1 Oct 2021

 

Tonight, BepiColombo will perform the first of six Mercury flybys, each honing the spacecrafts’ trajectory with the ultimate goal of shedding enough energy – after its two years ‘falling’ towards the Sun – to be caught by the innermost planet’s gravity and remain in Mercurial orbit.

This first Mercury flyby will alter the spacecraft's velocity by 2.1 km/s with respect to the Sun, with the spacecraft passing just 198 km from the planet’s surface – half the altitude of the International Space Station – at 01:34 CEST on the morning of 2 October.

For all of this to happen, BepiColombo must approach the planet from precisely the right position, and this has taken months of meticulous planning from the Flight Dynamics experts at ESA's mission control in Darmstadt, Germany.

 

Ultra-precise navigation

Gravitational flybys require extremely precise deep-space navigation work, ensuring the spacecraft is on the correct approach trajectory. For this flyby, the requirement is for BepiColombo to fly just 200 kilometres from Mercury at its closest point, and here every kilometre makes a difference.

To make things difficult, BepiColombo is more than 100 million kilometres away from Earth, travelling at a velocity of 54 km/s with respect to the Sun, with signals taking 350 seconds (about six minutes) to travel from us to the mission, at the speed of light.

“Because of the remarkable precision of measurements from our network of ground stations and antennas all over the globe and the continuous efforts of the Flight Dynamics Navigation Team, our current knowledge of BepiColombo’s position is accurate to about 500 metres, and we know its velocity to the nearest millimetre per second,” explains Frank Budnik, Flight Dynamics Manager of BepiColombo at ESA’s ESOC Operations Centre.

 

A very close approach

One week after BepiColombo’s latest flyby of Venus on 10 August, a correction manoeuvre was performed to nudge the craft a little for this first flyby of Mercury.

One deep space station can tell you the distance to your spacecraft and how quickly it is moving along the line-of-sight, but it needed two stations for a complete picture of interplanetary motion, using the slightly different view from each ground station to get the spacecraft’s ‘perpendicular’ motion.

Now, ESA is preparing to build its fourth deep space station in New Norcia, Australia, increasing the ability of the Estrack network to be there for missions in flight now and in the future.

 

[Image]

(A) BepiColombo first Mercury flyby

(B) Cebreros station

(C) BepiColombo’s first Mercury flyby – key moments

(D) Ultra-precise navigation