By David A. Ord
….. Rosetta awoke, but only after an additional snooze!
Many nervous people at ESA control centre in Darmstadt were made to wait an additional almost 18, nail-biting minutes before Rosetta decided to indicate that it was indeed awake, alive and mostly well.
The small blip in the baseline signal as Rosetta hails Earth
After being placed in hibernation mode for a total of 957 days during which time communication was not possible, Rosetta finally delivered its message to an excited but mostly relieved audience. The ‘right on the money’ time would have been 17:45 GMT; but engineers knew that the spacecraft’s computer only checked the on-board clocks every 15 minutes. So, no one was at all concerned when nothing had happened by 17:50 GMT and the smart money went on 18:00 GMT.
Alas, Rosetta had something else in mind and like a true diva held the centre of everyone’s attention for 18 additional, very long minutes. The alarm had gone off, but it appeared that the snooze button had been pressed.
The Rosetta probe is on one of the most technologically advanced missions ever attempted: to chase and place a lander onto the surface of a comet travelling at almost 40,000Km/Hr.
The spaceship was sent into a slumber in June 2011 so it could save energy for its long journey to the 5 Km frozen rock, known as 67P/Churyumov-Gerasimenko.
Rosetta’s internal alarm clock gave the wake-up call at 10am GMT, but it took more than eight hours for mission control to get the confirmation it worked.
The “Hello, world!” message from Rosetta was transmitted onto the Rosetta Twitter and Facebook media sites by the European Space Agency engineers.
The relief and joy in the control centre that the Billion Euro spacecraft was alive
It turns out that although the alarm went off at the pre-set time, and the revival sequence began as scheduled, some malfunction caused a re-boot to start the sequence again; leading to an 18 minute delay.
Rosetta carried four quartz alarm clocks and required at least two to ‘ring’ simultaneously to minimise the risk of a false wake-up call. Now, a data dump from the main computer shows that it tried to wake up in September 2012, during its hibernation period and it fortunately did not succeed.
Other spacecraft flying beyond Mars orbit have all relied upon radioactive energy sources, but Rosetta took the ‘green’ option and is only equipped with solar power.
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 keeping the instruments from icing up
The first task after waking up for Rosetta was to turn on the heaters and slowly warm up the on-board instruments. A high priority was the star trackers, so that the craft could determine its position and orientation with regard to Earth. Again, the recent data dump from Rosetta shows that this did not go quite as planned in that the backup star tracker had to be employed. Further analysis showed that the primary star tracker failed because it had simply not warmed up sufficiently to operate correctly, even after 6hrs of heating.
After Rosetta had located itself by reference to the targeted stars and then determined its orientation, it fired small thrusters to position itself such that its radio antennae were pointed toward Earth. Only after this sequence could it send the eagerly anticipated message to the Darmstadt Space Control Centre.
So, in the coming months, the engineers have some fixing to do. Unscheduled re-booting is not a desirable trait; it could happen at a mission critical moment. In addition, a high pressure helium leak has been detected and couple of the ‘action wheels’ used for the crafts orientation and manoeuvring are playing up. However, none of these appears to be a show stopper and we can look forward to the next phases of the mission.
By May, all of the instruments will be have been gradually brought on-line and tested. As Rosetta approaches the Sun, power will cease to be a problem. When the craft was 800 Million Kilometres from the Sun, its 68 square meters of solar panel could only produce 450 watts, but in 2015, when it is only 195 Million miles form the Sun, the same panels will be generating some 8,700 watts.
Rosetta’s ultimate aim: to place the lander Philae onto the surface of Comet 67P
Although Rosetta is now ‘only’ 810 million kilometres from Earth, it has in fact travelled some 6.2 billion kilometres and been exposed to temperatures ranging from -270oC to +100oC. First sighting of the Comet 67P by on-board instruments may be in May, with the possibility of images in August. These would be the next major milestones before placing the lander Philae onto the surface in November of this year.
However, even by June, on-board instruments should indicate if the hydrogen/deuterium ratio of the water on the Comet67P is the same as that found on Earth. This is would be valuable information in supporting the theory that most of the water on our planet was delivered via Comet bombardment at an early stage of Earth’s development.
The Rosetta Mission can be followed on the ESA site.