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    Re: Celestial navigation in space
    From: Andrés Ruiz
    Date: 2011 May 13, 14:49 +0200
    Hi Frank, very interesting.
    Do You know, if the solver is not "blind", how many time it takes to calculate an image?
    Years ago I was interested in knowing something about the project http://www.astrometry.net/
    Andrés Ruiz
    Navigational Algorithms

    2011/5/13 Frank Reed <FrankReed@historicalatlas.com>

    There is a case of celestial navigation in space happening right now. The NASA spacecraft named Dawn is preparing to enter orbit around the large minor planet Vesta (which was discovered way back in 1807). They just turned on the navigation camera a couple of days ago and shot a photo of Vesta in front of a field of stars. From the photo the "navigators" at JPL can get a line of position simply by measuring the position of Vesta among those stars. The position of Vesta in space (3d, x,y,z position) is known so then you just draw a ray extending out from Vesta towards the opposite direction on the celestial sphere. Spacecraft has to be on that ray. Easy!

    JPL put together a press release on the first navigation photo yesterday since it also is the first actual photo of Vesta taken by Dawn:
    Scroll down to the enhanced image.

    I got to wondering if I could do the navigation here and figure out where Vesta lies among the stars as seen in that photo. I tried the manual approach for a few minutes and gave it up as hopeless. Ah, but wonders of the Internet, there's an app for this. If you upload a picture to flickr.com and then post it to the astrometry group, it gets run through the solver at astrometry.net and their software will figure out what you're looking at without even knowing whether the photo is ten degrees wide or ten minutes of arc wide. It's pretty damn impressive. It takes a couple of hours for the server to get around to solving a particular image. Here are the results for the Dawn navigation image:
    In case that link doesn't work, I'm attaching a screen capture. Among the brighter stars in the image are xi, psi, and chi Scorpii. The RA and Dec of Vesta in the photo (now just interpolating by eye) are 16h 7.8m, 10d 17.3' S. With an accuracy of about 1 minute of arc in the angular position and a distance off of about 750,000 miles that pins down the position of Dawn to within about 200 miles (in directions perpendicular to the line of sight to Vesta). Obviously with just a little more effort, there's enough information in that photo to narrow the position to within about 20 miles.


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