Today, at 2 p.m. EST, Webb fired its onboard thrusters for nearly five minutes (297 seconds) to complete the final postlaunch course correction to Webb’s trajectory. This mid-course correction burn inserted Webb toward its final orbit around the second Sun-Earth Lagrange point, or L2, nearly 1 million miles away from the Earth.
The final mid-course burn added only about 3.6 miles per hour (1.6 meters per second) – a mere walking pace – to Webb’s speed, which was all that was needed to send it to its preferred “halo” orbit around the L2 point.
“Webb, welcome home!” said NASA Administrator Bill Nelson.
If I may re-phrase this question a bit more specifically: We know that it will be several months of testing and calibrations before they start doing any “real science” with this telescope. But surely (?) that must include gathering images from afar and examining them for their calibration and testing work. Will any such images be made public during this time?
I don’t know the answer, but @wolfpup 's diagram has “first light” (usually an important milestone for a telescope) at L+59 and “observatory available for ISIM activities” (ISIM is the JWST instrument package) at L + 70, so somebody is getting test images sooner than the L+180 we’ve been told when the 'scope is “fully commissioned”.
Oh I understand there are still numerous setup chores remaining (so we’re told).
Of course I’d love to see imagery as much as anyone else, but mainly I just want to know when we can relax about the prospect of suddenly finding out the thing isn’t going to work as advertised.
I read a description somewhere describing a major part of that “calibration” work. I don’t recall if I saw this somewhere on the official Webb site, or earlier in this thread, or somewhere else.
So they have these 18 separate mirrors that they’ve deployed with nanometer positioning precision. But they still need to get all 18 mirrors micro-positioned so, you know, they will all focus a distant star down to a single point on the receptor instead of seeing 18 separate stars.
So they will point the telescope at some known star, and then tweak the precise micro-positions of all 18 mirrors until they get all 18 images to coincide onto one point.
Well, lessee. L2 is 1.5m km from Earth, and JWST is orbiting L2 at (using @wolfpup’s diagram) a diameter of 1.5m km (radius half that) so max distance is about 1.68m km or 5.6 light seconds @ 300km/sec lightspeed. Total napkin calculation, but I’d think close.
One factoid I found interesting in the What Now for Webb program I linked to above was that the orbit around the L2 point is about 180 days, very leisurely indeed.
I don’t know orbital mechanics really well, but it’s not actually orbiting around the L2 point, there’s no mass there for it to orbit, it’s orbiting around the sun, and doing so at an orbital velocity that would put it into a higher orbit were it not for the gravity of the Earth. Its orbit is inclined, so that, from the Earth, it seems to trace a circle in the sky, but that circle isn’t actually the route that it takes around the sun.
The inclination of the orbit is what makes it seem to orbit a point, so I don’t think that the distance from that point (the amount of inclination of the orbit) would have any effect on how long it took to make that circle.
Though, now I wonder why it is 180 days, rather than a whole year.
They’re not related at all. The earth’s orbit, and its mass, determine where the second Lagrange point is located, that’s all. The JWST’s orbit is indeed really around L2, and the speed of its orbit is determined only by its radius from L2 at any given point and the gravitational balance at L2. JWST’s orbit is very slow both because it’s quite far from L2 (in a very elliptical orbit that takes it as far as about 1.5 million km) and because the effective gravitational balance at L2 is very weak. By happy coincidence, this is probably very helpful in minimizing the amount of tracking it has to do to maintain a fixed aim.
Its orbit is basically perpendicular to the plane of the earth’s orbit (see the diagram I posted earlier) and is held within its own orbit by the usual orbital dynamics, and the orbital plane is held in place by the balance of the earth’s and sun’s gravity against the centrifugal force of moving with the same solar period as the earth while in a higher orbit.
