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NOTE: I think either possibility suggested below is possible, I'm not looking for arguments why one of them would be your favorite like this answer. Instead, I'm looking for a fairly conclusive answer; either by ruling one out beyond reasonable doubt, or better yet by supporting your answer with authoritative sources discussing these particular artifacts in these specific images.

For discussion of the GIF below, see:

  1. this answer

  2. New Horizons Mission page February 8, 2019 New Horizons' Evocative Farewell Glance at Ultima Thule; Images Confirm the Kuiper Belt Object's Highly Unusual, Flatter Shape

  3. New Horizons’ Evocative Farewell Glance at Ultima Thule

I see a lot of white dots that repeat in each frame, moving steadily left to right. But I also see a lot of similar white dots that do not.

Going frame by frame, the ones without the collective left-to-right motion often appear in one frame only, but in some cases I can convince myself (but not others) that a dot may appear in two adjacent frames, just moving in a different direction.

Which makes me wonder if it could be debris around the asteroid, or between it and New Horizons, basically anything close enough that it doesn't appear fixed on the celestial sphere.

One of the reasons for all of the occultation observations in 2017 and 2018 (see also Timing shadows from the Kuiper belt! Any news? Did it work? and Will the upcoming observations of occultation by “Ultima Thule” (2014 MU69) be of a single object, or two?) of MU69 (Ultima Thule) was to check for the possibility of a debris field around MU69 that might make a very close flyby dangerous. None were discovered but the unusual shape was suggested by the strange occultation results.

Question: What are these white dots? Real objects, noise or cosmic rays in the silicon sensor, or something else?

Borrowed from this answer to the question Will there be back-side photos of Ultima Thule? and originally from New Horizons’ Evocative Farewell Glance at Ultima Thule

New Horizons flyby of Ultima Thule

also viewable in YouTube

Mission scientists created this "departure movie" from 14 different images taken by the New Horizons Long Range Reconnaissance Imager (LORRI) shortly after the spacecraft flew past the Kuiper Belt object nicknamed Ultima Thule (officially named 2014 MU69) on Jan. 1, 2019. The central frame of this sequence was taken on Jan. 1 at 05:42:42 UT (12:42 a.m. EST), when New Horizons was 5,494 miles (8,862 kilometers) beyond Ultima Thule, some 4.1 billion miles (6.6 billion kilometers) from Earth. The object’s illuminated crescent is blurred in the individual frames because a relatively long exposure time was used during this rapid scan to boost the camera’s signal level – but the science team combined and processed the images to remove the blurring and sharpen the thin crescent. This is the farthest movie of any object in our Solar System ever made by any spacecraft. The images reveal an outline of the “hidden” portion of the Ultima Thule that was not illuminated by the Sun as the spacecraft zipped by, but can be “traced out” because it blocked the view to background stars also in the image.

Credits: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/National Optical Astronomy Observatory

Food for Thought:

Asteroid caught red-handed ejecting particles!

From Space.com's Surprise! 'Active Asteroid' Bennu Is a Rare Particle-Ejecting Space Rock:

enter image description here

This view of the asteroid Bennu ejecting particles from its surface on Jan. 19, 2019, was created by combining two images taken by NASA's OSIRIS-REx spacecraft: a short-exposure photo, which shows the asteroid clearly, and a long-exposure version, which shows the particles clearly. Other image-processing techniques were also applied, such as cropping and adjusting the brightness and contrast of each layer.(Image: © NASA/Goddard/University of Arizona/Lockheed Martin)

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  • $\begingroup$ I'm still debating if this is better asked here or in Astronomy SE. It's on-topic in both places, arguments can be made both ways. Any thoughts? $\endgroup$
    – uhoh
    Commented Feb 9, 2019 at 3:06
  • $\begingroup$ I think noise can be eliminated as, noise effects randomly to many pixels randomly distributed. And Cosmic rays, well until and unless one crosses Oort Cloud, I doubt cosmic rays can effect so many pixel in just one second in the image sensor. New horizon rotates at about 30 degrees per second, and looking at total rotation of celestial sphere it appears to be 1 second video to me. As to what those white dots are, could it be that the ultima Thule reflection from sun just over exposes the surrounding pixels shadowing the stars for some duration? $\endgroup$
    – zephyr0110
    Commented Feb 9, 2019 at 5:48
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    $\begingroup$ @Prakhar the individual images have just become available now including individual timestamps and exposure times! pluto.jhuapl.edu/soc/UltimaThule-Encounter $\endgroup$
    – uhoh
    Commented Feb 10, 2019 at 4:43
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    $\begingroup$ The JHUAPL website says "Many background stars are also seen in the individual images", but it would be good to know the field of view and magnitude limit of the exposure/binning to be sure that many of the objects are stars. $\endgroup$
    – JohnHoltz
    Commented Feb 18, 2019 at 2:28
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    $\begingroup$ @JohnHoltz Assuming these are full frame images, I think all the objects that move together smoothly can't be noise, or hot pixels, and so probably all of them are stars, and their collective motion is the rotation of the spacecraft itself. But that's just my opinion. I'm asking about the similar brightness dots that appear in one frame only, and don't follow any pattern. $\endgroup$
    – uhoh
    Commented Feb 18, 2019 at 2:32

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Since no one has answered yet, I'd like to add some helpful information here that's too long to fit as a comment.

Note that the spacecraft was rotating backwards from what would have been desired.
I stacked the images that show Ultima Thule and re-registered them to remove that rotation.

Green dots mark stars (present in multiple frames) and red dots mark noise.

The stars marked with dashes are the two that show (by occultation timing) that Ultima is prolate rather than spherical.

While the ultimate question of what causes the noise still stands open, this may be helpful towards that end.

enter image description here

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  • $\begingroup$ Thanks and Welcome to Space! I like this analysis, it makes it much easier to see what's happening than staring at the GIF all afternoon. ;-) $\endgroup$
    – uhoh
    Commented Mar 21, 2019 at 3:54
  • $\begingroup$ just fyi in case you didn't already know the original (un-GIFified) images can be found here: pluto.jhuapl.edu/soc/UltimaThule-Encounter $\endgroup$
    – uhoh
    Commented Mar 21, 2019 at 3:58
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    $\begingroup$ I wish someone would re-do the video to stabilize the stars (they move right 6 pixels per frame, which is a lot at this low resolution). $\endgroup$
    – amI
    Commented Mar 21, 2019 at 4:00
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    $\begingroup$ Yes - I used the JPL jpegs. Being .jpg they may already have artifacts that make it harder to evaluate the type of noise. $\endgroup$
    – amI
    Commented Mar 21, 2019 at 4:03
  • $\begingroup$ update: the raw images are now at pluto.jhuapl.edu/soc/Arrokoth-Encounter $\endgroup$
    – uhoh
    Commented Feb 19, 2020 at 12:50

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