Kuiper Belt Object 2014 MU69
Preface (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 a summary of what I knew about this fly-by of 2014 MU69, going into the Lunar and Planetary Conference (LPSC) of 2019. I subsequently wrote a “post conference” summary and many editions and deletions later, the foreshortened, simplified result appeared in Ad Astra Magazine: STILL SEEKING NEW HORIZONS: Ad Astra – Summer 2019 page 48.
A great deal of the detail that I included in this preliminary draft was left out of the final product. The length, format and tone were, of course, the choice of the editor. Since this early pre-conference composition contains but little of the final result, I take the liberty of presenting it in my own publication (again Goingwalkabout.blog.)
I will also be adding original commentary and explanation, so I reckon that it is original material by any definition. My intention is to also present (later) the post-conference story – partly in original form – which contained quite a bit of detailed and interesting information that did not fit Ad Astra’s “tone and format”. Not to criticize Ad Astra, you understand, but only to display my own interest and enthusiasm for richer detail. And, I want to present some of that in my own publication, free to read (and – alas – to write for) though it may be.
Before the Lunar and Planetary Science Conference in 2019
New Horizon Fly-by of Kuiper Belt Object (486958) 2014 MU69 (a.k.a., Ultima Thule)
By Steve Campbell
February 2019
After a successful fly-by of Pluto in July 2015 (to read more, please see Sneaking Up on Pluto, Part I and Part 2), the New Horizons space probe followed up with another encounter on New Year’s Day 2019. This target was a much smaller body known as 2014 MU69 and frequently referred to by its nickname “Ultima Thule” (and subsequently “officially” called 486958 Arrokoth – as of late 2019)
The first opportunity for publication and presentation of those results will be the Lunar and Planetary Science Conference and will take place between March 12th and March 18th, 2019. The limited data transmission capability from New Horizons had the result that less than 1% of encounter data had been received when the LPSC abstracts for presentations were due. As one abstract’s introduction states,
“Therefore, many new, quite substantial results not available at the time of this abstract submission will be presented in the actual review talk.” (4)
Pluto and MU69 are both Kuiper Belt Objects. I will now explain what that means.
The Kuiper Belt
As of the 1930’s, the Solar system had been thought of as divided into the realm of the planets and the distant Oort Cloud. Dutch astronomer Jan Oort deduced this spherical “halo” of objects by studying the movements and behavior of long-period comets. A third zone between those two was later postulated to explain the many “short-period” comets that tend to be near the planetary plane and have periods of 200 years or less. Since the discovery of Pluto it had been suspected that a torus of such small bodies existed. It was named for Gerard Kuiper (1905 – 1973) who was a Dutch-born American astronomer and planetary scientist of great distinction. In an appropriate coincidence, Jan Oort was one of Kuiper’s teachers in Holland in the early years of the 20th Century.
Subsequent discoveries of Kuiper Belt objects (now in the thousands) have led to a more detailed division of zones within and near the belt. KBOs are distinct from comets only because they are so far from the sun that they do not produce the comas or tails.
Centaurs: These are KBOs orbiting in the zone of the outer planets, thought to have been scattered there from the Kuiper Belt by the gravitational influence of (primarily) Neptune. From there they may be further thrown sunward by the other giant planets.
Resonant: This applies to a KBO whose orbit crosses that of Neptune. These will not be thrown sunward further since their orbital periods are such that they always find themselves far from Neptune. Pluto is the prototype Resonant KBO and has an orbit that takes it around the sun twice for every three orbits of Neptune.
Cold Classical KBOs: These are KBOs that stay in the zone outside Neptune’s influence and thus avoid being re-directed by that planet. The “Cold” qualifier refers to the fact that these orbits are not inclined to the orbits of the planets and are near-circular.
Hot Classical: Similar to the “Cold” bunch, but these KBO’s have elongated orbits that tend to be substantially inclined.
Scattered: As you might expect from the name, none of the restrictions applied to the other categories apply to this bunch. These are the population that wind up as Centaurs or short-period comets.
New Horizons Probe
The idea of a Pluto exploration was not as a “flagship” mission like the Voyagers (Grand Tour) or the Cassini Saturn orbiter. Its goal was to target a fly-by of Pluto directly, but also to carry on into the Kuiper Belt and visit other, as yet undiscovered bodies. The budget was a shoestring compared to many previous missions and while the instruments were advanced and powerful, many compromises were made to balance the launch and timing of the mission, the way data were collected and transmitted and the cost and reliability of hardware. The vast distance to Pluto was compounded by the fact that the multiple gravity-assists enjoyed by the Voyager probes, were not possible. Only Jupiter was reasonably positioned for the “slingshot effect” — and that only for a limited time.
The result was a small spacecraft, launched away from Earth by the most powerful rocket available, adorned with extra strap-on boosters and outfitted with a third stage to achieve a record-fast trajectory.
The probe pointed its instruments by rotating the whole spacecraft – meaning that all data collection was done while the antenna that would transmit to Earth was pointed off at deep space. The images and data for the entire fly-by of Pluto and later 2014 MU69 had to be recorded and “played back” later when instrument-pointing was no longer in operation and contact with Earth could be restored. Payload considerations limited the size of the high-gain antenna, and hence, the data transmission rate. As with Pluto before, the entirety of the MU69 fly-by data will not be on Earth until well over a year after the fly-by (August/September of 2020). Besides the slow data rate, there was an additional delay because the probe had slipped behind the Sun, as viewed from Earth, in January.
Figure 1: The New Horizons Probe. Credit: NASA
The instruments are described here as quoted from the New Horizons web site (1):
“The New Horizons team selected instruments that not only would directly measure NASA’s items of interest, but also provide backup to other instruments on the spacecraft should one fail during the mission. The science payload includes seven instruments:
Ralph: Visible and infrared imager/spectrometer; provides color, composition and thermal maps.
Alice: Ultraviolet imaging spectrometer; analyzes composition and structure of Pluto’s atmosphere and looks for atmospheres around Charon and Kuiper Belt Objects (KBOs).
REX: (Radio Science EXperiment) Measures atmospheric composition and temperature; passive radiometer.
LORRI: (Long Range Reconnaissance Imager) telescopic camera; obtains encounter data at long distances, maps Pluto’s farside and provides high resolution geologic data. SWAP: (Solar Wind Around Pluto) Solar wind and plasma spectrometer; measures atmospheric “escape rate” and observes Pluto’s interaction with solar wind.
PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation) Energetic particle spectrometer; measures the composition and density of plasma (ions) escaping from Pluto’s atmosphere.
SDC: (Student Dust Counter) Built and operated by students; measures the space dust peppering New Horizons during its voyage across the solar system.”
The alert reader will note that the same antenna (REX) that returns data to the Earth is also listed as an instrument. It was used to measure the changes in an Earth-NH transmission as the signal was eclipsed by Pluto’s atmosphere and surface and the same situation was also measured at Charon.
The data capacity of the probe is only 8 Gigabytes. While that is not impressive in today’s realm of technology, it must be remembered that New Horizons was launched in 2006. And, as is typical in spacecraft design, the technology designated for use in the probe was “frozen” some years before that.
To put that in perspective, available memory for the Voyager probes (Launched in 1977) was actually on a ½ inch, 8 track magnetic tape with a total capacity of about ½ Megabyte and a top baud rate of 56kilobits per second (2). Since the technology was frozen five years before launch, it was some, 17 years out-of-date by the time Voyager 2 flew past Neptune in 1989.
2014 MU69 (AKA Ultima Thule)
The New Horizons space probe was a resounding success as it flew by Pluto. The long-patient team of planetary scientists were then eager to find new targets. After all, the probe was still in good shape, with enough fuel to change its direction somewhat and find more distant targets in the Kuiper Belt. There was, however, no known object in a suitable position to arrange that.
Extensive observations by ground-based telescopes found more KBOs but none within reach of the probe. So, a Hubble Space Telescope survey was commissioned by the New Horizons team and found three valid candidates – of which 2014 MU69 was targeted. (5)
Before the encounter this KBO was examined by telescope observations and distant examination by the spacecraft itself, whose battery of instruments includes a telescope (LORRI), which while not nearly as powerful as the major observatories on Earth, it had the advantage of being very much closer.
There was also a campaign of occultation observations to characterize the shape of the body and assist in targeting by precisely measuring the orbit. This is a time-tested method to characterize the shape and refine measurement of an asteroid by precisely timing the disappearance of a star that is expected to pass behind the target. The “shadow” of such a small body – itself, so very far away – is about the same size as the KBO to be measured. It is nonetheless miniscule and fleeting on the face of the Earth and is best measured by teams of observers, spaced appropriately – in this case about 4 kilometers (2.5 miles apart). They must travel to the expected path of said shadow, bring their own portable equipment and deal with weather, geography and bureaucracy. Four such occultations were observed with varying success and were, taken together, sufficient to pin down the rate of movement of MU69 and to indicate that the KBO was either a pair of close-orbiting bodies or an elongated shape, possibly a “contact binary”. For results, please see figure 2, below.
Figure 2: Results of occultation observations indicated a close binary, elongated shape or a contact binary nature of the KBO. The latter proved to be true. Credit NASA
So, rather ironically, to pin down the parameters needed to make the fly-by a success required the efforts of observers – all the way back on the Earth – mostly armed with 16-inch telescopes and other equipment that (the author estimates) could be purchased for the price of a late-model used car.
Characterization
The orbit of MU69 around the Sun is so nearly circular that the difference is less than 5 percent. This would seem to indicate that this is a primordial object, formed where it is found now and not a displaced asteroid that might have been ejected from the inner solar system, for example.
Because it is a Classical KBO, the two bodies making up the object are thought to be formed where they currently are and gently captured into a binary system where they eventually merged to form a “contact binary”. Even at this early stage the KBO is being referred to as “pristine”.
In other words, this is not something that was formed elsewhere and then cast out by gravitational interactions the outer planets. Nor was it altered much since its formation.
The spectral analysis revealed that the composition is similar to Pluto. That “dwarf planet” has been altered a great deal by differentiation as a planetary body, by an acquired atmosphere and by interaction with it large satellite, Charon.
Distant, low resolution images sent to Earth by the probe — just before the encounter – pictured the target planetoid as a close binary. The initial images (at better, but still lower resolution) pictured MU69 as a “contact binary”. That is, two planetoids so close in orbit around a common center that they are in physical contact. Please see Figure 3.
Figure 3: MU69 Credit: Nasa
The KBO is much darker than all the images herein indicate. Those pictures are “stretched” to show the difference in brightness. The true colors range from dark red to really dark red. The red color is contributed by nitrogen-rich organic compounds called tholins, which are described by the following quote from NASA
“…in the late 1970s, scientists made an organic substance in the laboratory that matched the reddest asteroids. The substance’s color spectrum ranged from yellow to red to black, and was termed “tholin” by Carl Sagan in 1980. Scientists measured and modeled the optical properties of tholins, and found that the tholins matched the observed red color of the majority of the most distant asteroids.”
The New Horizons probe has been determined to be approaching from almost a “pole-on” direction. This will limit the imaging of the other side which is mostly unilluminated at this point in the KBO’s orbit around the sun.
A great deal more data, images and analyses were presented at the Lunar and Planetary Science Conference in Houston on March 18 – 22, 2019.
To Be Continued…
References
- New Horizons Spacecraft: http://www.nasa.gov/mission_pages/newhorizons/spacecraft/index.html
- Voyager Memory and Data Rate: http://itknowledgeexchange.techtarget.com/storage-disaster-recovery/nasas-voyager-used-8-track-tape-to-go-into-space/
- New Horizons Mission: Dawn and New Horizons Missions Sept. 3, 2015 Lunar and Planetary Institute, Lunar and Planetary Institute September 3, 2015 NASA’s Exploration of Ceres and Pluto: An Update, Dr. Paul Schenk
- LPSC abstract 2152.pdf: THE GEOLOGY OF 2014 MU69 (“ULTIMA THULE”): INITIAL RESULTS FROM THE NEW HORIZONS ENCOUNTER. J.M. Moore, et al
- Planetary Radio: Countdown to Ultima: Alan Stern and New Horizons 12/26/2018 https://tunein.com/podcasts/Science-Podcasts/Planetary-Radio-p53414/?topicId=128373557
- STELLAR OCCULTATION RESULTS FOR (486958) 2014MU69: A PATHFINDING EFFORT FOR THENEW HORIZONS FLYBY. M. W. Buie1, https://www.hou.usra.edu/meetings/lpsc2019/pdf/3120.pdf
- Tholins: https://www.nasa.gov/topics/solarsystem/features/identify_saturn_moon_surfaces.html
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