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Comment by surfsvammel | Hacker News Reader

Won't it be pretty close to earth in its final position at L2? If so, is there no chance that there will be future missions to do service to it? If so, maybe we can get a photo of if then?

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It's not that close. The moon is 384,400km away, and JWST will be 1.5 million km away (i.e., around 4x as far as the moon). It would be a pretty serious undertaking to get to it and get back.

Distance isn’t the best proxy for difficulty of a space mission. The delta-v to get to low lunar orbit is higher than the delta-v to get to L2. The delta-v required for lunar landing is considerably higher. And all of these manoeuvres are much less than the delta-v required to get to LEO.

That depends. If we're talking a manned service mission, then distance is a factor as it would mean a several month trip using current vehicles.

That being said, and I haven't done the math, but I think a Falcon Heavy and Dragon could do it at least in terms of delta V; although there are numerous problems with that like the fact that the dragon doesn't have a proper airlock for starters.

Maybe Hubble could take a snapshot or two?

With Hubbles's highest resolution optics, the JWST will be about one-tenth the size of a pixel.

Scale. We are just a bit of noise in an impossible to understand large, complex system.

It is crazy to think about.

That is staggering. Just, wow.

Hubble can't see the Apollo descent stage on the moon either, and that is a LOT closer than L2.

How can it make out galaxies at billions of light years away, but not a truck sized craft at a few million miles?

I haven't done the math myself, so I'm certain one side of that equation is wrong, but that seems hard to believe!

Here's the math[1]. Seems believable to me. The Web telescope is miniscule conpared to faraway galaxies, and it doesn't emit much light.

[1]: https://mobile.twitter.com/gfish/status/1479339620785524736

I did a little math of my own, and found that while the ratio of these distances is on the order of 10^15, the ratio of the sizes of these objects is on the order of 10^19.

In other words, Webb is only one TEN THOUSANDTH the apparent size of a galaxy that's a _billion light years away_

That's just mind numbing to consider!

Well galaxies at billions of light years distance are also 100000s light years wide. Rest of the math you can do about resolution now!

It’s interesting that you are expressing an opinion about something that you could calculate in the same amount of time it took to write down that opinion.

It's been over a decade since my last astrophysics class, but I appreciate you being charitable about my assumed knowledge! I think if anything this thread shows:

1) how absolutely mindblowingly big space is

2) how bad humans really are at intuiting things at the scale of space

It seems like they are going for the "the easiest way to get a right answer is give the wrong answer on the internet" approach.

Cunningham's Law

https://en.wikipedia.org/wiki/Ward_Cunningham#%22Cunningham'...

To be fair, for someone who is not well-versed in either optics or space physics, calculating the answer is not that straight-forward.

You can do pretty well just comparing the ratios of the size to the distance, right? Or is it more than that?

That's a start, for sure, but to do a super accurate calculation (the kind I feel unqualified to carry out), you need to take into account apparent size, redshift, whether or not there's gravitational lensing, interstellar dust in the way, etc etc.

But I want to reiterate that what's meaningful about this discussion is in part how unintuitive things at the far edges of our scales of perception really are. It's a muscle that, left untrained, will lead you to make incorrect characterizations like the one I made.

>a truck sized craft at a few million light years?

JWST is three light seconds from earth

Awful typo on my part! Definitely face palmed when I caught it.

galaxy_width / truck_width = 3 × 10^20

TLDR: Not really. At the distance of the JWST, a single pixel on the hubble's camera is about 700m across, or roughly 30 times the size of the JWST.

The L2 point is about 1.5 million km from earth [0]. The smallest size Hubble can resolve is about 1/20 arcsecond [1]. It's in a pretty low orbit, so is effectively "on earth" relative to the distance to the L2 point where JWST is. Calculating the size of an angular measure at a given distance [2], you get about 727m [3].

[0] https://en.wikipedia.org/wiki/Lagrange_point#L2_point

[1] https://illuminateduniverse.org/2019/04/11/angular-resolutio...

[2] https://astronomy.stackexchange.com/a/20728

[3] https://www.wolframalpha.com/input/?i=tan%28%281+arcsec%29%2...

No L2 is on the further side of the moon.

Maybe when we have active bases on the moon or Mars would a rendezvous with L2 be feasible but not before. Hubble is in low earth orbit and far more accessible.

Not exactly. Actually it's on the opposite side of the Earth relative to the Sun. So the 3 are always on one line but it has nothing to do with the position of the Moon. (Maybe you thought it was at the L2 of Earth-Moon system?)

Oof, I knew that. JWST is at the earth-sun L2, that fact just didn’t enter my mind trying to figure out how far away it was.

Given that SpaceX is hoping to start initial Mars missions by the 2030s and JWST is expected to last much longer than 10 years due to fuel savings on the L2 injection burn, it wouldn't be too outlandish for a servicing mission to be attempted by then, a vehicle capable of the 3-6 month trip to Mars is obviously capable of a 2 week trip to L2 and back.

On the other hand, with how delicate the optics and sunshield are, a servicing mission might not be worth it. At that point NASA might find it more productive to build a larger telescope, except this time it could again be designed to be manually deployed and serviced, thus not needing the complex automated deployment that delayed the JWST so much.

L2 is 1.5 million km away. For perspective, the moon is 384 000 km away. That's nearly 4x the distance. It took Apollo astronauts 3 days to reach the moon.

So it will take almost 2 weeks to get to JSWT, and two weeks to come back.

Presumably such a mission would be unmanned and therefore a one-way trip.

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