41
$\begingroup$

Reading Wikipedia's article on the James Webb telescope, I'm struck by the following information. I don't understand how it arose as an optimal and/or preferred mission choice.

So,JW is in an unstable orbit at Lagrange L2. Unlike many other missions, which can last decades longer than planned because their main need is low level electrical power, JW also needs thruster fuel to regularly ensure it stays in its designated position.

Relevant data:

According to Wikipedia:

The telescope's nominal mission time is five years, with a goal of ten years .... after a 6-month commissioning phase. JWST needs to use propellant to maintain its halo orbit around L2, which provides an upper limit to its designed lifetime, and it is being designed to carry enough for ten years.

JW launch mass is about 6500 kg. The total mission cost is about $9.7 bn, with thruster fuel for 10 years. The telescope's thruster fuel for orbit maintenance is 238.5L/274kg (159L/159kg hydrazine and 79.5L/115kg dinitrogen tetroxide) at launch.

Other past missions:

Other flagship missions have spectacularly outlived their planned missions.

  • Hubble (planned 15, actual 30+)
  • Voyager (planned 5, actual 50+)
  • Cassini/Huygens (planned 4, actual 20)

Question:

Even allowing that some of these past "planned" missions may have been durations for primary mission only, with an extension expected, the overall impression is that missions can outlast and outperform their original planned duration, and be repeatedly extended thereafter, because of their value once deployed.

But JW was deployed with a single and absolute limitation point - its thruster fuel supply, which means that however successful, its duration is limited to apparently 10 years. Not long at all, for NASA's current flagship project, and one that judging by Hubble could still be valuable and quite possibly still working fine, in 20 or 30 years.

Assuming that halo orbit corrections occur at a relatively constant rate, it would have only taken another 274kg to build in capacity for an entire second decade. Around 4.2% total payload increase, for a doubling of viable mission time - and many missions have been valuable after 10 years, one may assume JW would also be. Also new or successor $10bn projects are not readily flicked into space on whim, or on short timescales, and are subject to political uncertainty, so there's a time, opportunity, and cost incentive, to make as much as possible from this one.

I don't imagine the current launch strains Ariane 5 so much that an extra 100 - 300kg was completely non viable.

Even if it did, the payoff of any extra months or years must surely be huge and I'd expect it to have been preferable to find a telescope mass, or launch fuel, saving in some other area - a small reduction in some other component, in exchange for doubling of mission outside time.

But despite all this, they decided not to.

Why?

$\endgroup$
11
  • 4
    $\begingroup$ Movable components like gyros and reaction wheels that wear out, and electronic components that are bombarded with cosmic rays, are designed in JWST to last at least five years, with 10 years as a goal. They're usually quite conservative regarding the fuel, so probably that is not what will limit the duration. Fault tree analysis and failure probability vs. all other system constraints (and cost) is a whole field in itself. $\endgroup$
    – pela
    Dec 15, 2021 at 11:25
  • 2
    $\begingroup$ There was an experimental robotic refueling program at NASA when I was at Goddard. If I remember correctly from random conversations with the JWST team, it was considered as a potential long-term candidate for that program, should it succeed. Part of the thinking may be simply that refuelling will be a possibility if the active cooler and other stuff survives 10 years. There was some doubt about the the solar shield opening up at all :) $\endgroup$ Dec 15, 2021 at 19:47
  • 5
    $\begingroup$ On the topic of "spectacularly outlived", back when I was in the business of calculating space mission life expectancies, the main reason everything massively outlived the expectations we calculated is that the way our models assumed satellites are operated bear no relation to how satellites actually are operated. In particular, the models assumed spacecraft would be driven at maximum capacity until something broke, at which point the whole mission would be scrubbed. Instead, what we actually do is carefully limit the demands placed on wounded satellites, and expend enormous effort... $\endgroup$
    – Ryan C
    Dec 16, 2021 at 18:54
  • 3
    $\begingroup$ in keeping them operating a somewhat reduced capacity for as long as physically possible. This goes beyond the usual answer of "in order to be almost certain of achieving 5 years, you naturally end up with quite a high probability of surviving much longer," because we don't just scrub missions when the orbit insertion goes wrong or the mirror isn't polished right -- we put in massive overtime fixing or working around problems, carefully and actively fighting to preserve and maintain partial capacities and avoid declaring failure until the last possible moment. $\endgroup$
    – Ryan C
    Dec 16, 2021 at 19:10
  • 1
    $\begingroup$ @OrganicMarble ohhhh, that hurts. that's awful. $\endgroup$
    – Ryan C
    Dec 16, 2021 at 22:17

2 Answers 2

11
$\begingroup$

I suspect that the culprit is the MIRI instrument.

Of all the scientific instruments of JWST, this is the only one for which passive cooling is not enough. Active cooling requires equipment with moving parts. AFAIK, it was one of the key technological risks and challenges encountered in the program, and correspondingly, if overcome satisfactorily, one that contributes to make the JWST unique as a space telescope, one that arguably allows JWST to look further than any past telescope into the past of the Universe.

The question then is, if they lose the MIRI after 10 years (10.5 years after launch) would it be worth to add some fuel for future possible extensions for the remainder of the scientific goals?

This is not easy to answer for me, but looking at the required man-power projected in the latest GAO's report (Fig. 5)

enter image description here

I would say that, it would be more clever, politically, to "hide the margin" than to publicize right now (e.g. by proposing to add more fuel) the possibility of "extended use". Let's not forget that, of these 274 Kg of fuel, the budgeted annual in-orbit consumption can be as little as 1.3% (2m/s of Delta-v out of the 150 m/s total). For details see Dichmann et. al. (2014) Station-keeping Monte-Carlo simulation for JWST

$\endgroup$
4
  • 1
    $\begingroup$ That last sentence ("Let's not forget") is thrown in without background or link, but seems quite relevant to the question. Can you expand on it a bit, maybe also add a link covering the 1.3%? $\endgroup$
    – Stilez
    Dec 15, 2021 at 15:41
  • 1
    $\begingroup$ @Stilez, done (the paper linked concluded that, conservatively, it's 2.4 m/s/y). $\endgroup$
    – Ng Ph
    Dec 15, 2021 at 16:10
  • 1
    $\begingroup$ So you are suggesting they are trying to hide a couple of thousand person-years of work in order to lower project cost and later appel to the sunk cost fallacy in order to get the additional money approved after all? $\endgroup$
    – Nobody
    Dec 15, 2021 at 20:02
  • $\begingroup$ In case anyone else has the same complaint: I got a 404 when I opened the first link, but opened it again and it worked fine. $\endgroup$
    – llama
    Dec 16, 2021 at 18:05
9
$\begingroup$

There's already great answers on the technical side, but it might also be useful to note what the JWST was supposed to be, as the original designs were much different than what we ended up with. The 90's were a period where NASA's goal was to be "faster, better, cheaper", which is extremely ironic given that many have lamented the development of it as a never ending series of disasters which "ate astronomy" (https://www.nature.com/news/2010/101027/pdf/4671028a.pdf).

The project was conceived in 1996, estimated at $500 million USD, and was initially planned for a 2007 launch. It ended up costing ~19x more than estimated and took ~2.3x longer than expected (assuming it launches at the end of '21). (https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#History)

In that light, even though 10 years seems like a short life-span, it makes a lot more sense when viewed from the perspective of the initial planners. Again, this was when they thought it would be fast and cheap; it doesn't need to last like the Hubble because it was thought we'd be tossing equivalent sorts of projects up left and right. You might compare the initial conception of JWST to the Spitzer (5 year planned primary mission). Do a job well and quickly move onto the next thing.

Now, if the initial designers knew in advance what the project would become, obviously things would have been different. And of course there were modifications to the plans made along the way. But as other answers point out, to extend the life would require many different changes and prolong the project even longer. At some point you need to go ahead with a plan. There will always be seemingly good justifications for changing something but then you don't end up with anything.

Finally, it should be said that while there were many, many hiccups along the way, if all goes to plan it will be an amazing piece of science!

$\endgroup$
11
  • 3
    $\begingroup$ @Ng Ph The problem is, we could have done hundreds of small missions for the same price, covering a much broader range of investigations, as well as learning how to do big things without the bloat. As SpaceX has shown with rockets, the way forward is to build many, fail frequently, and learn. JWST should have been budget capped and canceled when it overran. Missions "too big to fail" stand in the way of real progress. $\endgroup$
    – John Doty
    Dec 16, 2021 at 14:26
  • 3
    $\begingroup$ @JohnDoty You're right, but although that approach can work just fine in a privately owned company, there's no way you could apply it to government-funded projects. A project that gets cancelled after spending all the money without producing an awesome machine will be seen by most taxpayers as an utter waste of money. If you planned to produce ten cheaper JWSTs at 1bn each, expecting the first five to blow up (cf. Starship development history), it would have been more efficient in theory, but it would never get past the blowing-up phase in practice. $\endgroup$
    – TooTea
    Dec 16, 2021 at 15:08
  • 3
    $\begingroup$ @TooTea For payloads, don't start at 1bn. Have a program, not just isolated stunts. Do the smallest thing that will advance the state of the art. Make it a bit bigger and more capable next time, keeping the core development team in place. Iterate. ISAS in Japan used to do this, and managed to capture the public imagination while doing it (consider the immense interest in Hayabusa). They were feeding NASA, too: Chandra was only possible because ASCA proved the tech. Unfortunately, the merger with NASDA, creating JAXA, killed this approach. $\endgroup$
    – John Doty
    Dec 16, 2021 at 15:34
  • 2
    $\begingroup$ @TooTea there is one historical example of a "successful cancellation" of a US mega-project with an expected cost that seemed to inflate with time, the SSC. But that may have been because of several factors particular to it including being pushed by just one US state as opposed to the way NASA was (for better or for worse) distributed across the back yards of many US congressmembers. $\endgroup$
    – uhoh
    Dec 16, 2021 at 20:53
  • 1
    $\begingroup$ @NgPh The most common reason for NASA space mission failure is that NASA never even tries. Why? Because they use "parametric costing" based on previous missions, and that tells them they can't afford many missions. Bloated missions like JWST inflate the estimates, so they try fewer missions and allocate too much money to them, which feeds the bloat. $\endgroup$
    – John Doty
    Dec 17, 2021 at 23:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.