Jubilation as Philae probe touches down on comet

There were jubilant scenes at ESA’s mission control in Darmstadt, Germany, as the Philae lander confirmed it was safe and sound on the surface of Comet 67P Churymov-Gerasimenko shortly after 1600GMT.

On release from Rosetta, the lander was around 22.5km from the centre of the 4km wide comet, with a nervous 7hr wait for the mission team to hear whether their calculations were correct.

Now secured onto 67P, Philae will set to work with its suite of 10 scientific instruments to investigate the composition of the comet, which like all comets, is one of the oldest and most primitive bodies in the solar system, being formed from the material left over after the condensation of the Sun and planets from the original nebula of dust and gas.

A more through understanding of the comet’s composition is expected to give scientists further understanding about how the solar system was formed, and shed further light onto the origins of life itself.

Rosetta itself carries 11 scientific instrument packages and its 10-year journey to rendezvous and orbit 67P has been made possible with IFMS (Intermediate Frequency and Modem System) technology pioneered by BAE Systems in 1999.

Nick James, BAE Systems’ lead engineer for the project explained that IFMS has three functions: to uplink tele-commands, such as a sequence of time-coded commands to tell Rosetta when to fire thrusters, or when to separate the lander; to downlink telemetry, such as images and scientific data; and provide very accurate ranging and tracking measurements.

He said: ‘Before IFMS came along, the ground stations [of] ESA and other agencies had lots of different bits of dedicated hardware that were associated with the specific standards that were used by different spacecraft.

‘A spacecraft in near Earth orbit (NEO) would use one kind of modulation scheme and once sort of data coding type, whereas spacecraft far away in deep space will use a different kind because they have different requirements. The NEO ones want to send data quickly, whereas the deep space ones need to send data through very, very noisy channels and so by the time the signal arrives at the Earth its very weak.

‘The idea of the IFMS is it basically does everything, it’s a signal processing platform that sits in the ESA ground stations and, depending on how its configured, it can demodulate and decode data from a huge number of different spacecraft.’

Being able to track Rosetta, which is travelling at speeds of up to 55,000km/h, is key to the mission, which flew a complex trajectory involving a number of planetary fly-bys of Earth, Mars and Jupiter before rendezvousing with 67P.

‘With Philae [today], all of the motion that the Philae lander will have comes from the trajectory the main spacecraft Rosetta has at the release,’ said James. ‘So effectively Rosetta is on a collision course with the nucleus to release the lander. All of the knowledge of where the spacecraft is and how it’s moving comes from very careful monitoring of the radio signal from the spacecraft.’

James explained that there are three methods of doing this, with two involving radial measurements and the third based around interferometry.

The first radial technique measures the Doppler frequency shift of the carrier in order to gauge velocity.

James said: ‘We can do that incredibly accurately because it’s a system where the spacecraft effectively turns the uplink carrier around and sends it back down to us…[in doing so] we can measure the velocity down to tiny fractions of a millimetre per second in the radial direction and that’s on a spacecraft that is maybe doing 30km/s relative to the ground station.’

Ranging is also conducted by sending a sequence of data bits up to the spacecraft, which are then turned around and sent back to Earth.

‘We very accurately measure the time delay between transmitting and receiving and - when you know the speed of light - that can then give you the range. You measure that with an accuracy of about a metre and that is independent of where the spacecraft is, so it’s a metre at billions of kilometres.’

A third method used to ascertain the exact location of Rosetta involves triangulating its position with ground stations by taking a signal from Rosetta and determining the time delay between the arrival of the signal at each station.

‘That [time delay] gives you one side of a triangle,’ said James. ‘You’ve got a long baseline, so the baseline between the two stations is of the order of about 10,000km or so, so you can do the geometry, you can get a very accurate angle measurement…the kind of angle we can measure there is of the order somewhat better than a millionth of a degree.’

All three methods have been used extensively to track Rosetta’s rendezvous with the comet, measure its orbit and help the flight dynamics team at ESA determine the gravitational field of the nucleus.

‘They [needed] to do that in order to improve the accuracy of the landing,’ said James. ‘The small perturbations that are imposed on the spacecraft by the gravity of the [comet’s] nucleus are important when it comes to the landing because they will affect the actual position of the landing site.’

IFMS is set to be replaced in 2015 with TTCP (telemetry telecommand and control processor), which James said will have approximately 30 times the bandwidth and much higher processing speeds.

‘The IFMS equipment was actually developed back in 1999…and if you imagine how PC technology has developed in 15 years [then] that gives you an idea of the fact that we’re putting some quite complex algorithms on some very old hardware, which is quite challenging.’