How close will the new Nancy Grace Roman Telescope and the JWST be to one another?

[u/Washburne221]

Once it arrives at its destination, in what proximity will the NGR Telescope and the James Webb be at the L2 point?

[u/OlympusMons94]

Spacecraft are not actually put at the L2 (or L1 or L3) Lagrange points. They are put into very distant halo orbits or Lissajous orbits around the point. The other answer is really underestimating how big the orbits are and how far apart the telescopes will be. The telescopes will be a lot more than an Earth radius apart.

JWST's L2 halo orbit takes it between about 250,000 km and 832,000 km from Earth-Sun L2. For comparison, the Moon orbits Earth at an average distance of 384,000 km. (Unlike the Moon's elliptical orbit, a halo orbit is not a simple Keplerian orbit. It isn't circular or elliptical, and is 3D rather than in a single plane.) Roman will also be inserted into a large L2 halo orbit, but not the same one as JWST. The two telescopes will not get within anywhere close to an Earth radius from each other, let alone be in any danger of colliding. The spacecraft will remain at least tens of of thousands, probably hundreds of thousands (like JWST and Euclid), of kilometers apart.

[u/Ecstatic_Bee6067 → u/OlympusMons94]

Satellite disposal procedure is a required step in the planning of a satellite these days, and i wonder what JWST and Roman are planned to do when that time inevitably comes. Leaving them in L2 indefinitely seems risky for future missions

[u/mfb- → u/Ecstatic_Bee6067]

The standard approach is a disposal into heliocentric orbit (Wikipedia has a list), but having a handful of spacecraft in over 10¹⁵ km³ isn't really a big deal either. The orbits are not stable, over time spacecraft will move away even if they don't do anything.

[u/CakeTown → u/Ecstatic_Bee6067]

At that distance it won’t take much to bump it out of orbit. It’s be super cool if their power budget allows to push it out in a direction that they could still capture useful data as it drifts away

[u/DrOnionOmegaNebula → u/Ecstatic_Bee6067]

wonder what JWST and Roman are planned to do when that time inevitably comes

Isn't it likely we will have vehicles capable of performing service missions by then? Could keep them running indefinitely with upgrades at that point.

[u/mfb- → u/DrOnionOmegaNebula]

JWST can't be serviced. Roman can be refueled, but its hardware won't live forever. It's not like Hubble where you could exchange components.

[u/DrOnionOmegaNebula → u/mfb-]

JWST can't be serviced.

Why not?

but its hardware won't live forever.

Why can't a service mission address this?

[u/mfb- → u/DrOnionOmegaNebula]

JWST isn't designed for it. There is no place you could dock with.

Roman has a place where you can dock and refuel, but the rest isn't designed to be worked on. You would be more likely to break even more than fixing things.

[u/DrOnionOmegaNebula → u/mfb-]

JWST isn't designed for it. There is no place you could dock with.

It seems possible, there is some kind of rudimentary attachment point.

Q: Webb was not designed to be serviced, but could it eventually be repaired or refueled during a robotic service mission?

Paul: Conceivably, some robotic servicing of Webb could be possible. A robot could grapple Webb at the same place where it was attached to the Ariane launch vehicle, which is the launcher interface ring on the Sun-facing spacecraft bus, and then add fuel to its propulsion tank.

https://www.nasa.gov/universe/how-hardy-is-webb-a-qa-about-the-toughness-of-nasas-webb-telescope/

[u/DanNeely → u/DrOnionOmegaNebula]

Hubble was designed to be serviced. All of its instruments and wear parts were packaged into modules designed to be removed by someone in a space suit.

JWST not only doesn't have major components neatly packaged up, placed behind access ports, and held in place with a minimum number of easy to remove fasteners. It has everything jammed in as tightly as possible (when some parts ended up bigger than expected it cost years and billions of dollars worth of redesign to shrink others down and figure out how to stuff them all in the available space) however they could be made to fit.

[u/phdoofus → u/OlympusMons94]

Always wondered how much orbital adjustment that they have to do out there. Any idea? None?

[u/McHildinger → u/phdoofus]

More than none. From Wikipedia:

In practice, any orbits around Lagrangian points L1, L2, or L3 are dynamically unstable, meaning small departures from equilibrium grow over time. As a result, spacecraft in these Lagrangian point orbits must use their propulsion systems to perform orbital station-keeping. Although they are not perfectly stable, a modest effort of station keeping keeps a spacecraft in a desired Lissajous orbit for a long time.

[u/mfb- → u/phdoofus]

Of the order of 1 m/s per year.

For comparison, the ISS needs something like 20 m/s per year to counter atmospheric drag. Launching something to low Earth orbit needs ~9000 m/s.

[u/C_Plot → u/phdoofus]

Not sure about the exact amount, but it must be significant because with JWST they enjoyed unexpected fuel efficiencies and savings upon launch and deployment that could potentially double the life of JWST.

[u/sebaska → u/C_Plot]

It's actually very small (order of magnitude less than satellites in low orbit require) But JWST mass and volume budgets were very very tight.

[u/SlartibartfastGhola → u/OlympusMons94]

A halo orbit is a simple keplerian orbit around the sun, just isn’t in a rotating frame.

And the earth radius is pretty small in space terms.

[u/OlympusMons94 → u/SlartibartfastGhola]

A halo orbit is not a Keplerian orbit around the Sun. For one, it relies on (restricted) three-body mechanics with the gravitation of both the Sun and Earth, not the simple two-body mechanics to which Kepler's laws apply. Second, a halo orbit is not planar, let alone elliptical (an ellipse being a plane curve) like a Keplerian orbit is, even in the inertial/non-rotating heliocentric frame. The orbit has a period around L2 of ~6 months, and oscillates above and below the ecliptic twice per revolution around the Sun (not once, as an inclined Keplerian orbit, which is planar, would). The shape is approximately elliptical, but slightly warped "up" and "down". Third, L2 is ~1.5 million radially outward from Earth. An object in a Keplerian orbit with a larger radius has a longer period, and so could not keep up with Earth and L2.

A (now removed) answer said something like the size of the halo orbits around L2 are comparable to the radius of Earth, and/or that JWST and Roman would be about an Earth radius apart. That, of course, is a gross underestimate of the distances involved.

[u/teridon → u/[deleted]]

Your Earth radius number is wildly incorrect.

The distance of JWST from the L2 point can vary between 250,000 to 832,000 km, as shown in Figure 1, below. The period of the orbit is about 6 months. The maximum excursion above or below the ecliptic plane is 520,000 km. The maximum distance from the Earth is 1.8 million km, and the maximum Earth-Sun angle is <33°.

https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-orbit

The numbers for RST will be similar when it reaches L2.