l was trying to figure out why, based on the latest posted JWST speed number, I was getting an expected orbital period of a little more than a year. The speed figure is probably wrong, as it hasn’t changed since early yesterday morning, but it surely couldn’t have changed all that much!
Turns out, the diagram I posted earlier had a lot of early preliminary information and some of it has changed. The orbit is actually about half the size shown, an ellipse of 832,000 km by 250,000 km. This yields a circumference of about 3.65 x 106 km. Using the last posted speed as an approximation, the period now works out to about 209 days.
I suspect this remaining discrepancy is mostly because JWST is currently fairly close to its eventual orbital apogee. As it falls toward perigee it will trade off gravitational potential energy for kinetic energy and speed up, then reverse the process and slow down again, but its average speed should be a bit higher than what it is now, leading to the expected period of about 180 days.
I’m finding the whole thing a bit confusing, Lagrange points and all. Looking at this diagram of gravitational gradients you can see that L1, 2, and 3 all are saddle shaped, with two forces inward at right angles of two outward. I’m guessing the inward forces are what’s making JWST orbit the point and it might have a tendency to drift inward or outward due to perturbations from the Moon passing by every month.
L4 and L5 are even more puzzling with no inward forces at all. I would imagine station-keeping fuel must be spent at least occasionally. Space is hard.
I believe the stabilizing forces there are Coriolis forces.
I don’t know enough about Lagrangian mechanics to fully explain the difference, but I believe the short and simple answer has to do with what happens when the object drifts. All the Lagrange points except L4 and L5 react to drift with essentially positive feedback – the more you drift, the faster you roll off the balance point. L4 and L5 have negative feedbacks.
The orbits at L4 and L5, taking all those factors into account, are stable but can be amazingly complex. You can play around with them at the link below. I believe the parameter tmax is the amount of orbital time you want to simulate.
Is this true, about its tangential velocity changing wrt apogee and perigee? It would be true if JWST were orbiting a mass with gravity, but that’s not the case here with L2.
My brother (USAFA zoomie) did his thesis on the 3 body problem, I’ll ask him.
Interesting question. It’s not a Keplerian orbit, so maybe not. Lagrangian orbits are weird things, and don’t follow repeated paths, but I still find it odd to consider that there can be any elliptical orbit that doesn’t change velocity between perigee and apogee.
“Expect to see the first light for James Webb around L+36 days (early February), with an initial calibration mosaic. It won’t be pretty — yet. Further alignment will result in improved images, and we can expect the first science observations by this summer.”
How bad is it if one of the mirror segments don’t align properly? Does it scrap the whole telescope, or just collect 1/18th less light with a corresponding loss in resolution?
IIRC we will not see pictures until late spring/early summer. There is a lot of fiddling it seems to get it up to spec and start giving us really good photos. Nothing is wrong with it (as far as we know). It just takes time to get it into peak working condition. Everything needs to work at crazy levels of precision and it takes time to dial that in…especially when the thing you are working on is a million miles away.
(I suppose they could snap a photo today but it would be almost worthless and I doubt they want to send the public a fuzzy image from such an expensive telescope…best to wait)
NASA actually has a rule that all data collected from any of their missions must be made public within at most a year after being collected, and the principle investigators can release it earlier if they want. On many NASA missions, the principle investigators have, in fact, chosen to make all of the data available to the public immediately. So even if the images taken at “first light” don’t look very impressive or awe-inspiring, we might still get to see them soon, and will certainly get to see them eventually, if we really want.
Of course, “made available” doesn’t necessarily mean “with a big splashy press release, and posted on the front page of nasa.gov”.
That’s when they said it would start doing science, but I’m betting we get pictures well before that. They need everything to work at crazy levels of precision to do the serious science, but parts of it will deliver before then. It has to be very cold for the longer IR wavelengths to behave well, but JWST can see all the way up to orange wavelengths, and could deliver pictures that we the great unwashed would get excited over, well before optimal temperatures are reached.
Now, fate will tell us, and my drivel is of no real consequence. But my fingers are crossed for at least some token kind of eye candy in February or so.
Meanwhile, the “Where is Webb” page seems to have gone dormant, no longer updating now that Webb is at L2.
I’m hoping there will be on-going regular public updates to keep us interested folk aware of what’s going on with Webb day-to-day in the immediate near future.
Is there actually anything much happening in the next few weeks that us layfolks would find interesting?
I thought about this a bit more. Lagrangian orbits are hellishly complex things that I don’t pretend to fully understand, but I think one can reach a conclusion from first principles.
Consider that any object in any orbit must be experiencing a centrifugal force pulling it away from the center of its orbit, and this must be balanced by an equivalent gravitational pull from the center. It cannot be any other way. This is further confirmed by looking at the gravitational potential contours posted previously. The halo orbit which is roughly perpendicular to the ecliptic takes the JWST both “in front” of L2 and behind it, where the gravitational potential is higher than at L2. This manifestly must be true throughout its orbit. And the farther it is from L2 along the earth’s orbital tangent (within some relatively short distance) the higher the gravitational potential energy and therefore the lower the kinetic energy (i.e.-speed). So I conclude that the normal rule for tangential velocity at apogee vs perigee applies.
Another way of looking at it is that the above supports the idea that a weight on a string, set moving so it describes an elliptical orbit, is a valid analogy. There is no mass at the center of its orbit, but it describes a Lagrangian type orbit due to the balance of forces between the string and the earth’s gravity. One observes that in a shallow ellipse, it moves much faster along the long side of the ellipse where it’s closest to its rest point (perigee) than the short side where it’s farthest (apogee). Indeed if the ellipse is squashed completely flat, you have a pendulum, which has zero motion perpendicular to its plane, and the velocity of the orbiting weight at apogee is precisely zero.
If I’m wrong about any of this I’d welcome any correction.
