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?!

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.

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.

More recent Facebook posts today Wednesday 12 November 2104:

Posted at 08.25 UT:Rosetta mission update 08.25 UT Wed 12 Nov: separation of the lander Philae is due in 10 mins. Because of the distance of Rosetta from Earth, confirmation signal will be take 28 mins to get to Earth, so earliest we'll know about the separation is 09.03 UT. It is planned for Philae to take a farewell picture of Rosetta, so hopefully I can post that sometime today. Decision to go for landing was taken despite a problem on the lander. There is an upward facing cold gas thruster (i.e. small rocket engine) designed to fire on contact with the surface to stop the lander bouncing in the very low gravity. This could not be activated. However there are backup systems - ice screws and harpoons - so fingers crossed - it's going to be a long day.

Posted at 09.05 UT:

Rosetta mission update 09.05 UT: telemetry has confirmed that the Philae lander has separated from the Rosetta spacecraft, and is on its way down to a landing on the icy surface of Comet 67P in about 7 hours time. From here on everything is automated, and governed by Isaac Newton (i.e gravity).

A busy time for the operations team at the European Space Operations Centre (ESOC) in Darmstadt, Germany.
The above video may help in understanding what's going on today - 12 November 2014 (landing day hopefully).  An element of marking time is provided by the comet nucleus, which rotates about every 12 hours.

Recent updates that I've posted on Facebook - today 12 November 2014 is hopefully the day when the robotic lander Philae will land on a comet nucleus.

Posted 22.45 UT, Tuesday 11 November:  Rosetta comet landing update at 22.45 UT (UT=GMT). First Go/No Go decision was a Go at 19.00 UT, which was to do with the orbit aspects - Rosetta is confirmed after orbit determination to be on the right trajectory for lander delivery. Next Go/No Go decision is at 00.00 UT, which will look at the health of the lander, and the readiness of the uplink of commands to the lander. If all goes well separation will be at about 08.35 UT tomorrow (Wed), with touch down around 16.00 UT  tomorrow afternoon. Sleep well everyone.

Posted 06.15 UT, Wednesday 12 November:Rosetta comet landing update at 06.15 UT, Wed 12 Nov: midnight Go/No Go decision was a Go - commands to control lander delivery were ready to uplink. Next decision concerning the health of the lander came at 03.00 UT which again was a Go - amazing given that the lander has been dormant for the 10 year journey. So all's well at the moment for a separation of the lander at breakfast time (UK) today. The operations team are manouevring Rosetta into the delivery trajectory as I write. Follow live ESA webcast at http://rosetta.esa.int/.


Posted 07.05 UT, Wednesday 12 November:

Final decision to Go for separation and landing just announced by the Main Control Room in Darmstadt (07.05 UT). Separation will be at 08.35 this morning. Confirmation of landing (or otherwise) should come at about 16.00 this afternoon. All we now is some luck at touchdown - no boulders or cliffs please.

Prepare to witness history in the making!  After its 10 year journey, the culmination of the Rosetta mission will take place in a week's time with the separation of the Philae lander from the 'mothership' during the morning of Wednesady 12 November.  The lander will head for the primary landing site, called Agilkia (after an island in the River Nile) - see images of site in the 'Picture gallery' section of this website.  If all goes well, the robotic lander will a uplink a confirmation of a successful landing to the orbiting mothership, which will then be relayed to Earth. This should be received by about 16.00 GMT on Wednesday 12 November.

I wish the European Space Agency, and the merry band of spacecraft operators at ESOC in Germany, every success in this last crucial event of the Rosetta mission! 

This time last week, by coincidence, I was busy compiling an entry for the Virgin Galactic/Landrover competition to win a sub-orbital space flight on Virgin's Spaceship 2.  However, due to recent tragic events, I noticed today that the competition web page has understandably been suspended until further notice. 

Despite the recent accident, surprisingly I still felt 'up for it' - although I think I would now have great difficulty in persuading 3 of my FaceBook friends to accompany me in the unlikely event that I actually won the competition.

Anyway, this blog post is not about me.  It's just a rather inadequate expression of condolence for the families and friends of the 2 test pilots who were involved in the recent accident, when the vehicle broke up during a rocket powered flight test.  As most of you may know, Mike Alsbury was killed, and Peter Siebold was seriously injured in the accident.  Both were around 40 years old, and married with 2 children.  Although, as test pilots, they were very much aware that their jobs entailed risk, nevertheless this does not soften the blow for their families.  I for one hope that this project continues, and is ultimately successful in being a first small step in opening up the new frontier of 'space' to tourism.  Maybe this vision can be a worthy legasy for Mike Alsbury?

I felt a picture of the vehicle (above) in magnificent flight is much more appropriate than images of debris on the desert floor!