Herschel and Planck safely away The European Space Agency's Herschel and Planck space telescopes safely blasted off this afternoon at 13:12 GMT from the ESA spaceport in French Guiana. The Ariane 5 carrying Herschel and Planck ESA reports: "Herschel, the upper passenger, was the first to separate from the upper stage of the Ariane 5 at 13:37 GMT at an …
The article is correct. The L2 point of a body (in this case the Earth) is generally regarded as that between it and the body _it_ is orbiting, not another unstated body.
Nor is the Sun-Earth L2 point in the Earth's shadow and screened from the Sun. At that distance the Sun's disc is larger than that of the Earth, so there is no shadow. Probes there that are sent to get away from the Sun's emissions need shading on the Sun-facing side. In any case for CMB observations ideally you would want to protect yourself from the Earth, not the Sun, but that makes communications with Earth impossible without at least one relay probe.
If you're going to be pedantic, make sure you get it right...
The Earth/Sun L2 is about a million miles from Earth, i.e. four times the distance of the moon. Earth has four times the diameter of the moon. These fours cancel. As seen from L2 the earth and the sun will be about the same size in the sky (like the moon and the sun as seen from earth) which puts earth on the verge of not being able to totally eclipse the sun so from time to time the eclipse may be partial or annular. Is this OK, or likely to cause a problem, or total bull?
The Earth's angular diameter at L2 is 30 arcminutes, the Sun's is 31.5. So if you were sitting right at L2, you would gain some pretty valuable shielding.
However, L2 is a saddle point and satellites need to orbit it. In the case of Herschel, this is at a mean radius of about 800 000 km. Hence the Sun pops out from behind the Earth. But L2 does have a major advantage for shielding because the main heat sources - Earth as well as Sun - are always in the same part of the sky relative to the telescope. So you can just put a single heat shield on one side of your spacecraft and hide behind it. Look at e.g. the design of JWST - naked primary miror hiding behind a big parasol, basically...
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>> The SAPPEUR language destroys unneeded data structures as quickly as possible, thereby saving energy
Teh. Destroying information actually generates heat. Just leave those bits where they are...
But seriously, SAPPEUR is actually being deployed? It apparently allocates mucho stuff on the stack; you know what happens to your real-time algorithms if your stack grows quickly. I remember something about an ESA SWENG rule saying "no dynamic data structures in high-assurance real-time programs" (not that I have every written any of those) and "no C++ either".
Also,
"Reference Counting is realtime capable (as opposed to garbage collection)"
Uh... yeah. But there is a very good a reason why mark & sweep garbage collection exists, as opposed to vanilla reference counting garbage collection, right?
It comes down to money in the end - they're both going to L2 and an Ariane V launch costs about €200M so it's a substantial saving, bringing the total cost of both missions down from €1900M to €1700. Is it worth the risk? Well, Ariane V had a shaky start, I remember many people in the community being worried when ESA made the decision to launch both together, which is now maybe ten (or more?) years ago. But there's only been one failure in the last 30-something Ariane V launches so I think the risk/benefit looks reasonable from today's point of view.
The general attitude with these gigabuck science missions is to take the risk. When you've had a 20-year gestation period, and the satellite design gets frozen many years before launch, the technology will always be dated by launch time. Even if you had the funds to build a copy of the satellite, it'd be even further behind by the time it got launched, so better to look ahead and get working on the next generation. The only mission I can think of that was replaced by a near-copy was Cluster.
@David Wilson: Well said sir! That sort of common sense could go a long way in some space agencies :)
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