on the one hand, BepiColombo has to be moving quickly enough to match velocity with Mercury in the first place. But — and this is equally critical — it can’t be moving too fast, or Mercury’s gravity field won’t be able to capture it. If you watch the video below, this takes some serious orbital engineering. Here’s a hint: Don’t stop watching when it looks as if BepiColombo has entered orbit of Mercury. Wait until you actually see the word “Arrival” on-screen.

The second reason Mercury is so hard to hit is because gravitationally speaking, Mercury is a cotton puff orbiting a bowling ball. It takes six total flybys for BepiColombo to enter Mercury orbit, with a slowly decreasing interval between each. The first four flybys are each separated by 8-14, while the last two flybys occur quickly. The 4th is on 05 Sept 2024, the fifth is 02 Dec 2024, and the sixth is 09 Jan 2025. At this point, BepiColombo must be in orbit, right?

Nope. After the sixth flyby, BepiColombo will be a bit ahead of Mercury. It won’t actually arrive at Mercury until 05 December 2025, almost a full year after the final flyby, when Mercury will weakly capture the satellite and a chemical rocket will be used for final orbital insertion maneuvers. Prior to this point, the spacecraft will rely on an ion engine rather than a conventional rocket for propulsion.

Ion engines don’t emit much thrust — the engine on BepiColombo is the most powerful ion engine we’ve ever mounted on a spacecraft, and it emits a whopping 290mN, where one Newton is defined as the amount of force required to give a 1kg mass an acceleration of one meter per second per second. The massive advantage of ion engines is that they can achieve substantial acceleration or deceleration for minimal fuel cost, provided you can do so over a long period of time. Accelerating and decelerating for Mercury orbit fits the bill nicely, and I suspect that the spacecraft spends at least some of the time after its 6th flyby decelerating to allow Mercury to catch up and capture it.

The goals of BepiColombo are to study the formation of a planet so close to its host star, specifically Mercury’s form, geology, internal structure, crater formations and exosphere (the cloud of atoms around a planet too diffuse to be called an atmosphere, but nevertheless distinct from the vacuum of space). It’ll also perform some experiments to verify aspects of Einstein’s theory of relativity.

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