2014 MU69 "Ultima Thule" flyby, For discussion of the encounter as it happens Options

[dudley]

Odd, but I'd received the impression that an image with about twice the resolution of that shown yesterday was likely to be seen today.

Looking at the above 3d image, it appears as though the possible large depression on the 'Thule' lobe could be unusually deep and steep-sided.

[fredk]

According to Emily, we were supposed to get another 140 m/pixel image overnight, which would be similar resolution to the higher-res one from yesterday.

[HSchirmer]

Looking at the above 3d image, it appears as though the possible large depression on the 'Thule' lobe could be unusually deep and steep-sided.

It helps to consider that the two lobes of Ultima Thule are, roughly, the size of the nucleus of Halley's Comet.
Which raises the possibility that it could also be a comet 67P style sublimation pit.

But that triggers some really interesting questions about the scale of KBO building blocks-

Comet 67P shows evidence that is formed out of ~10 foot/3 meter units, "dino-eggs" (aka "accretting pebbles").


UT is about 10x the diameter of comet 67P, so, roughly, the "pits" on UT are around the size of 67P's lobes.

One big question is whether is UT is composed of 67P sized chunks, or from 10m "pebbles", or a mix of both.

Another big question is, which follows from observing the "diamond in the sky" rubble pile asteroids Bennu and Ryugu, is, what is causing the lobes of Ultime Thule to hold their shape? Are Ultima and Thule just "pebble piles" which hold their shapes because they have acquired a tholin surface skin?

[kenny]

Pluto, and therefore Ultima Thule, are both very much in the plane of the Milky Way now,

As I understand it, the Milky Way (our galaxy) is inclined at about 60 degrees to the Plane of the Ecliptic in which Pluto, Ultima Thule and New Horizons currently reside.

[Hungry4info]

Right but the two planes intersect, and at that intersection, in the crowded star fields of the galactic centre, it's difficult to detect solar system bodies amidst the glitter of lights.

[dudley]

Which raises the possibility that it could also be 67P style sublimation pit...

Quite possible. An impact that large, in proportion to the size of Thule, might have shattered it. Is much sublimation expected at 44 AU from the Sun?

[WTW]

Quite possible. An impact that large, in proportion to the size of Thule, might have shattered it. Is much sublimation expected at 44 AU from the Sun?

And we need to be thinking in terms of dynamics of materials at near absolute zero. (That react differently than materials heated by high-velocity impacts, radiation, etc.) Maybe even a "low velocity" impact could add significant heat relative to those low ambient temperatures, and cause structural as well as chemical changes.

Likewise, could heating from radioactive decay of things like 26Al be very localized within those bodies -- causing such things as outgassing from certain areas and not others?

[HSchirmer]

Is much sublimation expected at 44 AU from the Sun?

Yep, quick double-check shows that common ices are still volatile way past 44 AU.

N2 sublimation- 22 Kelvin/160AU
CO sublimation- 25 Kelvin /120 AU
CH4 sublimation- 31 Kelvin / 80 AU

Using Cometary Activity to Trace the Physical and Chemical Evolution of Cometary Nuclei

[mcaplinger]

I assume that there's a very good reason for not using thumbnails and I'm very interested to learn what it is.

Not having to have the ground in the loop to decide what to send down would be one reason. Especially when you're six light-hours away.

[Req]

Not having to have the ground in the loop to decide what to send down would be one reason. Especially when you're six light-hours away.

No need for the ground to be in the loop for the operation I described. What I described is a simple sequence of software that the spacecraft can run to produce a data product that could be less than 300 bits which would tell them if, and in exactly which images the highest resolution pass captured UT. It's not unprecedented, this has been a common practice during the opening days and hours of multiple recent missions and encounters(thumbnails in general that is, not the extra reductions I described to further minimize the size of the useful dataset in the case of this particular mission.)

[mcaplinger]

What I described is a simple sequence of software that the spacecraft can run to produce a data product that could be less than 300 bits which would tell them if, and in exactly which images the highest resolution pass captured UT. It's not unprecedented, this has been a common practice during the opening days and hours of multiple recent missions.

Which missions are those?

Maybe they just didn't think of it or they couldn't make it fit in their FSW. There's a general bias to minimize work done on the space side even if work on the ground side is 10x harder.

We use thumbnails on MSL, obviously, but usually not to pick and choose what to send down once we see the thumbnails. Sometimes, not often.

[Req]

Which missions are those?

Maybe they just didn't think of it or they couldn't make it fit in their FSW. There's a general bias to minimize work done on the space side even if work on the ground side is 10x harder.

We use thumbnails on MSL, obviously, but usually not to pick and choose what to send down once we see the thumbnails. Sometimes, not often.

MSL is the one that comes to mind instantly, however I've seen the same practice being used on Cassini for encounters, ROSETTA did it, and I just assume it's being/was used for insight, since phoenix did the same. Perhaps the piece that I'm missing is just your third sentence about 10x ground. Thanks for that insight.

[mcaplinger]

however I've seen the same practice being used on Cassini for encounters...

Did Cassini use thumbnails? Maybe, I wasn't aware it did.

NH probably wasn't designed with a flyby of a small body with such uncertain centering in mind, I don't think they had so much of a problem at Pluto (but I could be mistaken.)

FWIW, I argued long and hard that thumbnails for MSL Mastcam were not worth implementing. In hindsight I was wrong, but the use case of ground-in-the-loop selective transmission doesn't end up getting used that often, and we had to work fairly hard to get the thumbnail products small enough to be cheaply downlinked. They are mostly useful just to show that a sequence executed as designed, but that's fairly expensive for what amounts to debugging messages (though one can amuse oneself by building mosaics of thumbnails while waiting for the full products, etc.)

[nprev]

You may have answered your own question. Req. We don't know how the flight software of NH is configured, so it's possible that adding such a feature would incur operational risks that we with our limited (as in zero) understanding of the FSW do not know. Additionally, time and labor are limited resources, and presumably something like this has gotta rank pretty low on the priority list.

EDIT: I see that Mike has answered you as well.

Moving on...

[WTW]

Did Cassini use thumbnails? Maybe, I wasn't aware it did.

NH probably wasn't designed with a flyby of a small body with such uncertain centering in mind, I don't think they had so much of a problem at Pluto (but I could be mistaken.)

FWIW, I argued long and hard that thumbnails for MSL Mastcam were not worth implementing. In hindsight I was wrong, but the use case of ground-in-the-loop selective transmission doesn't end up getting used that often, and we had to work fairly hard to get the thumbnail products small enough to be cheaply downlinked. They are mostly useful just to show that a sequence executed as designed, but that's fairly expensive for what amounts to debugging messages (though one can amuse oneself by building mosaics of thumbnails while waiting for the full products, etc.)

Emily mentioned that histogram data are included in the image metadata that is being downloaded before the rest of the images are -- which should be enough to determine which frames contain UT image content, and to help prioritize them.