The Rosetta team here at ESOC are preparing for a thruster burn tomorrow, 18 June, designed to manoeuvre the deep-space probe onto an altered trajectory that will take it to the desired fly-by point on 10 July (burn starts 08:24 CEST).

Now here's an interesting bit of space science: based on data gathered during this month's complex optical navigation campaign, the spacecraft is currently predicted to make closest approach at just 2639 km from Lutetia - quite a bit closer to the asteroid then the hoped-for 3160 km.

Closer would be better, don't you think? So why are the Rosetta team burning fuel to take the point of closest approach 526 km further away from the target?

It turns out that 2639 km would be too close - more details under 'Full story' -- Daniel 

Exploded view of Rosetta showing internal and external components. There are 24 10N thrusters mounted externally.

Yesterday, the flight dynamics team working on Rosetta issued their most recent optical navigation campaign report, which gave a prediction of the fly-by distance based on 38 NavCam and 15 OSIRIS images acquired in the past several weeks.

If no thruster burns are conducted, Rosetta is currently predicted to make closest approach at 2639 km from Lutetia at 15:45:00.03 UTC (17:45:00.03 CEST), with an uncertainty of 7.47 seconds. But, as ESA's Andrea Accomazzo, Rosetta Spacecraft Operations Manager, explained in an email, this distance is too close:

If we fly closer than 3160 km, then the asteroid image will 'fill up' the cameras' field of view at closest approach. And then not only will we not see the full asteroid but, since the camera image guides the spacecraft attitude, we also would not know at what spot on the asteroid surface we are pointing. Moreover, the science teams have prepared commands for the spacecraft's instruments assuming a certain pre-set distance range from the asteroid.

If we do not pass by within this planned range, we might hamper their observations. It is true that we will use fuel for this correction, but it is very little compared to entire mission and it is far below the amount allocated for this flyby. To be precise, we will only use something in the order of 300 g of fuel.

Andrea said the trajectory correction manoeuvre will start at 06:24 UTC (08:24 CEST), with the thrusters switching 'ON' 8 seconds later. The burn will run for 188 seconds, and will be commanded automatically since Rosetta will not be in ground station contact at the time. Full results will be downloaded during the next ground contact a few hours later via DSA-1, ESA's 35m deep-space station at New Norcia, Australia.

The Rosetta Flight Dynamics head, Trevor Morley, added that:

Although the main reason for tweaking the Rosetta trajectory is to increase the fly-by distance, a secondary reason is to ensure that the solar phase angle passes through zero so that at about 18 minutes before closest approach (at a separation distance of 16 400 km) the asteroid, as viewed from the spacececraft, will be seen fully illuminated [and better for observations].

The results of the thruster burn will be analysed to generate new fly-by predictions, but by tomorrow at lunchtime the spacecraft should be lined up to pass by Lutetia at 3160 km at 15:44:55.57 UTC (17:44:55.57) - all figures subject to final confirmation.

This story illustrates well the deep cooperation required to make space science a success.

Extensive planning for Lutetia flyby started months ago at ESA and within the various science teams working on her instruments. Based on this preparation - and like Goldilocks' porridge - the flyby must be conducted within a certain range of distances - not too far, not too close - just right.