STEREO-A's Heliospheric Imager captured Comet ISON tracking left to right as it heads toward its close encounter of the hot kind: it will graze the sun on Thursday.
The video covers Nov. 20 to Nov. 22, 2013.
The smaller moving body seen in the middle ground is Comet Encke. The sun is off-stage right. The instrument visualizes the solar wind, which is also kool.
BTW, Mercury is not farther from the Sun than Earth. It's the perspective. Here is the position of the two STEREO satelites as of 27 Nov 2013. Put yourself at A (the one Ahead of the Earth and look back toward the Earth and you will see why Mercury looks farther from the Sun.
Thanks for this TOF. I was unaware that we had placed sattelites of any kind along our orbit, so far from Earth itself.
ReplyDeleteIf the Earth is 93 million miles from the Sun, then from the image STEREO would have to be much further from Earth than that.
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DeleteTOF, these satellites are not parked in L4 and L5 Lagrange points. They are actually orbiting the Sun roughly along Earth's orbit. You can see this by changing the dates on the "STEREO Orbit Tool" page at http://stereo-ssc.nascom.nasa.gov/cgi-bin/make_where_gif
DeleteYou can see, for example, that the two objects will pass each other around 20 March 2015.
There are Lagrange points in gravitational system: the original ones are the leading and trailing Trojans in Jupiter orbit, as a result of the Sun-Jupiter balancing points. The one's you are thinking of are the Earth-Moon Lagrangian points. But there are also the Sun-Earth Lagrangians.
DeleteThe STEREO satellites are not on anything's Lagrange points.
DeleteThe Sun-Earth Lagrange points are "fixed" relative to the Earth; the L4 and L5 Lagrange points stay roughly 93 million miles ahead of and behind the Earth.
The STEREO satellites, however, are moving relative to each other and to the Earth. They will pass each other 20 March 2015 (something Lagrange points cannot do), and they will eventually pass close to the Earth as well.
I looked into this further, and found an early planning document for the STEREO effort dated Dec 1 1997. It is a PDF of a photocopy, so you cannot search the text in it. But it is a worthy read. (I saved a copy to disc, because this kind of stuff can disappear into the digital dark age abyss, like comments have done at First Things!)
ReplyDeletehttp://stereo-ssc.nascom.nasa.gov/publications/stdt.pdf
Page 36 of the PDF (p. 28 of the report) has this:
"We recommend that the STEREO mission consist of two identically instrumented Sun-pointed spacecraft at 1 AU. The spacecraft should slowly drift away from Earth, so that after 2 years, STEREO #1 will lead Earth by 45° and STEREO #2 will lag by 60°."
Page 39 of the PDF (31) has this:
"Phase4: Global Solar Evolution and Space Weather (after day 1100, α > 180°) When the separation of each STEREO spacecraft from the Sun-Earth line becomes greater than 90°, events on the far side of the sun that launch particles toward Earth will be visible for the first time."
I also read something in there about "adopting a heliocentric orbit", but I cannot find that now.
So it seems from this that the plan was not to use Lagrange points originally. This doesn't of course mean that they didn't decide to adopt Lagrange points. It does look from the orbit movies that they started them out close to L4 and L5. But from the position calculator at the NASA site, they have long since moved away from them, which was the plan according to the doc. I cant help wondering now whether L4 and L5 are "reserved regions"... a big spherical barb wire fence with laser guns LOL... a stable bit of real estate (unreal estate?) like that could be very valuable. Maybe they were told "um no, you WONT be setting up residence in Lagrange 4 or 5".
It is great finally to find where YOS writes. It is a surprise to me to find that you write SF, but I am going to have to check it out now (I think it has been 35 years since I read anything SF).