Tomorrow, Rosetta will conduct a massive orbit correction manoeuvre (OCM) deep in our Solar System, switching on its thrusters for a bit less than 8 hours to reduce its speed relative to comet 67P by a whopping 291 metres/second (m/s) – one of the longest burns in ESA spaceflight history.

The burn starts at 15:23 UTC (17:23 CEST) on board and will be carried out autonomously; the commands were already uploaded on Tuesday.

The burn will take place during ground station visibility, via ESA’s 35m ground station at New Norcia, Australia, meaning that the Rosetta mission control team at ESOC will be able to receive telemetry (on-board status data) and monitor progress as it happens.

The massive burn marks the first of a trio of ‘big burns’ planned for 21 May, 4 June and 18 June; these are designed to reduce the relative speed with respect to the comet by 290.89, 270.98 and 90.76 m/s, respectively. (The engineers refer to these numbers as the ‘change in velocity’, or ‘delta-v’.)

On 7 May, Rosetta kicked off the current series of 8 OCM manoeuvres (through to end July) with a test burn, targeting a delta-v of just 20 m/s.

“We were especially keen to see how well the propulsion system functioned,” said Sylvain Lodiot, Spacecraft Operations Manager. “The burn went well and we’re on track now for the big ones coming up.”

(See Thruster burn kicks off crucial series of manoeuvres for details on why the orbit correction manoeuvres in May, June and July are so crucial to successfully arriving at the comet on 6 August.)

The manoeuvres aim, between now and the end of July, to reduce the distance between the two and lower the spacecraft’s speed relative to the comet.

“The manoeuvres, carried out with 10N thrusters, will bring the spacecraft a step closer to the comet, so that, upon arrival, early August, we should be at 100km distance and 1 m/s relative velocity,” says Sylvain.

On 21 May, before the burn, Rosetta will be 1 005 056 km from the comet, and will be moving with a relative speed (Vrel) of 754.1 m/s. The one-way radio signal time will be 27 min:50 sec.

We’ll provide an update on burn results later on Thursday.

Follow @esaoperations via Twitter for updates.