Is there something inherently more difficult about servicing satellites in the 2nd Sun-Earth Lagrangian point?

Question

There are several questions already asked on here about the potential for servicing the James Webb Space Telescope. This question asks what happens if the JWST needs repair. Basically, there are no plans to ever service the telescope because of its distance. To ensure robustness, engineers have planned for every eventuality they could imagine, and now we simply have to have faith in all of the engineers who worked on the telescope, and just hope it works perfectly (which seems to be the case as of now). This question explains that there may be a docking ring on JWST for possible future missions. This question is pretty much more of the same. Yes, JWST has a docking ring, but there are no plans to service it due to exorbitant costs. Finally, this question is just more of the same.

Now, none of the answers on these questions answers my question, which is, Is there something particularly difficult about servicing a mission at the Sun-Earth L2 point? Now, I understand that the L2 is good distance from Earth, about 1.5 million KM, but is distance the only thing making missions to the L2 difficult? I read in one of these questions that servicing JWST would be more difficult than achieving Low-Earth Orbit, but less difficult than landing on the moon, despite the moon being about 1/4 the distance to the L2. I am not 100% sure why that is, but I imagine it's because landing on another astronomical body and then relaunching to get off of that body are technological marvels, much more difficult than docking with a virtually gravity-less object. So, obviously, distance alone is not what dictates the difficulty of a mission.

So, my question, to belabor the point, is is there something specific about the L2 point, besides its distance, that makes servicing missions in that orbit especially difficult? For example, does the fact that it is an unstable Lagrange point make it more difficult than servicing a mission in a stable Lagrange point? Or are there dangers exclusive to L2 not found in other Lagrange points, or at least in other stable Lagrange points. Or, is this merely a case of the difficulty being it's distance from Earth?

Answer 1

Distance certainly is a key factor. It took JWST a month to go from launch to L2 pseudo-orbit insertion.

Another key factor is that to date, the only successful satellite servicing missions have been performed by humans. Repairing a vehicle in a pseudo-orbit about the Sun-Earth L2 point would entail a long journey if humans were involved, with no option of quick return in case something goes drastically wrong.

Another factor is that how to do it without humans is not yet known. DARPA and NASA have two automated satellite servicing missions in the work, DARPA's Robotic Servicing of Geosynchronous Satellites (RSGS) and NASA's Restore-L mission. Both are years behind schedule and hundreds of millions of dollars over budget.

The International Space Station is the quintessential example of a cooperative target. The ISS broadcasts its state and the GPS satellites used to determine that state to nearby spacecraft, enabling use of relative GPS. (There is no GPS, absolute or relative, at Sun-Earth L2.) The ISS is painted with markers that makes visual navigation much easier. Finally, astronauts / cosmonauts aboard the ISS command the ISS to go into free drift mode when a rendezvousing spacecraft gets sufficiently close.

Space rendezvous with a non-cooperative target is considerably more difficult than is rendezvous with a cooperative target, and rendezvous with an uncooperative target is even more difficult. Until those two missions mentioned above (RSGS and Restore-L) work I would say we do not know how to do it without humans being intimately involved.

Answer 2

Getting to L2 in a timely fashion is more challenging than going to the Moon. The Apollo missions took about 3 days to get to the Moon. 4.5 days is how long the LRO took to orbit the Moon when launched from Earth (It used a slightly slower, but more efficient trajectory). James Webb took 29 days to get to L2!

L2 is so far out that even in the best case scenario it is essentially a 1-2 month mission. The fuel requirements are less than landing on the Moon, but the life support costs and risks are far higher.

The instability, combined with the lack of actual gravity, makes getting there in a timely fashion a bit harder than getting to the Moon.

There is no more inherit danger to going to L2 than there is of orbiting at the Gateway orbit around the Moon for 2 months, except the distance makes aborting harder.

It takes about 0.9 km/s to orbit the L2 in JWST's orbit. Orion's total Delta-v is about 1.34 km/s. It has enough fuel to get there, but not enough to return. Realistically the only vehicle in the prototype or later phase that could do this is Starship, or perhaps Orion with a larger service module.

Answer 3

As difficult and expensive as it is to make a complex spacecraft operate for years without significant problems, it is even more difficult to design it to be serviceable in space.

Why is Chandra in a high orbit? The official excuse is that it's better for the science, but the real reason is that it was originally planned to be serviceable. But the serviceable version's cost was never feasible. The way we got the cost down was to make up a requirement that it be in an orbit that the Shuttle couldn't reach.

Answer 4

Apart from the distance (which creates issues for manned crews and large time lag for unmanned probes) there are a couple of other issues:

L2 issue:

• L2 is unstable (It's like trying to keep a ball on top of another ball - any small difference will tend to increase over time)

JWST issue:

• The JWST is reliant on very low temperatures and very sensitive equipment, therefore firing any thrusters or creating any bright lights around it will be likely to damage or break it.

Overall any service mission would be expensive, delicate and come with a high risk of failure. Given advancing technology and dropping launch costs will likely allow us to make better telescopes in the future; the cost-benefit analysis will almost certainly work out in favour of putting money towards a new telescope rather than servicing JWST.

Answer 5

One part of the original question asked, "...servicing JWST would be more difficult than achieving Low-Earth Orbit, but less difficult than landing on the moon, despite the moon being about 1/4 the distance to the L2. I am not 100% sure why that is.." and nobody seems to have directly addressed this yet.

There are a bunch of reasons, some of which have been pointed out. But the main reason is the usual one for space flight: Delta v. In other words, it is the amount of fuel required which all needs to be first lifted into low Earth orbit.

The Delta v to the L2 point from LEO is a bit more than 3 km/s. That doesn't seem too bad when you think about the fact that it is 7.5 km/s to get from Earth's surface to LEO (plus more to deal with atmospheric losses). But the difference is that you need to lift all the fuel for that 3 km/s up into LEO first. That's a big heavy load that you are carrying through that 7.5 km/s Delta v.

But this is a bit easier than landing on the Moon, as the poster says. Getting from LEO to low Moon orbit takes a Delta v of about 3.8 km/s. The main reason it is bigger than getting to the L2 is that when you reach the Moon you need to burn fuel to slow down in order to capture into orbit. Actually reaching a point about 20 km (say) above the Moon's surface actually takes a bit less Delta v as reaching L2. It's slowing down for the capture that makes it take more. Then to land on the Moon you need to slow down more, to the tune of another 1.7 km/s. So the total Delta v from LEO to Moon landing is around 5.5 km/s - considerably bigger than reaching L2.

What all of this adds up to is that whatever you are sending out there needs to launch on a pretty big rocket. This is why the JWST needed an Ariane 5. That comes at a very high cost. Now add to this the difficulty of an autonomous docking, with no possibility of help from ground controllers because of communication lag. It pretty much has to be robot mission. Sending people out that far would be an Apollo mission level of difficulty (smaller Delta v, longer mission time, so bigger, more massive space craft). We don't have any existing spacecraft that could get people out there and back.

Answer 6

I could add to previous answers, that we have really big loss with the retirement of the Space Shuttle, even considering its costs and limitations.

• The Shuttle was the only[1] space ship capable of reaching high orbit (700km, when servicing Hubble) that had a large internal space for humans to live and work.

In space, long means more than about a week (the Shuttle was capable of completing up to 30 day missions).

With the current tiny capsule ships, it is possible to make a large habitat module and to use some big space tug to reach the target orbit.

But capsule ship based architecture of such missions really means a scaled down version of a lunar or martian ship, or basically ISS with a huge engine module and large volume of fuel.

• ISS is capable of making a half year fully independent (isolated) trip. You'd only need to add big engines and fuel tanks.

So, you could estimate how much a human mission would cost.

For robotic missions, or to be more precise - TELErobotic, it is still an open question, even considering that the maintenance of Manhattan program machines in the 1940s was 100% remote, because it was just impossible FOR HUMANS to work under extremely hard radiation conditions.

In MP the solution was that all machines consisted of replaceable modules, which were specially designed to change a whole module when needed using telerobotic technologies.

Some of similar techs were used later in ICBMs. (You may read about that in hard scifi - generation ships, which have stocks of supplies of spares in an inert gas, for very long storage. This is used by the military).

For me it's sad, that after more than 70 years, we've not progressed to fully telerobotic support/repair missions in space.

Many experts think that for now it will be cheaper to create and launch new such space machines (like Hubble or Webb), than to repair the old ones. This opinion is well supported, by experience with tv/telecommunication satellites, which usually become obsolete before they are broken.

I'm not sure, for me this could also be just conservatism.

To be precise, many commercial satellites for geostationary orbit, are basically ready platforms to be used for scientific missions. There even exist a few examples of scientific sats, made just on a commercial platform. The main requirement of such convertible platforms are they have apogee engine(s), and are launched to low orbit than transfer to GSO themselves.

[1]: Mean modern time. - Few decades ago, existed Apollo, which flight to Moon orbit; first modifications of Soyuz space ship was made for Moon flyby (incapacious, but have all need equipment for such flight); existed TKS - soviet heavy ship, which launched by Proton rocket and was much more capable than current stripped down Soyuz; even seriously considered lunar Gemini, and this program was cancelled only after real Apollo hardware make first flight.

But none of current ready space ships are ready for more than LEO without huge works, like make/certificate new human grade rocket, make additional booster (tug), make new control system.

Even polar orbit is out of reach.

Answer 7

For servicing the JWT with humans, an almost certain requirement, I would consider a tug to bring the telescope to LEO, where humans are much more protected from radiation and their mission would be much shorter. The humans could possibly use an existing spacecraft, much less life support system requirements, and less total mission mass. You only need a tug to bring JWT back and return it.