With about 4500 km separating Rosetta from comet 67P/C-G, the spacecraft will conduct the last of four FAT – ‘Far Approach Trajectory’ – orbit correction manoeuvres later today.

The manoeuvre aims to further reduce Rosetta’s speed with respect to the comet by about 4.82 m/s, such that Rosetta ends the day with a relative speed of 3.5m/s at a distance of 3500 km.

The four FAT burns have been carried out weekly this month, and all have proceeded nominally; the ‘Close Approach Trajectory’ (CAT; pre-insertion and insertion) burns will follow on 3 and 6 August, which will provide further changes in velocity of about 3 and 1 m/s, respectively.

Rosetta and comet 67P/C-G to scale, assuming Rosetta is orbiting at a distance of 10 km, and the comet is about 4 km wide. Credits: ESA.

Rosetta and comet 67P/C-G to scale, assuming Rosetta is orbiting at a distance of 10 km, and the comet is about 4 km wide. Credits: ESA.

“Today’s burn starts at 10:38 UTC [12:38 CEST] on board, and is planned to last 16 minutes and 35 seconds,” says Sylvain Lodiot, Rosetta Spacecraft Operations Manager. “All systems on the spacecraft are performing well and the entire team is counting down now the kilometres separating Rosetta from its destination.”

The one-way light time today is 22 mins:26 secs.

Brightest beacons

In the week to 20 July, telecommunications with Rosetta was provided by ESA’s 35m deep-space station at New Norcia, Australia, and NASA’s DSN (Deep Space Network) stations at the Goldstone complex in California.

ESA’s Cebreros, Spain, and Malargüe, Argentina, tracking stations are also being used for the highly sophisticated and very accurate ‘delta-DOR’ (delta differential one-way ranging) navigation technique.

Delta-DOR involves using signals from a quasar located in another galaxy to refine and correct location information derived from Rosetta’s radio signals (see ‘Brightest Beacons’).

Are we there yet?

On 6 August, Rosetta will arrive at the comet at a stand-off distance of 100 km and a relative speed of just 1 m/second.

Arrival will mark the start of an intensive period of scientific data gathering to further characterise the comet and determine candidate landing sites for the Philae lander{LINK}.

In the following weeks, Rosetta will lower its orbit, aiming to get as low as just 10 km; many unknowns remain about 67P/C-G, including its mass, gravitational field and outgassing activity, all of which must be determined and understood before Rosetta can move in that close.

Mass is possibly the most uncertain of all the cometary parameters (because the density is extremely uncertain) yet is the single most important parameter to be estimated from the standpoint of early navigation activities at the comet.

The latest science data – including recent images indicating the comet is a complex ‘contact binary‘ – is enabling the mission team to refine models of the comet. Based on the best estimates of the comet size and shape available before the recent images, the mass is estimated at 3.14×10^12 kg. This will surely change as we learn more about this enigmatic object.

The next OSIRIS images are expected tomorrow.

On 23 July, the comet and Rosetta will be some 403.5 million km from Earth.