Commissioning update

The Gaia project team provides an update on the ongoing commissioning activities of ESA’s billion star surveyor…

The work done to bring online all components of the Gaia service module, which houses equipment needed for the basic control and operation of the satellite, has gone very smoothly. The chemical and micro propulsion systems function well, with the latter providing tiny (micro-Newton) thrusts to maintain Gaia’s spin rate, compensating for torques due to solar radiation pressure. The phased array antenna is operating very well, ensuring that we can maintain the high data rates that are needed to downlink all the science data. And the essential rubidium atomic clock is also working to specification.

Exploded view of Gaia spacecraft. Click for more information.

Exploded view of Gaia spacecraft. Click for more information.

The Gaia scientific payload is also functioning very well. This includes all 106 CCD detectors and the associated electronics units, as well as the seven on-board computers that manage the CCD’s. Alignment and co-focusing of the two telescopes through their movable secondary mirrors is working as expected. Following the last displacement of one of the secondary mirrors by just 3 micrometres, we are currently at the optimal image quality that Gaia can deliver, well balanced across the large focal plane and the three instruments. This is no small achievement considering the complexity of the optics!

However, a few other aspects of the commissioning have been progressing somewhat less smoothly.

In order to deliver exquisitely precise measurements of the positions of stars on the sky, we need in turn to know where Gaia itself is in space very accurately at any given moment. The distance part of Gaia’s orbit is readily determined from radio signals sent back and forth, but the position on the plane of the sky needs ground-based telescope observations of the satellite.

It turns out that Gaia is much fainter in the sky than hoped for, at magnitude 21 rather than 18, and thus the smaller 1 metre diameter class telescopes planned to be used by Gaia’s GBOT network  are not big enough to detect Gaia in a reasonable amount of time. But by shifting the bulk of the observations to the 2.0-m Liverpool Telescope on La Palma and ESO’s 2.6-m VST on Paranal, as well as introducing Very Long Baseline Interferometry radio measurements, the problem is now under control.

Near the beginning of commissioning, a steady drop in the transmission of Gaia’s telescopes was seen, due to water-ice deposits building up on the mirrors as trapped water vapour was liberated from the satellite after launch. The transmission was fully recovered following a decontamination campaign, during which the payload was heated to remove the ice from the optics.

Annotated diagram of the Gaia payload module. Click for more information.

Annotated diagram of the Gaia payload module. Click for more information.

Ice deposits are thought to play a part in another concern, in which unanticipated ‘stray light’ is seen hitting parts of the Gaia focal plane. Some of the stray light is thought to come from sunlight diffracted around the edges of the sunshield and entering the telescope apertures. There also seems to be a smaller contribution from night sky sources reaching the focal plane via unexpected paths.

Although the diffracted sunlight component was foreseen, we think that it is enhanced by reflections off ice deposits on the ceiling of the ‘thermal tent’ structure surrounding the payload, allowing it to reach the focal plane. It was hoped that the decontamination campaign would also remove this ice layer, but unfortunately the stray light is still there at the moment.

Careful preparations are being made for one more attempt to remove the water ice and, hopefully, the stray light. But in parallel, we are now continuing with the nominal commissioning and a detailed performance verification phase. Even if the stray light remains, the current best assessment is that degradation in science performance will be relatively modest and mostly restricted to the faintest of Gaia’s one billion stars.

We will, of course, provide an update on the blog when we have new information to share.

Posted on behalf of the Gaia project team.



  • Russ Ravella says:

    Finally! Thank you for the update. There are quite a few frustrated people (who foot the bill for projects such as Gaia) who have been “waiting in the dark” for far too long.
    Two questions re/ the stray light issue: 1) if the ice decontamination procedure doesn’t work, is there no other remedy? And 2), what does “… degradation in performance … relatively modest .. restricted to the faintest of Gaia’s stars …” mean when the whole basic POINT to Gaia was exactly the ability to collect that faint star data?? Also, how did something as fundemdntal as non-sequential ray tracing (including potential environments) in a faint light optical instrument not get done properly during design in the first place?? How about a non-political, straight forward, complete and accurate, honest response?

  • F. R. Velluv says:

    I just wanted to express my indignation with the current ESA public relations policies. It’s just intolerable to keep us feeding off rumors for months as if we lived in a dictatorship. It reminds me of long ago when I tried to collect information about the soviet space program through almost clandestine means in the 1980’s.

    Anyone with a minimal knowledge of Optics will tell you that a “stray light” issue in such a delicate instrument can range from the trivial to the catastrophic. I thought it was more on the catastrophic side given the ominous silence for WEEKS. Now after this “update” a really faint ray of light has finally reached my brain and all I can say is I haven’t the slightest idea if this thing is going to do its job or not.

    Can anyone up there among the fat cats of ESA Public Relations Office or whatever its name is, you know, the geniuses who can tell you with a straight face that losing HALF the information from HUYGENS is no big deal, can you PLEASE give us we poor taxpayers some REAL information once and for all? I mean numbers, real data, opinions from inside people… Can you spell “taxpayer” by the way? Can you grasp the very concept of Public Opinion? Do you have any respect for the lots of people in academia across Europe and everywhere, every astronomer biting their nails for months?

    OK, end of rant.

    • Nic Tamer says:

      A dictatorship ? Everything that is exaggerated is insignificant.
      You want to know “if this thing is going to do its job” ? Well, that’s exactly what a satellite commissioning is supposed to do. And there may be reasons why this takes time. You will know the fate at the end of the commissioning, like everyone else.
      Most probably, if there are no news yet, this must be because there is nothing new to report, no? Indeed, someone from ESA public relation could be paid (with public money) to communicate some useless news. Understood: you must be a taxpayer who complains paying not enough taxes!

  • KD says:

    Thanks for this welcome update.
    While I share frustration about the too-long absence of news since you announced the stray light issue, I find the “rant” in the previous comments quite excessive …
    I’m confident you do your best to manage optimally the mission as it is, and understand well that it is not anormal that such a delicate mission get some difficulties / unexpected issues.
    Reading calmly your update (frustration doesn’t help for that ;-), I think we get the answers asked in previous comments : Gaia works very well, focusing is done, stability good, CCDs working perfectly, etc… And for stray light, this diffuse light poses some problems for faint stars but the impact should be “modest” : I guess it may impact spectroscopy of faint stars (near mag 17) and possibly (?) positionning of stars near mag 20 when there is stray light (ie a few % of the time).
    That’s in my sense the big question and frustration about your “modest” impact and the lack of details: could you give please some better assessment of the “impact” as you understand it today?
    Thanks a lot, sincerely

  • Russ Ravella says:

    The “rant” in the earlier post is not only not excessive, it pretty accurately reflects the opinions of a whole lot of really, really frustrated people around the world. ESA has done an absolutely abominable job. As it always has. And people – the ones who PAY THE BILLS – have finally had enough. That is the current state of things. Like it or not.

    • Doug Schiffer says:

      I am happy to see an update on the GAIA commissioning effort.

      As for “rants” – too many people have no idea how complicated a project as sophisticated as GAIA can be. They expect quick answers and smooth progress – when there can be technological hurtles in all manner of things – component failures, launch difficulties and yes, design mistakes. If you think that something as complicated as GAIA can happen with no design errors – you are being foolish.

      Here’s hoping for speedy finishing up of the shakedown phase – and uneventfully getting to data acquisition. I for one am looking forward to a vastly improved galactic map.

      Best wishes to all on the GAIA team!

  • Russ Ravella says:

    Of course, the issue has absolutely nothing whatsoever to do with how difficult and complicated Gaia is. Everyone is perfectly aware of that. The issue is how poorly ESA has handled communications about the mission. As if the people to whom they are ultimately responsible are irrelevant. The people who pay for Gaia and who pay their salaries.

    Also, I happen to have 35 years of experience in space based optical instruments and am fully aware of the difficulties involved. Reducing funding for stray light analysis, then paying the price on orbit, is literally a classic programmatic error. I suspect that scenario played some role here. Those with actual experience in this area would have strongly counciled ESA to pay especially close attention to stray light on a mission like Gaia. And I’ll bet that happened and was ignored. ESA needs to start actually feeling “responsibility” so this never happens again. Their handling of it so far reveals just how little sense of responsibility their senior management actually feels.

  • Mark McCaughrean says:

    Thanks to the previous commenters for their feedback on this post. We welcome your input and understand your frustration, but as you can all appreciate, the commissioning and testing of a major astronomical satellite is a very complicated business.

    We’ve been trying to keep the public informed of progress and problems in broad terms, but hopefully it’s understandable that we can’t always post “complete and accurate” information, when that information is still being pulled together in a time-consuming and painstaking way by the scientists and engineers devoted to getting Gaia ready for science operations.

    For example, it takes days to heat the satellite for decontamination and then weeks for it to cool again for performance tests, and the tests themselves also take days and weeks to complete, analyse, and re-run if needed. The sometimes long gaps between updates on this blog reflects all of that.

    On the request for more something more quantitative, we’re only now entering the full performance verification phase, and so it’s not yet possible to give definitive numbers on scientific performance. Even then, the stray light varies with time as the satellite rotates and is not uniform across the focal plane or with position on the sky, and its impact will not be the same for all stars or all modes. So there are lots of variables in play.

    But what we do know is that the diffuse stray light currently seen is not a significant issue for the (relatively) brighter stars for which we can measure the most accurate positions, parallaxes, and motions, and which are thus used to reach many of Gaia’s key science goals. At the faintest end for both astrometry and spectroscopy, there will be an impact on the accuracy of measurements if the stray light remains, but current understanding is that that will be modest.

    But being more quantitative will have to wait until the entire commissioning campaign is complete: that’s what it’s there for. Hopefully that’s understood.

    To be clear, many people in the ESA project team, in industry, and in the wide scientific academic community involved in Gaia are fully engaged and informed, and are all working very hard to ensure that Gaia performs as well as possible.

    On the communications side, we don’t think we’re being evasive or “political”. The aim is to keep the public broadly informed of progress and all of the Gaia blog posts have been written directly by project team members themselves, including the Project Manager, Giuseppe Sarri, and the Project Scientist, Timo Prusti, or with their direct input, feedback, and approval.

    But it’s not obvious that sharing all of the detailed engineering and scientific data and our continually evolving understanding of it in the public domain would help the team get their work done any quicker or better, towards getting Gaia ready for science: that has to be their main priority at the moment.

    Prof Mark McCaughrean
    Senior Science Advisor
    Directorate of Science & Robotic Exploration
    European Space Agency

    • Jeff Green says:

      You may very well not be able to post complete and accurate information and it takes a long time to heat the spacecraft and longer to cool it, but it only takes a moment to make a blog post saying. “We have started a heating/cooling cycle to attempt to remove water ice contamination, it is unlikely we will be able to post any meaningful results from this until … ”
      It should be a firm project aim to have at least one authoritative blog post a week.

    • Richy says:

      funny how all these people think they are paying the wages of the ESA the tax paying public also pay for domestic waste collection. they think the are paying the bin man personally and think they own him talk like shit to him, but he is taking your rubbish away so you dont have rats and diseases your 1 doller a year he gets from you is nothing, without him you would drown in your own shit. without science you would be drowning in ignorence. this project is very difficult to do and you can only expect that there are going to be problems and it will be amazing if it works 100 % it was worth the risk and it has cost you and me very little. lighten up its a 5 year project just wait and see instead of coplaining and moaning and biting the hand that feeds you, information wise grrrrr

  • Russ Ravella says:

    Ok, now this is getting VERY frustrating. ESA, you have to be kidding with that patronizing, condescending response to serious, absolutely legitimate criticisms of your clearly inadequate and frankly botched communications on this project. You didn’t seriously address a single substantive issue – just more of the same BS you’ve been feeding the people paying your salary. More of the same vacuous fluff that has so many people upset in the first place. No wonder people are so angry. Given the tremendous financial pressure were all experiencing, I wonder how your cavalier attitude will effect public support for ESA budgets going forward. Perhaps this money can be better spent…

    • F. R. Velluv says:

      Relax, Russ. At least they haven’t switched into Clandestine Mode yet. Maybe they decide now to forget the blog, the twitter account and all these enormous sources of information, go Planck, and go on with it like an underground antiterrorist operation. In a few years your thirst of knowledge will be satisfied. Or not. Relax.

      -What’s going on with the mission? Anybody knows anything?

      -What, Gaia? That thing they launched years ago? Oh yes, I remember. Sorry, no information. There’s a total Planck-out, you know.

      Besides, in this case they don’t have to worry about somebody showing up in the South Pole or with some balloon somewhere beating them to the finish line.

      What amazes me most is how on Earth can I get almost real-time data from missions including some obscure ones, built, launched and payed by guys from Minnesota than from european missions. And I’m not talking about Gaia. This is an old story going on for years now.

      But even more amazing to me is the fact that I could follow almost DAY BY DAY the disaster with Galileo, I remember back in ’93 with no web, no nothing, when I was still in college, from text based terminals and time-sharing systems, you know the problem with the high gain antenna not opening, all the things they tried, shaking up the spacecraft, changing the attitude to heat it up and so on…. Never EVER I felt I was not being informed, Press releases were somewhat optimistic as always but the basic data was shared with the public and I was able to get it thousands of miles away for free and up to date.

      Even with HIPPARCOS I’d say I had more information. By means nobody would ever believe today like listening to foreign radios, signing up for mailing lists when mail was MAIL, not e-mail…. I remember some day that summer thinking ‘today there is another perigee burn’. I would calculate approximately the time with my 8-bit computer, then to get the bad news a few days later reading some foreign newspaper or something.

      Can anybody tell me please what’s the use of so many “twitter” accounts, “blogs” and so on if there’s no intention of sharing information with the public? What is this, China? Why all that Chernobyl mentality, you know, nothing happens, there’s nothing going on, if there is, well, it’s not important, and if it IS important and nobody can ignore it any more, then nobody is responsible of anything….

      I don’t want to be too critical. In fact I tend to like the ESA approach with press releases and so on. I mean the sober, no-hype style, and the intended target as intelligent people, not 7-year olds like the flood of american scientific news on the internet from universities, NASA, etc. I also like the lack of chauvinism and nationalism as opposed to the US and others, for obvious reasons in Europe.


      Can anybody explain to me why is that incompatible with transparency and open discussion of issues instead of this obscurantist, pseudo-totalitarian approach? I’ve never done public relations and I respect it and it must be hard too like any other job. I also understand the “sensitivity” and “diplomacy” with which that task must be carried out. Some companies, individuals, or even entire countries could be offended. Some people could lose a lot of money, prestige, and so on. But WHY is all that incompatible with openness and clarity and sincerity and why does it only happen here (ehem and Russia and China) and not in the US?

      I’ve had that feeling for years since the days of Giotto. Public relations at ESA are a complete mess. But now this is the last straw. You show up one day with a “tweet” dropping a BOMBSHELL telling the world in that matter-of-fact style that the mission we have been following for years, its design, construction, etc, one of the most sensitive optical systems ever launched to space has a stray light problem and you don’t tell us the magnitude of it, characteristics, periodicity, what are the usual suspects, what impact it will have on the mission, NOTHING. As the masters of Suspense you are, you keep the public in that state for weeks, while showing us some pictures now and then with ZERO informative content.

      Now at the end of april you finally remember us and post an “update” saying that everything is wonderful… except for the BIG THING which not
      only is still there but also is not well understood yet. But no-problemo: “degradation in science performance will be relatively modest and mostly restricted to the faintest of Gaia’s one billion stars”. “Current understanding is that that will be modest.”

      ‘Modest’ seems to be the keyword here. Modest. RELATIVELY Modest. Wow. As for the “faintest stars” we all thought that’s where the beef is. The faintest are the most abundant and generally the farthest and subject to deliver more information.

      I wish I am wrong but I would bet right now we have a crippled mission. That’s the conclusion you must draw with the scarcely available information. The only funny thing is that perhaps it’s not really that a big deal and only it seems so because of a public relations disaster.

      I sincerely hope so with all my heart.

    • Richy says:

      your being patronizing and condesing yourself wise up man

  • KD says:

    I just think it’s easy to say afterwards that more efforts should have been done on stray light … But there are thousands of complicated and challenging things to think about : how to know in advance the one which will pose problems ?
    Yeah, sure, stray light was looked at before, and more could have been done possibly, but many other things could have been done better. But then there are timining and budgetary issues, and priorisation is needed ! And who knows if it’s a design issue ? It may not be design-related …
    Whatever, hopefully consequences will be limited, and the galactic map will be done. Even if only 50% of the expected job is done, it’ll still be hundred times better than what we have today !
    Thanks to Gaia’s team and do your best to solve that as good as possible (and to communicate a bit more if possible 😉

  • Caroline Meisse says:

    2 years ago, I was working in Liège, on the environmental tests for the RVS. Within the period, I had to present a paper in Paris and took the train for this occasion. While waiting for my train, I met a homeless person. When he asked me what was my job, I explained but told him I was ashamed to work on a 1 billion dollar program, while people like him were still begging for money to survive. In this winter of Liège, he gave me the most extraordinary comment, that the overall scientific community should hear : “What you are doing is fantastic ! The money which is spent is spread out all over Europe, for everyone of us who are part of this GAIA adventure. Every encountered difficulty has to be taken as a challenge to search ideas, find a solution and finally succeed…”

    These intelligent words are taking sense in these days of commissioning. With the bright minds around this project, a good and safe solution will come out for sure.
    And like KD said, if only 50% of the job is done, it’s 100 times better than what we have today. So let’s take advantage ! The homeless person (Michel was his name) would have said the same, I’m sure.

    • Jon Brown says:

      It will be awesome just to see the STRUCTURE, the spiral arm-like patterns of stars’ concentrations and their fading off into intergalactic nothingness. to actually see it will bring satisfaction to me. I think “100 times better” star maps will do the trick if that be the case. New and more accurate artistic depictions of the Milky Way will be breathtaking and compelling, instead of today’s cheap and, many times, paper thin virtualizations (such the one NASA used in one of their fun applications). I appreciate technology, and I appreciate smart people that build things like GAIA. I’m sorry to hear angry voices amidst those of individuals who simply wait for a better glimpse at a universe that is worth beholding, sooner or later, by this technology or that. The thought is enough to eclipse my frets about countless, far-from-perfect teams and organizations, their foibles and accomplishments alike. Are scientists really going to KNOW in our lifetimes even 1% of what makes distant galaxies and all of matter in the cosmos tic, and (the most pressing question) how in God’s name we are even here to talk about it? Let ESA do their thing (they’re probably going to do a pretty decent job). But if I expect to actually see the REASON behind things like plain old existence, I would do better tending to myself if by any means I can be prepared for what I’m ABOUT to see.

      Thank you, ESA, for your services

  • David says:

    HI Guys
    Congratulation on the project success.

    I don’t understand why such a positive project that has the public heart and can be used to guarantee future project seem so far to be frustrating to the public.

    I would look at the success of the Kepler mission and how that was handled.
    Be negative or positive the public would be with you whatever the news.

    • Mikel says:

      >> seem so far to be frustrating to the public <<

      Three and a half weeks passed. Deafening silence only.

      Com-mu-ni-cate Esa!

  • Ted says:

    There seem to be two themes of criticism here. One is that the actual spacecraft design was incompetent, which I think is more than a bit unfair. They caught the transponder potential problem before launch. It didn’t blow up on the launch pad, as dozens of previous missions have done. The Fregat upper stage fired flawlessly, unlike the Hipparcos orbit correction burn and dozens of others. The Lissajous insertion burn was flawless, unlike the Mars Climate Orbiter and many others. Data communications has been successful as planned, unlike the Galileo failed high gain antenna or the Voyager failed receiver. Using that metric, I would say the glass is 99.997% full. The light leak is troubling, and (a) I’m hopeful that the engineers will develop a “fix” or “work-around” and (b) I’m as mystified as most people where this kind of design COULD have a light leak.
    The second criticism I have a little more sympathy for: In a mission where developments are as fast-moving as this, monthly updates aren’t enough. We WOULD appreciate having numbers attached to “relatively modest” and “faintest”, even if – at this preliminary stage – those numbers are subject to revision. Of course they are- this is science, and if we knew everything we wouldn’t be doing it.

  • Lorenzo Simone says:

    In the blog, the TT&C Transponder which performs the GMSK modulation at the highest bit rate never achieved before in the frame on Near-Earth mission, is not mentioned. This appears a little bit unfair when compared to the emphasis placed about the Phase Array Antenna.

  • lux says:

    I wonder where the 3-magnitude difference between the expected and actual brightness of GAIA is coming from.
    A 3-mag difference is a factor of 15x in brightness – not a small difference.


    • KD says:

      Good point, I asked myself the question and can see only one explanation : a calculation/simulation mistake …, as it is a much too high ratio for a physical explanation. That happens, but it’s surprising anyhow.
      Otherwise, when we’ll get additionnal news, especially on the stray light issue please ? We are starving for info : -(
      Thanks in advance, sincerely,

      • JP says:

        with my catastrophe-oriented simple mind: what if the sunshield deploys wierdly, such that the foil is teared apart and reflects the light elsewhere while lots of scattering occurs 😉

        • KD says:

          I thought about that but it seems impossible IMHO : either there is a major failure with the sunshield (which could explain the 15 factor) but the solar panel should be impacted (as well as the global performance of Gaia, which is not the case apparently) or it’s a minor sunshield problem potentially explaining the associated stray light, but that cannot explain the 15 factor in global luminosity … So I don’t think this scenario is credible. Let’s wait for additionnal info.

          • Terran says:

            Maybe the 15 factor luminosity comes from the new 42° tilt (instead of 45°), which was introduced to try and get rid of the refraction problem, which only occured at some point in the full turn of Gaia?
            I might have jumbled up , didn’t read everything in this blog again.

  • Anthony says:

    Crowd-Intelligence-Platforms for ESA Missions
    Dear Gaia team, most scientific projects suffer from insufficient budgets which again are a result of an insufficiently informed public. It is absolutely vital to future projects that current missions like Gaia strongly improve their information policy. Being an engineer myself I know that scientists and engineers like to tinker around with problems until they can proudly surprise the public with positive results. But the world has changed, problems are too complex for any one person or team to be resolved. Therefore I strongly advice that you open up a problem solving platform for the public, where you explain challenges in detail and allow for any interested person or party to contribute solutions. You would be surprised how quickly you will be able to solve almost any problem. Crowd intelligence is fascinating, as the individual might be absolutely incapable of contributing to complex scientific problem solving, but the network of minds is! Making all your problems transparent to the public, will not bring you shame but will trigger the human help factor, which is the most powerful motivation. It will make that people do not criticise the projects or faults but actively engage in problem solving. At the same time you will gain much more support for any future projects. Also you should invite the interested public to contribute their crowd intelligence to future mission challenges. BTW have you ever thought of crowd funding parts of your projects or full projects? You would surprised how quickly you will attract millions for important projects. There are many more people nowadays that understand science and space science in particular. Make use of the 99.9999% of the intelligence that have not been able to contribute yet. All you need is a basic web platform and a smart engagement process programmed as a web-flow. It’s a very easy to set up and very low cost tool but a huge lever! Try it and you will discover how much more efficient and effective even big science with the help of the global mind can be. 🙂 Anthony

    • KD says:

      Good point Anthony, it could be a great idea both for the Science and the communication of it !
      To be honest, I 100% support this project which I find wonderful and is “a dream” for science, with so much results expected.
      However, I’m deeply frustrated by the minimalist communication : it has been launched for 6 month and we still don’t know what’s happening. “It’s working, there is a stray light issue which could impact performance but you don’t know where it comes form, the impact should be more or less limited, we’ll know more later” and that’s basically it. Ok, but after 2 months (or more) of investigation, still 0 additional info on stray light : any progress in your investigation of the cause and the consequences ??? Whatever the final performance of Gaia, hopefully good, I think we just wait for 2 things : “do your best to get results as good as possible, and tell us where you are”. If results are good, wonderful. If they are bad, at least we will have seen all the efforts you did, we’ll understand the situation and we may even contribute to try and imagine solutions (as suggested by Anthony). In any case, there won’t be the frustration (that I share) and agressivity (that I don’t share) that ou face today.
      I know that you are in an engeeniering world, no problem with that, but having a Public Relation support would greatly help the team, the project and more generally ESA (which will for sure help funding the next missions !).
      Thanks a lot for your hard work and best wishes to all of us

  • Appreciating the hard work you put into your website and in depth information you present.
    It’s good to coe acrfoss a blog every once iin a while that isn’t the
    same out of date rehashed information. Fantastic read! I’ve saved your
    site andd I’m including your RSS feeds to my
    Google account.

2 Trackbacks

Comments are closed.