By David A. Ord
…… time has probably run out for the Philae lander
Rosetta’s lander Philae has remained silent since 9 July 2015. With each passing day, Comet 67P/Churyumov-Gerasimenko is getting further and further away from the Sun, and as such, temperatures are falling on the comet’s surface. This is a very critical time for Philae.
On the 10th of January this year, the Philae controllers sent what may be the final command to Philae in an attempt to invoke a response from the lander. Knowing that the craft’s momentum wheel was fully operational during its descent to the surface of the comet, it was decided to have one last try to activate it again in the hope that it might shift the lander’s position.
However, as of writing this article, no signal has been received by Rosetta, the orbiting ‘mother-ship’. And time is running out!
By the end of January, 67P/Churyumov-Gerasimenko will be more than 300 million kilometres from the Sun, resulting in an operating temperature below -51ºC on Philae such that all power will be lost and cannot be re-generated.
The command to activate the momentum wheel will, therefore, be one of the last attempts to obtain a response from Philae.
In the next few days, Rosetta will maintain an active listening schedule for the lander and will vary its approach and orientation to the comet in an attempt to optimise reception of any signal. But, the current silence is ominous.
The conditions that must be met for Philae to operate are an internal temperature above –45°C and more than 5.5W of power. As the comet moves away from the Sun and back towards the orbit of Jupiter, these critical parameters are unlikely to be met again during the project.
However, Rosetta itself continues to perform very well. The spacecraft has travelled an accumulated 23.3 billion Km and is currently some 330 million Km from the Sun, 227 million Km from Earth and moving at a speed of 23.3 Km/sec. The craft continues to carry out a whole raft of scientific experiments on Comet 67P/C-G.
Indeed, one of the latest announcements from the mission scientists is the confirmation of exposed water ice on the surface of the comet. Although water vapour is the main gas seen flowing from comet 67P/C-G, the great majority of ice is believed to come from under the comet’s crust, and very few examples of exposed water ice have been found on the surface.
However, a detailed analysis by Rosetta’s VIRTIS infrared instrument reveals the composition of the comet’s topmost layer; it is primarily coated in a dark, dry and organic-rich material but with a small amount of water ice mixed in. The ice is associated with cliff walls and debris falls, and was at an average temperature of about –120ºC at the time.
The heat from the Sun penetrates the surface of the comet and triggers the evaporation of the buried ice. While some of the resulting water vapour may escape from the nucleus, a significant fraction of it re-condenses in layers beneath the surface. Laboratory experiments suggest that as much as 80% of the released water vapour does not make it up through the dust mantle layer of the comet, but is re-deposited below the surface.
So, it is implied that the ice-rich layers of the comet are the consequence of cometary activity and evolution and not primordial layering at formation.
Despite retaining as much as 80% of its buried ice, the comet nevertheless has produced a spectacular tail of dust and water vapour.
An image taken by Alan Fitzsimmons, on January 19th of this year suggests that the tail of Comet 67P/C-G extends some 2.5 million Km. and this observation clearly indicates that the tail is now split.
Multiple tail structures are not uncommon in comets, and indeed have been observed during previous apparitions of Comet 67P/C-G. The two portions of the tail are attributed to different populations of dust grains swept away from the comet’s nucleus by the radiation pressure of the Sun over the course of its 6.5 year orbit.
It is thought that the dust grains from the second trail were ejected from the comet in March 2015.
From this point forward, it is expected that operating Rosetta will become increasingly more challenging. As the comet recedes from the Sun, the amount of sunlight falling on Rosetta’s solar panels will drop significantly. As the power drops, it will not be possible to operate all of the scientific instruments at the same time.
By September-October 2016, Rosetta and the comet will appear very close to the Sun as seen from the Earth, making the up-linking of operational commands and the down-linking of scientific data extremely challenging. On 1 October 2016, the spacecraft will enter a period of conjunction, i.e. it will be behind the Sun as seen from Earth.
To extend the mission would require an extensive hibernation period for the craft, but given the fact that Rosetta has been flying for 12 years, 2 of which were in the very aggressive atmosphere of the comet, it is highly unlikely that the craft came through totally unscathed. Therefore, it has been decided that the mission will be terminated in September 2016.
The plan calls for a slow approach to the comet, to around 10Km, which has not been possible for the past year due to the cometary activity. Then Rosetta will begin a slow descent towards the surface, taking scientific data at very low altitudes, and ultimately leading to a controlled impact, ending this landmark scientific mission with Rosetta joining Philae on the surface of the comet.
During the final ‘collision trajectory’, the spacecraft’s high-gain antenna will be Earth-pointing, making it possible to get back telemetry and scientific data all the way down to contact. However, once impact has occurred, it is highly unlikely that any further communication with the Rosetta will occur.
All in all, it is foreseen that Rosetta’s mission will end at the point of contact on to the surface of Comet 67P/C-G, a symbolic finale to an epic journey spanning almost 20 years of planning and preparation, and 12 years in space.