Foreword January 2021
As a lifelong Astronomy Nerd (perhaps not at birth, but not long after) I cannot help but notice how Planetary Science has advanced over the last half century. To say that much has been discovered is a ridiculous understatement. This theme of Solar System Astronomy can also be noticed in my other Categories (Comets, Planets, Asteroids, Urban Astronomy, Science and occasionally even in Going Walkabout, Energy, One Climate Fact and Humor*.
*These are the collections behind those icons on my homepage at Goingwalkabout.blog.
This is the sequel to my earlier post Kuiper Belt Object 2014 MU69
As related below the body now officially named 486958 Arrokoth was indeed described and analyzed at the 50th Lunar and Planetary Science Conference in 2019.
Prolog – March 2019
As expected, the LPSC has revealed much greater detail about the Kuiper Belt Object 2014 MU69 (A.K.A. Ultima Thule)
The flyby of this Kuiper Belt Object was declared a 100% success by the Principal Investigator Alan Stern. The closest approach was at about one fourth the distance as the previous KBO encounter with Pluto. This is not surprising when the new target is comparable to a small mountain on Pluto. That, and the fact that the velocity of the probe is about seven miles per second (yes, per second) complicates the task. Other compounding circumstances were the light level comparable to a moonlit night on Earth, a spacecraft design that does not allow continuous contact and a download rate that is agonizingly slow. As discussed earlier these were design trade-offs that made a long, difficult exploration effort possible – and quite successful – on a limited budget.
At the time of the Special Session at the LPSC, it had been 80 days since the encounter and considerable data had been received. However, more than a year separates us from the completion of that transmission. The state of knowledge at this time was summed up by the PI in the first presentation and those items are presented below along with details filled in by the presenters that followed.
An earlier comparison to a “snowman” shape were based only on a 2D view. Both the bodies that makeup this contact binary are thinner in the third dimension. The larger one (now popularly called “Ultima”) is actually shaped more like a thick hamburger patty. The smaller body (Thule) is more nearly spherical, but still visibly “flattened”. This is not unprecedented. There is a moon orbiting Saturn called Hyperion that has a similar shape to Ultima’s.
As promised, the resolution now available is greatly improved. The panel below is a stereo “cross-eyed” view. It takes a bit of practice, but it is possible to focus the left eye on the right image and vice-versa to get a clear and vivid 3-D view that appears between the two. Sit up straight and view with no tilt on the image. Hold your head very still and adjust by tilting slightly right or left. It should resolve into a clear and quite impressive 3-D image. I find it works best, with minimal eye-strain at about 2 feet (about 60 cm) away.
Figure 1: Raw images of the contact binary MU69. These are from the LORRI instrument and therefore lack the red coloring or the previous, low resolution image. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
- The crater on the smaller body (Thule) is seen to be quite deep. It may be an impact feature, but no one has dared to unambiguously claim so. Other craters may have been caused by faulting, perhaps during the merger. None of the few craters seen can definitely said to be caused by impact.
- A comparison was made to Phobos, the greater moon of Mars, and its biggest crater named Stickney. The two bodies are roughly the same size. Phobos is one of the darkest objects in the inner solar system and in that sense, also very similar to Thule. See Figure 2 for a side-by-side images.
Figure 2: Left: MU69-B (Thule) and Crater “Missouri” Right: Martian Moon Phobos and Stickney Crater
Phobos and the lesser moon of Mars, Deimos were discovered by American astronomer Asaph Hall in 1877. Hall was about to abandon his search for Martian moons, but was convinced to struggle on by his wife, Professor Chloe Angeline Stickney Hall.
Somewhere in the vast body of writings by Issac Azimov (and almost certainly somewhere in the shelves and piles of ancient paperbacks I still possess – and have forbidden my wife from recycling) is told the anecdote about a presentation wherein the speaker related this story and hoped – that when Phobos was ultimately imaged – a major feature might be named for the mathematician, suffragist and abolitionist who would not let Hall give up the search in which he ultimately succeeded. When the presenter (who may well have been Azimov, himself) admitted his ignorance of her name, an attendee stood up and shouted, “Angeline Stickney” (for she did not use her first name). And, in the fullness of time, it came to pass that the crater that dominates the surface of Phobos is so named.
More Points about 2014MU69 / Ultima Thule
- Bright spots tend to be in low zones. Troughs and mounds are clearly discernable.
- The merger of these two bodies into one was determined to be a “gentle one”. In the distant realm of the Kuiper Belt, and especially among the “Cold Classical”, objects that find themselves near each other tend to be moving with the same velocities. Willian McKinnon described this method of simulating the velocity of merger: “Walk into a wall”.
- No satellites were detected. Nor was there any detectable atmosphere.
- This may be because any satellites there might have been would have been ejected from orbit during the “merger” process.
- It is my (your humble narrator) own interpretation (2021) that this system once consisted of five or six main bodies – all orbiting in the same plane – that were slowly accumulated onto the larger of the two. And then the accumulated “hamburger patty” (whose shape indicates the co-planar rotation IMHO) and the smaller body finally merged. Tidal stresses in orbiting bodies tend to slow down the mutual rotation – thus bringing them all together.
- The period of rotation has been pinned down to 15.9 hours.
The New Horizons spacecraft probably has enough fuel to re-direct to another Kuiper Belt Object. There is not yet a target body known to be in reach and Ultima Thule was found at the limits of the Space Telescope capability. It is expected that the probe’s own long-range imager (LORRI) will be the instrument to find the next destination. It is estimated that one more target will be the last possible. There is no hurry, however, since the probe will be in the Kuiper Belt until 2027 or so.
See NASA video about MU69/Arrokoth
Ex Sciencia, Trivia,