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NASA WISE Telescope Starts Taking Pics
coondoggie writes "NASA said its Wide-field Infrared Survey Explorer spacecraft successfully popped the cover off its infrared telescope and began 'celestial treasure hunt' mission of sending back what will be millions of images of space. The WISE lens cap served as a safety system keeping the ultra-sensitive lens and telescope system safe until the spacecraft positioned itself correctly in orbit. The cap also served as the top to a 'bottle' that chille the instrument and detectors. This cryostat is a Thermos-like tank of solid hydrogen."
Thermos like. Or, if you don't want to step on anyone's trademarks, a Vacuum Flask. You can even see a picture of WISE's cyrostat at Wikipedia
Yes, they do need the hydrogen to keep things chilly. The vacuum is a nice insulator, but can't prevent all the heat from all that sunshine hitting the satellite (and other sources) heating things up- including those very, very sensitive detectors. Noise from heat is the enemy, which is why they've gone through so much trouble to keep the detectors cold and safe in a covered 'Thermos' until ready to serve.
The hydrogen is a nice heat sink in addition to the natural vacuum flask provided by Space, and is allowed to slowly boil off, carrying away what heat that makes it through. It's also why there's a definite time limit on the main part of the mission and effective lifetime; once they hydrogen is gone, the detectors begin to heat up, heat noise ruining all that lovely sensitivity.
I suspect that they'll continue to get some results after the liquid hydrogen is gone but, much like similar previous missions, the "warm" part of the mission will not achieve anywhere near the same results as the first "cold" portion.
Space, being a vacuum, does not dissipate heat all that well.
The operating temperatures will be 30–34K for the 3.3 & 4.7 m detectors, 7.8 ± 0.5K for the 12 & 23m detectors and 17K for the optical system, which are achieved using a two stage solid hydrogen cryostat providing a minimum mission lifetime of 7 months allowing for a single full coverage of the entire sky.
Update on The Wide-Field Infrared Survey Explorer (WISE).
and surely the hydrogen will now boil off?
Exactly what they want. As the hydrogen melts and boils off, it removes heat which keeps noise down and image quality up. When the hydrogen finally boils off, it'll greatly reduce the value of the telescope since the internal components will heat up. That's probably when they'll end the mission.
TFA specifies that the hydrogen will run out about ten months into the mission (including a month for systems check-out).
Allegedly real newspaper headline from 1998:
Man Struck by Lightning Faces Battery Charge
Doubtful. One thing you can be pretty sure of is that a mission that uses cryogenic expendables won't last much longer than planned. The tanks are built to a certain size and filled to brimming; working out how long that'll last is fairly straightforward. You might get perhaps 50% longer than calculated if you're very lucky (more likely you'll get less), but no way will it last five times longer.
As an example, the Spitzer Space Telescope was planned for 5 years before the cryogens ran out: it lasted roughly 6.3 years.