KBO encounters Options

[Alan Stern]

Great news, 5 occultations, so the obvious questions are why didn't any of the scopes detect it on 3rd June and what are the Sofia results?

June 3rd got the hazard search we wanted done but didn't put telescopes in the right place because back then we didn't have the MU69 orbit prediction well enough in hand. Subsequent HST June-July data helped with that.

[JRehling]

Considering how slow its orbital motion is relative to Earth's I would expect the latter is the main factor determining the speed of the occultation.

That's definitely true. In fact, with both objects revolving the same way, it's more the Earth's motion minus the occulter's motion (near opposition) that determines the speed of the occultation. This is the reason why Venus and Mars have longer synodic periods than Neptune and Pluto.

[jasedm]

Phenomenal and diligent work ensuring the occultation was observed. Congratulations all.

Does anybody know if there are any plans to provide 2014 MU69 with a name prior to the flyby? (can't seem to find anything online)

[Tom Tamlyn]

From an April 28 blog post by Alan Stern:

One last thing I want to tell you is something I get asked a lot about. Yes, we’re going to give 2014 MU69 a real name, rather than just the “license plate” designator it has now. The details of how we’ll name it are still being worked out, but NASA announced a few weeks back that it will involve a public naming contest. Look for more information on that in the fall.

https://blogs.nasa.gov/pluto/2017/04/28/no-...-back-on-earth/

Edit: I didn't find this through a search on the naming issue; I just happened to look for the latest NH blog post the other day.

This post has been edited by Tom Tamlyn: Jul 20 2017, 10:10 PM

[Tom Tamlyn]

Earlier today Dr. Stern mentioned that the June 3 observations "got the hazard search we wanted done," even though no occultations were observed.

The interesting posts unthread from HSchirmer starting here helped me to understand how negative occultation results can provide some information about 2014 MU69's orbit and perhaps some constraints on its size that confirms previous calculations.

I'm puzzled how the team could glean information about hazards in the absence of an occultation, except perhaps by grossly negating the existence of extremely large companion objects, the existence of which would already seem unlikely.

I'm also curious about the SOFIA results. Has an occultation not yet been confirmed by analysis, or have the SOFIA results possibly been overtaken in importance by the multiple observations taken on July 17?

[Hungry4info]

I'm puzzled how the team could glean information about hazards in the absence of an occultation, except perhaps by grossly negating the existence of extremely large companion objects, the existence of which would already seem unlikely.

With occultations in hand, we have a much better idea of the orbit and position of 2014 MU69. The non-detection from the first observations can then be "re-calculated" to show that in a chord that passes near the asteroid, no debris was seen. For example here are numerous observations of a similar event for 90 Antiope. Several observations failed to detect anything, so each non-detection can allow you to place upper limits to the amount of dust in the area near the asteroid.

Edit: Perhaps a more illustrative example is Chariklo, where stellar occultations that missed the main body still allowed for the detection of rings around it.

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[Tom Tamlyn]

Thanks, that's very helpful.

I had assumed that the sixteen inch Dobsonians deployed by the team weren't powerful enough to detect the occultation of rings or satellites on their own, but rather that the existence of rings/satellites could only be inferred by the crispness of the edge of the main body's image. I didn't have any basis for that assumption, and I see from an arxiv paper on the Chariklo ring discovery occultation that ring-only observations were made by instruments with an aperture of approximately ten inches.

I also found this discussion on the NH website:

"

Combined, the pre-positioned mobile telescopes captured more than 100,000 images of the occultation star that can be used to assess the environment around this Kuiper Belt object (KBO). While MU69 itself eluded direct detection, the June 3 data provided valuable and unexpected insights that have already helped New Horizons.

These data show that MU69 might not be as dark or as large as some expected," said occultation team leader Marc Buie, a New Horizons science team member from Southwest Research Institute (SwRI) in Boulder, Colorado. * * *

Besides MU69's size, the readings offer details on other aspects of the Kuiper Belt object.

"These results are telling us something really interesting," said New Horizons Principal Investigator Alan Stern, of SwRI. "The fact that we accomplished the occultation observations from every planned observing site but didn't detect the object itself likely means that either MU69 is highly reflective and smaller than some expected, or it may be a binary or even a swarm of smaller bodies left from the time when the planets in our solar system formed."

http://pluto.jhuapl.edu/News-Center/News-A...p?page=20170706

[HSchirmer]

[attachment=41796:Screensh...12.19.32.png]

Would a successful observation in the second or third occultation help ascertain where the first track actually was, and could they verify it was a 'picket fence' problem with the first one ?

Looks like the last observation benefited from a tighter fence...

Telescopes spaced only 4 or 5 km apart,
appears that they got occultation with five telescopes,
so size should be in the range of 16 km to 30 km.

And, now you've got really good data on the orbit and location for occultations next year.

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[JRehling]

Tom, the occultation / photometry paradigm permits almost unlimited resolution, in principle. This is how Kepler could determine the radii of planets over 5,000 light years away, Pluto and Charon were mapped during a series of mutual transits, and many asteroids have had their size and shape measured from Earth. The opportunities are, unfortunately, very limited and not readily of our choosing. It's a lucky coincidence that NH is barreling towards the Milky Way, which enormously increases the probability of occultations. If we'd sent a flyby past Pluto a few decades earlier/later, this would not have been the case.

Just last night, I completed a pair of images of Iapetus showing the steep difference in its brightness now versus 40 days ago. It'd be a tall order for an earthbound telescope to resolve Iapetus' surface features, but noting the difference in brightness was performed by Giovanni Cassini in 1705!

Absolutely no earthbound telescope built (or even conceived) could resolve a KBO at kilometer scale, but this occultation method would work even if the target were a thousand AU away.

[hendric]

If I did my maths right, to get 1km resolution at 30 AU requires a 716km aperture at 500nm wavelength. At 20cm (radio waves), it would take ~286,000km, so hypothetically possible if done with synthetic aperture using a lunar radio telescope.

It would be amazing if we got a Dactyl like moon, or a contact binary. My money though is on a fairly smooth surface made up of 2-3m sized objects stuck together, similar to what we saw on 67P. Of course, those won't be resolved by NH.

[Holder of the Tw...]

If I did my maths right, to get 1km resolution at 30 AU requires a 716km aperture at 500nm wavelength

Not quite so large as that. The 2.4 meter Hubble can already resolve Pluto, seeing features around 1000km big on that body. To resolve to 1 km you need a mirror about 1000 times bigger. So two to three kilometers.

Which is bigger than anything I've got.

[JRehling]

Note that HST's highest resolution photos of Pluto were taken in ultraviolet light – not a coincidence! Shorter wavelengths have smaller diffraction patterns and allow higher resolution, all other things being equal. In visible light, resolution would be about 25% worse.

I believe the construction of HST's Pluto albedo maps involved combining multiple images with a very well known shape model – Pluto had to be a sphere – which is a luxury one wouldn't have in trying to resolve the shape of a tiny body. So those maps had a better resolution than any one image of an unknown object could ever be.

[hendric]

Hubble's albedo maps used the mutual eclipses between Pluto and Charon to determine how much the system brightness dropped as each body was covered up by the other. With advanced geometry and lots of computer time, scientists were able to make a best-guess as to the actual surface. Pretty good job, IMO!

[stevesliva]

The target is binary, or oblong:
http://pluto.jhuapl.edu/News-Center/News-A...p?page=20170803

[Explorer1]

Gosh! I briefly thought this might be the case with the initial negative occultation result (i.e. the slimmer axis caused it to miss the star). Looks like we're getting two for the price of one!
Talk about good luck! (unless binaries are super common....)