By David A. Ord
…… August sees the beginning of the final phase of the Rosetta mission.
Well, having been amazed at the images of Pluto from the NASA New Horizons mission in the last few weeks (and maybe the very best is yet to be shown!) August sees attention re-directed to the ESA Rosetta mission as it enters the final phase.
As a reminder, Rosetta is currently in orbit around the comet 67P/Churyumov-Gerasimenko, having discharged the lander probe Philae onto the comet’s surface for a ride around the Sun. After more than 10 years travel covering an accumulated distance in excess of 7 billion Kms, Rosetta is in position to witness the comet pass through its perihelion (closest point to the Sun) on August 13th. The good news is that Rosetta is in fine health and all 11 on-board instruments appear to be in excellent working order.
The comet has a 6.5 year commute around the Sun from just beyond the orbit of Jupiter at its furthest, to between the orbits of Earth and Mars at it closest. Rosetta first rendezvoused with the comet at around 540 million Kms from the Sun. Travelling at around 120,000 Km/h, around their orbit, Rosetta and the comet will be 186 million Kms from the Sun at perihelion. For comparison, Earth orbits the Sun at an average of 149 million Kms.
The comet’s activity has been growing over the last year that Rosetta has been orbiting the comet. This is an incremental process brought about by the increase in solar energy incident on the comet, warming up its frozen ices which subsequently sublimate or vaporise. Rosetta has been witnessing this gradual rise, and it is expected that this activity will reach a peak during August and September. Perihelion will not necessarily mark the maximum activity since there will be a lag in the amount of thermal energy absorbed by the comet. Outbursts are possible, but unpredictable.
Comet 67P is not classed as a ‘sungrazer’, in that it will not be so close to the Sun that it will completely disintegrate, as did the recent Comet Ison. Indeed, Rosetta is scheduled to monitor the comet through to September 2016. However, this outcome cannot be totally ruled out and it has been documented that there is a 500m long fracture on the neck of the comet that will be watched with great interest.
No decision has yet been made as to how close Rosetta will approach the comet during this period of high activity, but currently it has not been possible to reduce the orbit to less than 150 Kms. One issue that has been a concern is that the star trackers which are critical for the very precise navigation of Rosetta have been confused by bright dust particles emitted from the comet. The idea is that they lock on to known stars and from that information, the exact position and orientation of the craft can be determined. Unfortunately, some of the material coming from the comet is being identified as a star, resulting in erroneous navigation data with very dangerous consequences. Because the signal time to earth is about 14 minutes, Rosetta must make autonomous decisions to protect itself and if in doubt keep a healthy distance from the comet.
However, even at 150km, most of the data being collected should be good enough. At some time though, the decision to move Rosetta as close as possible (it has been as close as 10 Km) may well be a gamble worth taking by the mission engineers. To some extent this decision would be simplified or perhaps not even necessary if the situation with the lander Philae on the surface of the comet could be resolved.
Philae was dropped onto the surface of the comet, but after its 7 hour ‘flight’ failed to secure itself and took at least 3 bounces before coming to rest. The on-board experiments were automatically started and believed to have been largely successful. However, as of writing this article, the exact whereabouts of Philae still remains unknown.
There was great excitement when Philae ‘reported in’ on 13 June 2015 after seven months of hibernation and sent data about its health. The lander was ready to perform its tasks, 300 million kilometres away from Earth.
Further data sent on 24 June did not suggest that the lander had experienced technical difficulties. Now, Philae’s internal temperature of zero degrees Celsius gives the team hope that the lander can charge its batteries; this would make scientific work possible regardless of the ‘time of day’ on the comet.
On July 9th Philae confirmed that a command to one of its instruments had been received and executed. However, since then, no further contact has been made.
Included in the data sent on July 9th was the information that the profile of the charging by the solar panels had changed and this may suggest that Philae had slipped or been moved; perhaps by gas emissions from the comet. This would have the knock-on effect that its orientation to Rosetta which has to receive its signal would have changed. In addition, the health of the transmitting and receiving antennae may be in doubt; with the suspicion that at least one transmitter and one receiver has failed.
It seems likely that Philae is still functional and although the mission will now focus its scientific priority on the orbiter, Rosetta will nevertheless continue to attempt a dialog with the lander. However, each attempt to resolve the communication problem may well require a special orbit by Rosetta in a particular orientation and this will take time away from the scientific schedule.
Given that Philae seems capable of accepting commands, there is a great hope that a ‘safe block’ of instructions stored within Philae can be executed with a single command which in turn would initialise all of the on board analysers as and when power became available. Getting the results communicated back from these tests may a tougher ask.
Many ground based telescopes are becoming involved in the observations of the comet, including the William Herschel Telescope, the Telescopio Nazionale Galileo and the Liverpool Telescope on La Palma, ESA’s optical ground station on Tenerife, in the Canary Islands and the NASA Infrared Telescope Facility on Mauna Kea, Hawaii.
Seen from the ground based telescopes, the tail of Comet 67/P is already in excess of 120,000 Kms. Uniquely, the make up of the tail, the coma and the body as seen from Earth can be compared with the up-close data produced by Rosetta. This ‘calibration’ process will allow a much more exact study of not just 67/P, but any other comet by ground based telescopes.
Around the time of perihelion, the comet can be observed from Earth in the early morning hours, just before sunrise. It will remain relatively close to the Sun in the sky, and thus observable in the early morning, for several months. Then, the comet will be in the night sky between December 2015 and March 2016, which will be the prime time for ground-based observations. Even so, ESA scientists suggest it will take an 8” (200mm) telescope to observe the object. Previous experiences suggest that those scientists are eternal optimists, so 8” is probably the minimum.
Perihelion of 67/P will occur at 04:03 local time on 13 August 2015 and ESA expect to release some images from Rosetta around lunchtime.