AIRBUS GROUP : Wake up Rosetta!

• Four alarms will be ringing on board the hibernating probe built by Space Systems (formerly Astrium)
• The space community will be listening out for signals from deep space on 20 January
• ESA comet mission enters its critical phase

On 20 January 2014, the global space community will turn its gaze to Darmstadt (Germany) in anticipation of signals from Rosetta, the comet probe developed and built by Space Systems (formerly Astrium) of Airbus Defence and Space for the European Space Agency (ESA). After around two and a half years in planned hibernation (957 days to be precise), during which there has been no contact with the probe, Rosetta is now set to wake itself up and enter its final mission phase to investigate the comet 67P/ Churyumov-Gerasimenko.

Rosetta is equipped with four alarms to make sure it wakes up!

"At least two of these alarms must ring in order to minimize the risk that the probe is activated at the wrong time by a malfunction in one of its four quartz clocks," said Gunther Lautenschläger, project manager at Space Systems.

On the day of the wake-up, Rosetta will be some 810 million kilometres away from Earth. However, the probe has travelled more than 6.2 billion kilometres since taking off aboard an Ariane 5 launcher from Kourou (French Guiana) on 2 March 2004. The probe will then be "only" around nine million kilometres from its target, comet 67P/Churyumov-Gerasimenko.

Rosetta faced challenging hibernation

Developing the probe's flight software proved to be one of the project's biggest challenges. This software underwent intensive testing on two fully equipped models of the probe for two years prior to launch, to make sure every imaginable malfunction was taken into consideration. Rosetta must be able to independently switch over to the relevant redundant component if necessary, or even switch to a special safe mode in an emergency.

Between June 2011 and January 2014 the probe travelled to regions very far from the Sun - to distances equivalent to Jupiter's orbit. With the level of solar energy available to it in this region of space so low (400W), Rosetta had to be put into hibernation. Everything was turned off so this small amount of energy could be dedicated to heating the probe.

But even in this standby mode, the probe needs around 390 watts of electrical power. Up to now, all spacecraft flying beyond the orbit of Mars have drawn their power from "radioactive" batteries, which convert the heat released during a substance's radioactive decay directly into electricity. Rosetta, however, flies on solar energy and therefore is a "green" probe, according to Gunther Lautenschläger.

The probe is fuelled by solar cells - developed for Rosetta through a major research programme that ran from 1990 to 1996 - which are optimised for conditions of very low light intensity and very low temperatures. Rosetta features two solar panel wings, each 15 metres long, with a total surface area of 68 square metres. At a distance of 800 million kilometres from the Sun, where the light is just four percent as intense as it is in the vicinity of Earth, these panels generate around 450 watts - which is more than the original calculations suggested would be possible. In summer 2015, when Rosetta approaches a distance of just 195 million kilometres from the Sun (putting it slightly beyond Earth's orbit), the solar cells will operate at their full electrical capacity of 8,700 watts.

Thermal control of the probe also poses a technical challenge. In the same way as Rosetta's distance from the Sun varies between some 195 and 840 million kilometres, Sun rays heat the spacecraft to varying degrees (-270°C/+100°C). In a ddition, the extent to which the on-board instruments and computers heat the probe's interior depends on their operational status. But despite these significant variations in operating conditions, the probe's interior temperature must stay relatively constant (5°C/45°C). To this end, a nd for the first time in Europe, special shutters were developed to open and close autonomously depending on the temperature.

Dust released from the comet's nucleus pose a potential problem. During its journey and when in orbit around the comet, Rosetta gets its bearings from the stars. To do this it uses a small telescope known as the star tracker. Starting in August 2014, once Rosetta is orbiting just a few kilometres from the surface of comet 67P/Churyumov-Gerasimenko, it is likely that dust will obscure the telescope's view. "Normal" star tracker software would not be able to tell stars from the fine particles of dust, and so would inevitably lose its bearings. This problem was solved for Rosetta by developing smart software capable of recognising any constellation, even if it is shrouded by up to a thousand "false stars" that would otherwise confuse the computer. The Rosetta mission will help scientists understand how our solar system was formed from primordial matter some 4.6 billion years ago. While the matter that ended up in the solar system's planets has been transformed by the actions of solar radiation and geological processes, comets act as "cosmic freezers", preserving this matter in its original state. "We're all waiting with baited breath for the signal that Rosetta is alive and well," said Gunther Lautenschläger. The engineers at Space Systems have a long day ahead of them on Monday. Although the alarms are set for 11am (CET), the first message from the comet probe back to Earth is not expected to arrive at the European Space Operations Centre in Darmstadt until that evening (between 6:30pm and 8pm CET). First, the probe will go through an unprecendented reactivation process: warming up, then assuming the correct orientation in space and pointing its two-meter-wide antenna towards Earth before finally sending its first "hello" back home. Although it will travel at the speed of light, this signal will still take 43 minutes to cross the vast expanse of space and reach Earth.

About Airbus Defence and Space

Airbus Defence and Space is a division of Airbus Group formed by combining the business activities of Cassidian, Astrium and Airbus Military. The new division is Europe's number one defence and space enterprise, the second largest space business worldwide and among the top ten global defence enterprises. It employs some 40,000 employees generating revenues of approximately €14 billion per year.