Months.
From this link:
oooooh, that’s why it takes 6 months to commission. Got it!
The chance of being hit by micrometeorites is no lower on station than it is in transit, and transit is a couple weeks out of a 10+ year projected lifespan. The sun shield is somewhat over-engineered so that as it inevitably gets holes poked in it, it will still provide adequate shielding. Micrometeorite strikes are not expected to be the lifespan-limiting factor, which is rather the fuel available for station-keeping.
What is the chances of it hitting another probe already at L2 ?
Apparently there are at least 2 there already (that we know about !)
Not quite zero, but pretty damn close.
We know about them because we put them there, and we know where they are.
L2 is not a point that you sit on, it’s a point that you orbit. There’s plenty of room. We also have a collection of items at L1 that we have no trouble keeping out of each other’s way.
I see today they are starting to deploy the ‘wings’ of the primary mirror. Let’s say hypothetically one or both of the ‘wings’ fail to successfully deploy…could they still do some limited science at this point presupposing they successfully achieve and maintain an L2 orbit?
Yes. A partial mirror still allows for some utility.
Since doing anything on the telescope is more difficult than doing it on the ground, I would bet converting to engineering units – which would include the ability to hold some kinds of calibration constants – would not be done there, it’d be done here. Probably not even a voltage coming down to us, probably a binary number representing a scaled value. Or something even more stingy – when you’re trying to minimize traffic and storage, you only report the change since the last reading, and maybe not even that if you haven’t measured a change. There would be less frequent handshaking of some kind to re-establish a baseline occasionally.
Also, probably not a thermocouple. Thermocouples don’t actually measure temperature. They give a voltage that depends on the temperature difference between separate locations, and you add another temperature measurement device at one of the locations to infer a temperature at the other. So, you have to have two thermocouples and one thermometer, in effect, to get the temperature at the other thermocouple. This is known as cold junction compensation or CJC. But I dunno, it’s possible there are reasons to go with thermocouples, and perhaps exploit solid state temperature measurement devices that stay with the electronics package or something.
It seems that when a probe comes to the end of its useful life it gets
moved to a heliocentric orbit.
So how does that happen ? Do they save the last drop of fuel for that, or
do they have a separate thruster thingy ?
And where is the new orbit ?!
Questions, questions.
In terms of weight, cost & complexity, saving the last bit of fuel would be much the better choice.
Googling indicates that this was done (via a 20-minute engine burn) with the Wilkinson Microwave Anisotropy Probe (WMAP) when its mission was declared complete in September 2010.
More good news for JWST: The Ariane put it in such a good trajectory that it had to use very little of its own fuel for orbital correction, meaning it’s going to have a much longer operational mission. Depending on the error, JWST could have had to use enough fuel to lower its expected lifespan to 5 years or less, with an expectation of 5-10 years of life. But now it looks like it will live for ‘significantly more than’ 10 years:
Port wing of the primary mirror fully deployed and latched. Remaining: Starboard wing deployment, alignment of individual mirror segments, and L2 insertion burn.
Question: how much pan and zoom does the main mirror assembly have? All the depictions I ever see is the mirror pointing directly forward. If the shield has to point forever toward the sun, doesn’t this drastically limit the range of the sky that can be imaged?
There is no pan or zoom. The entire spacecraft is rotated, albeit within significant restrictions. The telescope will always only see a disc roughly perpendicular to a line between it and the sun, but over the course of the year that disc will cover the entire sky.
Scott Manley explains. If I’ve done this right, the link jumps to the relevant bit of the video.
Great explanation, thanks!
Strange. That seems to be contradicted by this:
The telescope and scientific instruments started to cool rapidly in the shade of the sunshield once it was deployed, but it will take several weeks for them to cool all the way down and reach stable temperatures. This cooldown will be carefully controlled with strategically-placed electric heater strips. The remaining five months of commissioning will be all about aligning the optics and calibrating the scientific instruments.
Since they felt the need to de-orbit the WMAP, there is obviously some utility to doing so, but I wonder if it’s strictly necessary. L2 is inherently unstable, which is why the JWST needs to make constant orbital adjustments in the first place. Perhaps the orbit would eventually degrade on its own, though I have no idea how fast or how much. But if the JWST got sufficiently far away from L2, it would have a natural tendency to drift to a higher orbit, losing energy and slowing down until its orbital velocity around the sun matched its orbital radius.
This is actually a biggie that I had forgotten about. Each of the 18 mirror segments has 7 actuators, 6 around the perimeter of the hexagon, and one in the centre to adjust curvature. That’s 126 potential points of failure!
Sure but a single failure wouldn’t be very significant.
I would just like to say that I fucking love science.
The primary mirror has just been successfully deployed. Yay!
I happened to catch the live feed of the starboard wing this morning, I figured the angle would be higher than 90 – it ended up ~ 105 degrees.
Brian