As you no doubt recall, if you've been following this blog, Rosetta's Philae comet lander made a successful (and eventful) landing on Comet 67P on 12 November 2014. After a non-nominal landing it found itself in an unfortunate location and in an unusual attitude such that the performance of its solar arrays was critically compromised. As a consequence, once its batteries were almost exhausted, little Philae entered an automatic process which placed it into a period of hibernation - and I would have guessed that that would be the last we'd hear from it. The only hope was that as the comet moved closer to the Sun on its orbit, the sun's intensity and the lander's temperature would increase, making it at least possible that the batteries would recharge.
Well, much to the surprise of the ESA scientists and engineers, Philae 'spoke' briefly for 85 seconds, the signal being received at 20.28 UT on Saturday 13 June 2015 at the European Space Operations Centre. Among other things, it indicated that its temperature was a balmy -35 degrees C, and it currently had 24 W available from the solar array. The simple brief message suggests that Philae may yet recover from its long sleep. The comet is not yet at perihelion (the closest approach to the Sun - which occurs on 13 August). So hopes are high that Philae's work is not yet done.
In the mean time, the 'mothership' Rosetta continues to orbit the nucleus, imaging and recording every detail as the nucleus begins to warm up .
When I were a lad I grew up with the ‘old Solar System’, which had nine planets, and with mnemonics to help me remember their names and sequence – such as “My Very Easy Method Just Speeds Up Naming Planets”. Up to the present time, all of these bodies have been visited or orbited by spacecraft – bar one – humble Pluto. However in the early noughties, NASA decided to rectify this deficit, and the process culminated with the launch of the ‘New Horizons’ spacecraft on 19 January 2006. However, ironically in the August of the same year the International Astronomical Union decided to demote Pluto from full ‘planet’ status to a ‘dwarf planet’ producing the ‘new Solar System’ comprised of 8 planets (and in the process ruining all those long-standing mnemonics of old!).
So New Horizons has been travelling across the Solar System for the last 9 ½ years or so, and on the day of writing (11 June 2015) finds itself 34 days and about 40 million km (25 million miles) from its destination. Travelling at a speed relative to Pluto of about 13.8 km/sec (8.6 miles/sec) it will finally reach its closest approach to its target on 14 July 2015. Note that this is a fly-by mission – New Horizons does not have the rocket power or propellant to allow it to slow down and enter orbit around Pluto. So this means that operations to observe and measure the Pluto system is a rather brief affair, beginning on 4 July and concluding on the 20th. After the encounter the spacecraft will continue on its trajectory and will ultimately leave the Solar System.
As a consequence, the trajectory, and in particular the flight time from Earth to Pluto had to be carefully chosen. One of the main drivers is a trade-off between spacecraft reliability on the one hand, and ensuring that there is sufficient time to observe Pluto during the encounter on the other. Current technology and build techniques allow spacecraft to be manufactured with a typical expectation that they will continue to operate reliably for something like 10 to 15 years. Once they reach about the 10 year mark there is a probability that there may be sub-nominal operation of some components. However, with redundancy built into the design, it is reasonable to expect that the spacecraft’s performance is not critically compromised. So in choosing the New Horizon flight time, this important aspect of spacecraft reliability needed to be considered. For example, the mission designers could have chosen a larger boost out of Earth orbit to decrease the flight time to, say, 5 to 6 years, so increasing the probability of reliable operation at Pluto. The down side of this strategy would be an increased speed relative to Pluto, and consequently a shorter period for close approach operations. On the other hand a flight time of, say, 15 years would slow the spacecraft at encounter and so increase the opportunity to collect data. However the longer mission would then result in poor reliability and potentially spacecraft failure prior to the encounter.
The mission target Pluto was discovered in 1930, and is a small object about 2400 km (1500 miles) in diameter. It is currently about 30 times more distant from the Sun than the Earth, which means that the solar intensity is about 1/1000 th of that at Earth. As a consequence it’s cold (Pluto’s surface temperature is around – 230 degrees C). Images from the Hubble Space Telescope, and other ground-based systems, currently give Pluto a total of 5 moons. Charon is the largest of these, being almost the same size as Pluto itself, making the Pluto-Charon system to closest thing to a double planet in the Solar System.
The mission poses many challenges to the spacecraft designers and operations team. The thermal control design needs to accommodate the extreme variation in environmental temperature – from the significant solar heating experienced at Earth orbit to the intense cold at Pluto. The relatively low light intensity at Pluto impacts the imaging operations. One of the main factors is the light-time between the Earth and the spacecraft. A signal from Earth to New Horizons will take about 4 ½ hours, so the close approach operations cannot be controlled from Earth. This means that all imaging and other data gathering tasks have to be carefully planned prior to the encounter, and then all the time-sequenced commands required to carry out these tasks have to be uplinked to the spacecraft, so that whole process can be controlled automatically.
Closest approach to Pluto will occur at 11.50 UT* on 14 July, and data will be gathered by the spacecraft’s imaging systems – looking principally at Pluto, Charon, Nix, Hydra and looking for possible ring systems. Other information characterising Pluto’s surface composition, atmosphere and surface temperature will also be stored on board. Onboard storage is critical as there is no real-time communication with the ground.
After the encounter this stored data will need to be downlinked to Earth. The spacecraft has a 2.1 m (83 inch) diameter high gain antenna to help with this task. However, because of the extreme distance of propagation and the low power availability onboard the spacecraft, the data rate will be typically about 2000 bits per second (2 kbps). Comparison of this with your typical broadband data rate, say 20 million bits per second (20 Mbps), shows that your broadband’s performance is about 10,000 better than the downlink from New Horizons. The task is also limited because the large ground-based antennae of the DSN (Deep Space Network) need to be shared with other spacecraft missions. As a consequence, the process of downlinking the data products will probably take until the back end of 2016 – so don’t hold your breath!
But it will be worth the wait!
* UT (Universal Time) is the time system used by the astronomical community. It is essentially equivalent to GMT (Greenwich Mean Time). For those in the UK, British Summer Time BST = UT + 1, so the closest approach to Pluto will occur at 12.50 BST on 14 July 2015.
It is with great sadness that I announce the death of Peter Fortescue, who passed away peacefully on Wednesday 3 December 2014.
Peter was a good friend and colleague who I shall miss greatly. He was also an editor and coauthor of a standard textbook on spacecraft engineering and design – ‘Spacecraft Systems Engineering’, published by Wiley currently in 4th Edition, which is being translated into Russian and Chinese –
Global sales of the book since it was first published in 1991 have reached many tens of thousands of copies which makes it a ‘best seller’ for this category of book.
Peter was my ‘unofficial mentor’ when I joined the University of Southampton in 1987, and I am forever grateful for his help and kindness at that time in aiding my transition from the space industry to academia. He himself had made a similar transition from industry years earlier, and as an expert in control engineering he had fingers in many pies. But his main contribution, I believe, was that he established the now renowned Southampton courses in spacecraft systems engineering, along with John Stark, out of which grew the textbook mentioned above. Subsequently, the courses were established as part of ESA’s training programme and as a consequence many courses were presented to ESA staff over the years at a variety of ESA venues, including their technical HQ Estec in the Netherlands.
Thank you Peter for your kindness, and always giving generously of your time to advise and help. Your example was a great influence on my own development as a teacher of both students and industrial/agency engineers.
The Orion capsule, the USA’s next generation of manned vehicles after the Space Shuttle, was lofted into orbit yesterday for its first brief test flight.
Clearly, when you look at the vehicle, it closely resembles the manned capsule of the Apollo era and this ‘next generation’ seems at first thought to be a retrograde step. However, Orion is about twice the size of the Apollo capsule and can accommodate 4 astronauts in reasonable comfort. Another advantage is of course that its design integrates the latest technology into the system. But its main benefit is safety – a lesson learned from the shuttle era. In the 30 years of Shuttle operation the US human spaceflight programme achieved a great many spectacular successes, but it also saw some tragic lows with loss of 2 crews in 1986 and 2003. The issue was the Shuttle’s complexity – although this allowed great flexibility in mission capability, it also made the vehicle dangerous. This was primarily because it had no genuine escape system for the crew.
Taking onboard these issues, the system designers of Orion have gone for relative simplicity in order to improve crew safety. The final launch vehicle for the Orion spacecraft – which is yet to be developed – will have a simple multi-stage configuration with an escape tower system to drag the manned capsule clear in the event of a catastrophic launch failure. Also the heat shield configuration of Orion is simpler than Shuttle, and protected from debris impact risk on launch, so reducing risk on re-entry into the atmosphere. So the philosophy is to launch people on a ‘simple’ and very reliable expendable launch vehicle, and launch the required hardware for the particular mission separately on a relatively less-reliable heavy lift launcher – the two component parts coming together and docking in orbit afterwards.
Coming back to yesterday’s test, the Orion vehicle was launched at 12.05 UT atop a (stand in) Delta 4 heavy lift expendable launch vehicle (see video).
The system then executed 2 orbits of the Earth prior to a re-entry and splashdown at 16.30 UT. The second orbit was elliptical with a high point of 5,800 km to increase the re-entry speed to around 30,ooo km per hour – the resulting 2,000 degrees Celsius providing a rigorous test of Orion’s thermal protection system. The principal objectives of yesterday’s events were to test the capsule’s heatshield and parachute systems, both of which performed successfully. A NASA summary of the Orion system in general, and yesterday’s test in particular is given in the second video.
Although yesterday’s test was encouraging – the US human spaceflight programme is up and running – nevertheless there is long way to go. The Orion system comprises two main components – the capsule which was tested yesterday and a service module which is yet to be built. The service module (again similar to its Apollo predecessor) is the necessary system required to provide essentially life support and propulsion for the Orion capsule. The European Space Agency (ESA) and Airbus recently signed a contract with NASA to build a service module based on ESA’s Automated Transfer Vehicle (ATV), and this is scheduled to be ready for an unmanned test flight in 2017/18. You may recall an earlier blog post about this (24 September 2014) ‘Chance encounter with a rocket man’ when I ‘bumped into’ a NASA engineer on a walking holiday in West Wales who was working on adapting the Space Shuttle orbital manoeuvring system engine for use on ESA's ATV?
And then of course there is the requirement to develop a man-rated launcher for the Orion capsule/service module system. This is currently in the hands of US private industry, and a first unmanned test flight of this element is pencilled in for 2017/18. The first crewed launch of the whole system is not expected until around 2021, so don’t hold your breath! Given that the last flight of Shuttle was 2011, that’s a hiatus of 10 years in the US human spaceflight programme which is disappointing, in my opinion. The pace of events seems to be very slow, and with the vagaries of funding for such projects with changing political administrations in Washington DC, it does make you wonder whether the US will ever acquire sufficient momentum to take the next major steps in solar system exploration. I say this not as a critic of the US space effort, but as a supporter who wishes they’d just get on with it!
Now that the ‘dust has settled’ a little on the Philae landing on 12th November – what next for Philae?
Someone asked me recently why the lander was called Philae. Despite knowing all the ins and outs of the landing operations and onboard instruments, I had to admit that the answer to this question escaped me! Thank goodness for Google – in case you didn’t know, apparently Philae is an Egyptian island in the river Nile. An obelisk was discovered at the Temple of Isis on the island in 1815 by explorer and egyptologist William Bankes, and the hieroglyphs and Greek inscriptions on the Philae obelisk helped 19th Century scholars unlock the secrets of the Rosetta Stone and the language of ancient Egypt. However, I digress … getting back to the 21st Century Philae lander residing on the alien surface of a comet half a billion kilometres away.
While Philae made its descent to the comet’s, the orbiters narrow-angle camera tracked the ‘fridge-sized’ landing craft as it headed for its first touch-down. Below shows an amazing time-tagged mosaic of images of this event.
After its two rebounds from the surface, Philae finally came to rest on third touchdown in a rather unfortunate shady spot, surrounded by boulders and/or cliffs. And to make things more interesting it settled in a ‘non-nominal attitude’ – that is, not sitting comfortably on its three spindly legs as intended. This aspect has unfortunately complicated the deployment of some of the post-landing experiments, with the consequence that not all the intended surface science was achieved. However, the scientists managing these experiments assured the waiting media that at least 80% to 90% of the science was achieved, and indeed a great deal of scientific data were received during the 60 hours or so before the lander’s power source was exhausted
The Philae power system comprises a primary (non-chargeable) battery system intended to give 60 hours of surface operations, a secondary (chargeable) battery system for extended surface operations and a primary power source in the form of body-mounted solar arrays to convert sunlight into electricity to charge the secondary batteries. However, due to the cold shaded place where Philae came to rest, there is insufficient sunlight to charge the batteries. The realisation of this situation after landing meant that the surface science teams had to work rapidly to devise a comprehensive plan to achieve as much as possible in the brief period before the primary battery expired – allowing some residual power, of course, to ensure that the results could be uplinked to the orbiting Rosetta spacecraft. Fortunately a great deal was achieved, and as the power levels dropped, Philae executed an automated process to put itself into hibernation. So the lander is not actually ‘dead’, and contrary to ‘popular opinion’ the lander mission may not be at an end. As the comet approaches closer to the Sun (closest approach between the orbits of Earth and Mars in August 2015), Philae’s temperature will rise, and solar intensity will increase by a factor of about 15. So there is a chance that we may hear from the surface of the comet again. Personally, I’m not sure how likely this is, but I probably wouldn’t bet on it – but it is possible.
In the meantime, the science teams have oodles of results to mull over in the coming weeks and years. Although some of these have been discussed in the media, many of them will require a great deal of work and further interpretation to provide a definitive view of the comet’s surface environment.
The results most highlighted by the media so far have been the claim that ‘carbon-based organic molecules’ have been detected independently by two onboard experiment packages. The COSAC instrument (COmetary SAmpling and Composition experiment) is designed to ‘sniff’ the comet’s thin atmosphere. If you’re into this stuff, COSAC is based upon a Gas Chromatograph/Mass Spectrometer (GCMS) system. The instrument’s principal investigators (PIs) could not definitively indentify exactly what molecules had been detected at the moment, and that they were ‘trying to interpret the results’. Another GCMS-based experiment called ‘Ptolemy’ later confirmed the COSAC finding, the PI commenting that “there is a rich signal there … there is clearly a lot of peaks (in the chromatograph’s trace). Sometimes a complicated compound can display a lot of peaks”. Ptolemy was programmed to ‘sniff’ the environment immediately after first touchdown, so presumably the observed surface impact cloud gave it plenty of raw material to work on.
Results concerning the nature of the surface were returned by an instrument called MUPUS (MUlti-PUrpose Sensors for surface and sub-surface science). This experiment deployed a hammer, which detected a dust layer about 10 to 20 cm deep overlying what is believed to be a hard surface of water ice. At the extremely low temperatures currently being endured by Philae, water ice is very hard – having characteristics similar to sandstone. Another key objective of MUPUS was to drill a sample of ‘soil’ and deliver this to COSAC’s oven for analysis – but unfortunately no such sample was delivered.
Reading all this back, I have to say the tone seems a little negative – but let me say that this is not intended. I think it’s amazing what has been achieved under very pressured and trying circumstances. And also it really is too early to understand the huge amount of information that Philae’s 60 hours of surface life has given us. It will take a while for it all to be assimilated, but I think there will some interesting and unexpected outcomes over the months to come. Congratulations to all on both the engineering and the science teams involved in the Philae landing mission!
Those of you who have perused the pages of this blog may have noticed that I am also involved in another book, apart from How Spacecraft Fly, which is a technical textbook called Spacecraft Systems Engineering (SSE) published by Wiley. SSE is now in 4th Edition and is soon to be translated into Russian and Chinese. In the context of Wiley’s business, as a publisher of textbooks, it is considered to be a ‘best seller’. But (unfortunately) this does not mean it sells millions like Harry Potter – so I’m not going to be a millionaire any time soon. However, it does sell globally in the thousands each year.
I’ve been involved in SSE for the 3rd and 4th Editions as principal editor and coauthor – the book has about 700 pages so I haven’t written the whole lot. I share the authorship with a merry band of about a dozen technical experts (to whom I am very grateful), so that we can cover the engineering design of spacecraft at a level which is hopefully useful to people involved in the process. So unless you like equations, or maybe you’re having trouble sleeping (!), you shouldn’t consider buying a copy. It also received an award for scientific literature, which required me to make a very enjoyable trip to Vancouver in 2004.
As the main editor, I enjoy the opportunity to provide input on the matter of book covers, and for the current edition we chose a rather splendid depiction of the Rosetta spacecraft. Remember – this was 2011, and we had no idea whether the ambitious and risky mission envisaged for this spacecraft would be achieved. So the question – “would we have a failed spacecraft mission on the cover, come 2014?” – did cross our minds. However, recent history has affirmed our choice as a good one, thank heavens. Similarly the 3rd Edition cover (published in 2003) featured an event not due to take place until July 2004 – the Saturn orbit insertion burn of Cassini – an NASA/ESA mission. Again would the event depicted be successful? Fortunately, once again, the answer was ‘yes’!
If SSE goes to 5th edition, upon which mission should we bestow a good omen by featuring it on the cover?!
The inside story of the Rosetta mission to date - an ‘interview’ with Paolo Ferri, Head of the Mission Operations Department at ESA
Paolo Ferri has been a colleague and friend over a number of years now, since he was a contributor to the Space Systems Engineering courses I used to organise (prior to my retirement in 2010) at ESA Estec, ESA’s Technical HQ in the Netherlands. As Head of the Mission Operations Department at ESA’s European Space Operations Centre (ESOC) located in Darmstadt, Germany, he has managed the team involved in the Rosetta mission since its launch in 2004. Consequently, he has fronted many of the recent PR events as a consequence of the amazing success of the Rosetta spacecraft and Philae landing missions. News of these events has reverberated around the world, and made a huge PR impact for ESA, which is very much well-deserved – Rosetta in my view is the most daring and complex robotic mission ever.
I always think of Paolo as a good friend, but also a very humble guy. Now that he has established his place in the astronautics history books, I’m sure he will handle his new-found fame with good humour and humility as he has always done in the past. Characteristically, despite the fact that he must be very tired after his recent very long working days (did he get any sleep in the couple of days leading up to the Philae landing on 12 November?), he has agreed to provide an input to this blog, for which I am very grateful. The format is along the lines of questions (me) and answers (Paolo), so I hope you enjoy this brief insight into recent events from the ‘horses mouth’, so to speak!
GS: Looking at the mission overall, what were the most critical events?
PF: The two things that really worried me beforehand were the hibernation phase and the initial comet phase, the former since we had to leave the spacecraft alone for 31 months, without any contact. No mission had done this before its primary scientific phase. Also, Rosetta is a 3-axes stabilised spacecraft, and to leave it alone we had to spin it up so that we could deactivate the on-board attitude control. Finally we were at distances from the Sun that no solar-powered spacecraft had ever reached before. The reactivation on 20th January this year was probably the most crucial moment in the whole mission – no signal would have meant a complete loss of the mission. Receiving the signal was big relief and a great joy!
The initial comet phases were also critical because once again Rosetta had to do something no one had tried before – flying in the proximity of a comet. This is very different from normal spaceflight. The comet is a small object, with a faint and very irregular gravity field. Additionally it also has gas and dust around it that escapes with high velocity. We had to create a model of the dynamical environment and of the comet (mass, gravity potential, rotation axis, centre of mass, gas and dust flow dynamics, surface characteristics, etc.) such that we could properly fly around it. And to build this model we had to fly around it!
GS: The rendezvous manoeuvre with the comet was the first such event for the Rosetta mission, after the previous fly-bys. What were the special challenges?
PF: The rendezvous manoeuvre after the exit from hibernation was large (a ‘delta-V of about 800 m/s was required) and had to be done with small thrusters (4 thrusters, each with a thrust of 10 Newtons). So we had to split it into many burns (during the period May to July 2014), the longest of which was more than 7 hours. Before hibernation we had done a similar manoeuvre and had experienced several anomalies in the behaviour of the thrusters, so we were very worried this time. This concern was made more acute since now the thrusters were performing in worse conditions (the feed pressure in the propulsion system now being lower than before). However, thanks to a re-tuning of the thruster controller software that we performed in February, everything went very smooth, much to everyone’s relief.
GS: Is the spacecraft in good shape after its 10 year journey?
PF: The spacecraft is very healthy. We had two major problems in the early years:
1) a leak in the high pressure part of the propulsion system. This was isolated, but it prevented us from attempting a re-pressurisation of the tanks for the recent rendezvous manoeuvre. This meant that we have to carry out the rest of the mission in “blow-down mode” – that is, the operating pressure of the thrusters decreased with the fuel consumption (as the tanks contain less fuel and the gas that provides the pressure expands to a larger volume). For the time being the thrusters are working very well at around 7 bar of pressure (they were designed to work at 17 bar and are qualified down to 10 bar).
2) two of the reaction wheels started to show a noisy behaviour that is known as "cage instability". Over the years of hibernation, using an engineering model on the ground, we developed a new strategy for the operation of the wheels at low speeds, and this seems to be working, with no sign of further degradation over the past year. Also, just in case, we have developed a back-up attitude control mode that uses 2 wheels and thrusters, in case two of the four available wheels break down.
GS: What did you think when you first saw the comet nucleus close up?
PF: I can't describe the feelings. My first rational impression, though, was that this surface is ‘alive’, and very young – that it keeps changing shape and appearance very quickly. This is plausible, as the processes that change it are linked to the revolution period around the Sun, which is only 6.5 years. I am convinced the surface will change rapidly in the coming months and we will see a different comet after perihelion (closest approach to the Sun) in August 2015 compared to the one we see today.
GS: The comet orbit phase is unusual due to the very 'low energy' spacecraft dynamics. Again what were the special challenges?
PF: See my explanation above related to the comet modelling. Another challenge was the precise navigation required. Normal radio frequency measurements are not enough to navigate around the comet, since what counts is the relative position and velocity between Rosetta and the comet, and the comet has no radio transponder! So our flight dynamics experts had to develop optical navigation techniques. They used images taken by the on-board cameras to identify and recognise landmarks on the surface. They continuously compared the landmark positions over successive pictures and managed so to perform "triangulations" that allowed us to reach the necessary navigation accuracy.
GS: Why did you choose such a long descent duration for Philae, given that the unpredictable disturbances caused by the comet’s environment build up more over the longer time period?
PF: There were two main reasons:
1) the lander separation mechanism is designed to impart a variable ‘delta-V’ to the lander in the range 0 to 100 cm/s (well, more or less in this range). However it also has an emergency mechanism, based on springs, that was designed to ‘jettison’ the lander in case the prime mechanism did not work. This emergency system gives the lander a fixed ‘delta-V’ of 18 cm/s. Following intense discussions in the last year the lander team requested us to find (if possible) landing strategies that would require a delta-V of 18 cm/s, such that, in case the prime mechanism did not work, the emergency system would still provide the lander with the same ‘correct’ ‘delta-V’ for the landing. Given this decision, we had to find a trajectory that allowed landing with a small (18 cm/s) difference in velocity between the lander and the orbiter at separation. At the same time we did not want Rosetta to perform the separation on a ‘kamikaze’ orbit – i.e. one that would bring the spacecraft to a collision with the comet in case something went wrong with the planned post-separation orbit manoeuvre. So we had to separate very far away from the surface (22.5 km), such that Rosetta flew no closer than 2.5 km from the surface in case of post-separation problems.
2) the second reason is related to the accuracy of our navigation. You are correct in saying that a longer descent will accumulate larger errors in the lander trajectory. However the errors in the Rosetta navigation, i.e. the accuracy with which we can predict the position and velocity of Rosetta at the time of separation, are much smaller if we fly at larger distances from the comet. So, in the end the higher altitude reduces the size of the landing error ellipse (the largest contributor to it being the accuracy of the position, velocity and direction of the lander at separation time, which are all dictated by the Rosetta orbit).
So, that's the reason why in the end we had to drop the lander from such and incredibly large distance. It wasn’t easy, but our Flight Dyamics people are incredible - they assured me that we wouldn’t miss the comet, and I trusted them!
GS: What are the most significant future events from an operations point of view?
PF: After the end of the Philae relay phase, Rosetta will stay in a 20 km orbit for several weeks. Then, depending on the comet activity, it will increase the distance but continue following the comet in its journey towards the Sun. After perihelion in August 2015, we will try to go closer again as the activity decreases.
GS: What will we learn from the science?
PF: Well, it's not to me, the bus driver, to say what my passengers are up to! However in general Rosetta is about learning everything about comets. And since these objects are among the oldest objects in the solar system, the aim is to attempt to understand from what type of material the solar system was made, how it evolved, and how the planets formed. Also, are comets the source of the abundant water on the surface of the Earth, and maybe of life?
GS: ESA has truely 'come of age' with missions like the Titan landing in 2005, and now the Rosetta mission, which I rate as one of the most complex robotic missions. Why, in your view, does ESA not get the same kudos as NASA in the general sphere of astronautics? Is it a deficit in the ESA PR effort?
PF: I think Rosetta has changed this situation in the past few days. The resonance around the world, even in the US, was incredible.
GS: What does current analysis tell us about what actually happened at touch-down?
PF: We are learning more and more about this. We also have incredible pictures that show the lander flying over the surface after having touched down once! We are also close to finding the exact place Philae ended up after its two long jumps (it is very far from the initial touch down point!). What happened is simply that the harpoons did not fire at all. So Philae bounced, keeping its attitude since the flywheel was still rotating. However the first touch down on the surface automatically stopped powering the flywheel, which slowly reduced its rotation, thereby transferring angular momentum to the Lander body, which started rotating. After the first long jump that lasted almost 2 hours, Philae touched down once more and bounced again, but this time for a very short jump of 6-7 minutes, before stopping in its final position, most likely leaning against a rock or the wall of a crater. Fortunately we never lost the signal, and we had all the 5 foreseen contacts during the 2.5 days on the surface. We were less lucky with the illumination conditions, which in the final location are very bad. So after the primary mission on batteries is over, we most likely won't have much of a chance to recharge the batteries. However this was always considered a ‘bonus’ – the solar cells were anyway too weak to perform many useful operations, even in best illumination conditions. We will learn more (and perhaps we'll have to correct the current understanding as I described above) once we get a final localisation with the Osiris pictures.
GS: Finally Paolo, will you see more of your family and friends post-landing?
PF: While Philae was active on the comet I spent most of the time in ESOC. I was with Philae on the comet, and slept 10 hours in 3 days. My family and children saw me only in TV interviews. Now that Philae sleeps I am at home, answering congratulation messages and catching up with work that last week had to wait for this historical phase to finish. So again not much for my family in this period I’m afraid. My team is still very busy with the continuation of the Rosetta mission, which is challenging enough even without having to take care of Philae in parallel. But I hope they can get some rest and family time over Christmas!
Rosetta landing mission update 13.10 UT Thursday 13 November: a (rather confusing) image taken by the Philae lander from the surface of Comet 67P. The end of one of the 3 landing legs can be seen in the foreground. Current analysis suggests that Philae landed at 15.33 UT yesterday but then bounced to an undetermined height (100s of metres?) before landing again at 17.26 UT nearly 2 hours later. It then bounced again before settling a few minutes later at 17.33 UT. As far as I know, it is still not anchored. Also I don't think its precise position on the surface is yet known - the comet will have rotated while the lander was 'airborne' for 2 hours after the first bounce, so it will not be anywhere near the first touchdown site. I think there is a degree of confusion, which will hopefully be resolved soon. Despite this however, still an amazing achievement!
Facebook post 10.00 UT Wednesday 12 November: Image of the Philae lander taken from the Rosetta spacecraft, as the lander began its descent at 08.35 UT this morning. All antennae and landing gear in place
Facebook post 14.40 UT Wednesday 12 November: Rosetta lander mission update 14.40 UT: ESA confirms that the link with the Philae lander has been established successfully - comms with Earth has to be via the Rosetta spacecraft in orbit. The dynamics team have just confirmed that the lander is on the correct trajectory. Finally landing is expected at 15.34 UT, so confirmation signal should be received at about 16.02 UT. Everything is going really well - although of course noone has control over what the lander may encounter on touchdown.
Facebook post 16.15 UT Wednesday 12 November: Rosetta lander mission update 16.15 UT: we have confirmation that the lander is on the surface of the comet and providing telemetry. So looks like we're down and alive - absolutely amazing!
Facebook post 16.45 UT Wednesday 12 November: Rosetta lander mission update 16.45 UT: a more detailed (but still early) look at the state of the Philae lander indicates that it may not be securely attached to the surface - no indication that the harpoon anchors have fired. This is causing some concern, and the lander controllers have a difficult decision as to whether to attempt to fire the anchors again. The lack of security is an issue as, due to the very low surface gravity, the weight of the lander is tiny despite the fact that its mass is around 100 kg. So firing the harpoon anchors now may make the situation worse. They are still expecting to receive imagery from the surface this evening.
Graham Swinerd - I hope to use this page to highlight current major events in space and spacececraft.