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Dawn approaches Ceres, From opnav images to first orbit
Phil Stooke
post Jan 14 2015, 02:13 PM
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"Is it safe to say that the Dawn team is looking at the 25 pixel image of Ceres as we speak?"

You might very well think that. But I couldn't possibly comment.

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Superstring
post Jan 14 2015, 10:55 PM
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Is every image during approach going to be delayed several days? If so, is there a reason for that? Not to sound too impatient -- I don't recall that being the case for past missions so I'm just curious. And I may or may not have marked Jan 13, Jan 26, Feb 4, etc in my calendar... smile.gif
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Explorer1
post Jan 14 2015, 10:57 PM
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Pretty sure it was like that for Vesta too; thrusting takes a priority, since it can't point at Ceres and keep on trajectory at the same time. Dawn isn't Voyager, so no scan platform.
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djellison
post Jan 14 2015, 11:33 PM
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QUOTE (Superstring @ Jan 14 2015, 02:55 PM) *
I don't recall that being the case for past missions


It was very much the case for Vesta approach. Dawn doesn't operate an image policy like MER, MSL and Cassini.
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Superstring
post Jan 14 2015, 11:42 PM
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Ah, I guess my memory from the Vesta encounter is fuzzy -- thanks for the clarification.
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Mithridates
post Jan 14 2015, 11:52 PM
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During Vesta approach Dawn did the following:

http://dawn.jpl.nasa.gov/mission/journal_05_27_11.asp

QUOTE
To help target the probe for survey orbit, controllers have commanded it to observe Vesta once a week since the beginning of the approach phase on May 3. As we saw that day, the pictures allow navigators to gain a better fix on Dawn's trajectory relative to Vesta. So far, the images reveal little more than the desired important information of where Vesta appears against the background of stars. And yet, in a sense they show much more...


It was this image:

http://3.bp.blogspot.com/-d3pKJNvHCGg/Te2F...Sad%20Vesta.jpg

An image of Vesta from 1.3 or so million km away was taken on May 3 and released a week later. The next one was released on June 18th at a distance of 218,000 km, this one:

http://3.bp.blogspot.com/-XLZukRt5LlY/Tfw4...s1600/Vesta.jpg

Later on (though I don't remember where exactly I read it) the team said they didn't release the images in between because they were small, boring, untouched, etc.
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elakdawalla
post Jan 15 2015, 12:24 AM
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Because of the severe limitations imposed on the mission by the loss of the use of the reaction wheels, Dawn will be taking fewer approach images of Ceres than it did of Vesta -- I think there are only 8 opnav sessions planned before arrival (compared to 24 for Vesta), so it's far from every day. These limitations also mean fewer comm sessions. On the other hand, op nav images are by definition important for navigation, so I expect they are planned in such a way that they can be returned to Earth very soon after being taken. But it took them forever to release approach images for Vesta, and I don't expect Ceres to be any different.


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DrShank
post Jan 15 2015, 12:26 AM
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QUOTE (Explorer1 @ Jan 14 2015, 04:57 PM) *
Pretty sure it was like that for Vesta too; thrusting takes a priority, since it can't point at Ceres and keep on trajectory at the same time. Dawn isn't Voyager, so no scan platform.


Thats correct. The cadence of observations is that a set is taken for a hour or so on approach every 1 to 2 weeks, depending on the distance. >95% of mission time is spent ion thrusting and it can't point at ceres. There are a variety of exposures and filters and such during each sequence and it takes a few days to select the optimal exposure, remove camera artifacts, etc to release to the public. No worries, the team is eager to show the world what the target looks like! Once they settle into mapping phase, there will likely be a photo of the day release just as there was for Vesta.


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Mithridates
post Jan 15 2015, 12:57 AM
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QUOTE (elakdawalla @ Jan 15 2015, 12:24 AM) *
Because of the severe limitations imposed on the mission by the loss of the use of the reaction wheels, Dawn will be taking fewer approach images of Ceres than it did of Vesta -- I think there are only 8 opnav sessions planned before arrival (compared to 24 for Vesta), so it's far from every day. These limitations also mean fewer comm sessions. On the other hand, op nav images are by definition important for navigation, so I expect they are planned in such a way that they can be returned to Earth very soon after being taken. But it took them forever to release approach images for Vesta, and I don't expect Ceres to be any different.


I wonder. Because on the other hand Ceres is already at a distance where it has a similar resolution to the second Vesta photo, the one where the team thought it had become interesting enough to release.
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djellison
post Jan 15 2015, 01:07 AM
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QUOTE (Mithridates @ Jan 14 2015, 04:57 PM) *
I wonder.


I wouldn't. Images will show up when they show up. There's really very little point in try to guess when it'll happen.
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Explorer1
post Jan 15 2015, 01:25 AM
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Wonder no more: I have a strange sense that an image will be released on the 20th. wink.gif

https://twitter.com/NASA_Dawn/status/555511503369940992

It pays to check the most obvious source sometimes...
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Mithridates
post Jan 15 2015, 01:30 AM
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Nice! I was looking at that Twitter feed earlier in the day before they wrote that, glad to see they've given us a date.
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DrShank
post Jan 15 2015, 03:46 AM
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QUOTE (Mithridates @ Jan 14 2015, 07:30 PM) *
Nice! I was looking at that Twitter feed earlier in the day before they wrote that, glad to see they've given us a date.


nice to see these dates are being posted!


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TheAnt
post Jan 15 2015, 12:07 PM
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QUOTE (DrShank @ Jan 15 2015, 01:26 AM) *
Once they settle into mapping phase, there will likely be a photo of the day release just as there was for Vesta.


Close but not quite, due to the flywheel problem Dawn will have to use thruster power to reorient to send back to Earth.
To conserve fuel Dawn will be spending longer time taking images and do measurements, therefore we will be getting new images every second day at best.
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DrShank
post Jan 15 2015, 12:13 PM
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QUOTE (TheAnt @ Jan 15 2015, 06:07 AM) *
Close but not quite, due to the flywheel problem Dawn will have to use thruster power to reorient to send back to Earth.
To conserve fuel Dawn will be spending longer time taking images and do measurements, therefore we will be getting new images every second day at best.


yes, but by then there will be lots more archived to chose from to post on the Pic-o-the-Day site. Each orbit returns dozens of frames. The number will vary depending on altitude and which mapping phase we are in, of course, but I don't have those numbers.


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