Saturday Night Science: Ultima Thule Encounter

New Horizon and 2014 MU69 (artists's conception)(Saturday Night Science usually appears on the first Saturday of the month.  I have moved up the January 2019 edition one week to discuss the New Horizons spacecraft fly-by of Kuiper belt object 2014 MU69, “Ultima Thule”, on New Year’s Day, January 1st, 2019.)

In January 2006 the New Horizons spacecraft was launched to explore Pluto and its moons and, if all went well, proceed onward to another object in the Kuiper Belt of the outer solar system, Pluto being one of the largest, closest, and best known members.  New Horizons was the first spacecraft launched from Earth directly on a solar system escape (interstellar) trajectory (the Pioneer and Voyager probes had earlier escaped the solar system, but only with the help of gravity assists from Jupiter and Saturn).  It was launched from Earth with such velocity (16.26 km/sec) that it passed the Moon’s orbit in just nine hours, a distance that took the Apollo missions three days to traverse.

In February 2007, New Horizons flew by Jupiter at a distance of 2.3 million km, using the planet’s gravity to increase its speed to 23 km/sec, thereby knocking three years off its transit time to Pluto.  While passing through the Jupiter system, it used its instruments to photograph the planet and its moons.  There were no further encounters with solar system objects until arrival at Pluto in 2015, and the spacecraft spent most of its time in hibernation, with most systems powered down to extend their lives, reduce staffing requirements for the support team on Earth, and free up the NASA Deep Space Network to support other missions.

As New Horizons approached Pluto, selection of possible targets for a post-Pluto extended mission became a priority.  In orbital mechanics, what matters isn’t so much distance and speed but rather “delta-v”: the change in velocity needed to divert the trajectory of a spacecraft from where it is currently headed to where you want it to go.  For chemical rockets, like the thrusters on New Horizons, this depends entirely on how much propellant is on board, and this resource would be scarce after expending what was required for the Pluto mission.  New Horizons was launched with propellant to provide 290 metres/sec delta-v, but most of this would be used in course corrections en route to Pluto and maneuvers during the Pluto encounter (the scientific instruments are fixed to the spacecraft structure, which must be turned by firing the thrusters to aim them at their targets.)  Starting in 2011, an observing campaign using large Earth-based telescopes began searching for objects in the Kuiper belt which might be suitable targets for New Horizons after Pluto.  These objects are extraordinarily difficult to observe: they are more than four billion kilometres from Earth, small, and mostly very dark, and thus visible only with the largest telescopes with long exposure times under perfectly clear and dark skies.  To make things worse, as it happens, during this time Pluto’s orbit took it past some of the densest star fields of the Milky Way, near the centre of the galaxy in the constellation of Sagittarius, so the search was cluttered with myriad background stars.  A total of 143 new Kuiper belt objects were discovered by this search, but none was reachable with the 33 kg of hydrazine monopropellant expected to remain after the Pluto encounter.

It was time to bring a bigger hammer to the job, and in June 2014, time on the Hubble Space Telescope was assigned to the search.  By October of that year three potential targets, all too faint to spot with ground-based telescopes, had been identified and called, imaginatively, potential targets PT1, PT2, and PT3.  The course change to get to PT1 would use only around 35% of New Horizons‘ remaining fuel, while the others were more difficult to reach (and thus less probable to result in a successful mission).  PT1 was chosen, and subsequently re-named “2014 MU69”, along with its minor planet number of 486958.  Subsequently, a “public outreach” effort by NASA chose the nickname “Ultima Thule”, which means a distant place beyond the known world.  A recommendation for an official name will not be made until New Horizons reveals its properties.

The fly-by of Pluto in July 2015 was a tremendous success, fulfilling all of its scientific objectives, and in October 2015 New Horizons fired its thrusters for sixteen minutes to change its velocity by 10 metres per second (equivalent to accelerating your car to 22 miles per hour), setting it on course for Ultima Thule.  Three subsequent burns would further refine the trajectory and adjust the circumstances of the fly-by.  This was the first time in history that a spacecraft was targeted to explore an object which had not been discovered when launched from Earth.  After transmitting all the data collected in the Pluto encounter to Earth, which took until October 2016, New Horizons went back into hibernation.

In June 2018, the spacecraft was awakened and in August 2018 it observed its target with its own instruments for the first time.  Measurement of its position against the background star field allowed precise determination of the inbound trajectory, which was used in final course correction maneuvers.  At the same time, the spacecraft joined Earth-based telescopes and the Hubble in a search for possible moons, rings, or dust around Ultima Thule which might damage the spacecraft on a close approach.  Had such hazards been found, the fly-by would have been re-targeted to be at a safer distance, but none was found and the original plan for a fly-by at 3500 km was selected.

Although New Horizons is bearing down on its target at a velocity of 14.4 km/sec, it will remain just a faint dot until hours before closest approach at 05:33 UTC on New Year’s Day, January 1st, 2019.  Other than its position, brightness, and colour (reddish), little or nothing is known about the properties of Ultima Thule.  We don’t know its size, shape, composition, temperature, rate of rotation, albedo (reflectivity), whether it is one object or two or more in close orbit or in contact, or anything about its history.  What is almost certain, however, is that it is nothing like anything in the solar system we’ve explored close-up so far.

Its orbit, unlike that of Pluto, is that of a conventional, well-behaved member of the Sun’s extended family.  The orbit, which takes Ultima Thule around the Sun every 296 years, is almost perfectly circular (eccentricity 0.045) and close to the ecliptic (2.45°).  (By contrast, Pluto’s orbit has an eccentricity of 0.25 and an inclination to the ecliptic of 17°.)  This makes it probable that Ultima Thule has avoided the cosmic billiards game which has perturbed the orbits of so many distant objects in the solar system, making it a “cold classical Kuiper belt object” (the “cold” refers not to temperature but its analogue in dispersion of velocity).  What this means is that it is highly probable that this body, unlike the planets and moons of the inner solar system, which have been extensively reprocessed from their original constituents, has been undisturbed since the formation of the solar system 4.5 billion years ago and is a time capsule preserving the raw materials from which the inner planets were assembled.

In 2017, predictions of Ultima Thule’s orbit indicated that it would pass in front of, or occult, a distant star, with the shadow passing through southern Argentina.  Since the distance to the object and its speed in orbit are known reasonably well, simply by measuring the duration of the star’s occultation, it is possible to compute the length of the chord of the object’s path in front of the star.  Multiple observing stations and precise timings allow estimating an object’s size and shape.  A network of twenty-four small telescopes was set up along the expected path (there is substantial uncertainty in the orbit, so not all were expected to see the occultation, but five succeeded in observing it).  Combining their results  yielded this estimation of Ultima Thule’s size and shape.

2014 MU69 Occultation resultsThe best fit was to a close binary or “contact binary”: two lobes, probably originally separate objects, in contact with one another.  What does it actually look like?  We’ll have to wait and see.  The occultation observations found no evidence for rings, moons, or a dust halo, increasing confidence in the planned close fly-by.

Another mystery which will have to await close-up observation is the absence of a pronounced light curve.  An irregularly-shaped object like Ultima Thule would be expected to vary dramatically in brightness as it rotates, but extended observations by Hubble failed to find any variation at all.  The best guess is that we’re observing it close to the pole of rotation, but again it’s anybody’s guess until we get there and take a look.

Are we there yet?  No, but it won’t be long now.  As I noted, the closest fly-by will be at 05:33 UTC on 2019-01-01.  Most of the scientific data will be collected in the day before and after the moment of closest approach.  Coverage of this event will not be like what you’ve become accustomed to from other space missions.  New Horizons will be 6.6 billion kilometres from the Earth at the time of the fly-by, more than 43 times the distance of the Earth from the Sun.  It takes light (and radio waves) six hours to travel that distance, so anything transmitted to Earth will take that long to arrive.  Further, since the high-gain antenna used to send data back to Earth is fixed to the same spacecraft structure as the scientific instruments, while they are collecting data during the fly-by, the antenna won’t be pointed in the correct direction to send it back to the distant home planet.

After the scientific observations are complete, the antenna will be pointed at the Earth to send “quick look” data, spacecraft health information, and the first images.  These are expected later on the first of January and over the next few days.  To those accustomed to broadband Internet, these data arrive excruciatingly slowly.

Deep Space Network: New Horizons data downlink

Even with a 70 metre Deep Space Network antenna, the downlink rate is 501 bits per second.  If you have a 50 megabit per second broadband Internet connection, this is one hundred thousand times slower: comparable to the dial-up computer terminal (300 bits per second) I used in 1968.  It takes around an hour to return a single image, even in the compressed formats used for quick-look data.  Downloading all of the science data collected during the fly-by will begin on the 9th of January, when New Horizons returns to spin-stabilised mode (which requires no maneuvering fuel) with its antenna pointed at Earth, and is expected to take twenty months.  When the data download is complete, the spacecraft will be placed back into hibernation mode.  If another Kuiper belt target is identified which can be reached with the remaining maneuvering fuel before its nuclear power source decays or its distance to Earth becomes too great to return fly-by data (expected in the 2030s), it may be re-targeted for another fly-by.

Coverage of the New Horizons fly-by of Ultima Thule will be broadcast on the Johns Hopkins University Applied Physics Laboratory (who built the spacecraft and manages the mission) YouTube channel.  Here is a schedule of mission-related programming.  This is the mission Web site, with links to resources for the spacecraft and its destination.  This article by Emily Lakdawalla of the Planetary Society gives more detail about the encounter, when data and images will be returned, and what we can expect to see when.

I will post news and data as they arrive in the comments to this post.  If you wish to be notified when new comments are posted but don’t have a comment to add at the moment, simply post a comment consisting of the single word “follow” and you’ll receive notifications without your comment appearing.

Here is a Science Chat from September 2018 with New Horizons principal investigator Alan Stern looking ahead to the encounter with Ultima Thule.

This is a panel discussion at the American Geophysical Union meeting in December 2017 describing the preparations for the encounter with Ultima Thule and what may be learned from the fly-by.


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42 thoughts on “Saturday Night Science: Ultima Thule Encounter”

  1. Could a Falcon Heavy launch a large enough spacecraft to go into orbit around Pluto and do useful science?  How long would such a mission take (assuming it could use Jupiter to do a gravity assist).

  2. Here is a question and answer session from 2018-12-28 with the leaders of the New Horizons project.  Participants:

    • Principal Investigator Alan Stern – Southwest Research Institute
    • Mission Operations Manager Alice Bowman – Johns Hopkins Applied Physics Lab
    • Science Team Co-Investigator Kelsi Singer – Southwest Research Institute

    Moderator: Mike Buckley – Johns Hopkins Applied Physics Lab


  3. A new documentary about New Horizons with a total running time of two and a half hours titled Summiting the Solar System has just been released.

    Part 1 focuses on the exploration of Pluto.

    Part 2 concentrates on the history of the mission and the Ultima Thule encounter.


  4. Richard Easton:
    Could a Falcon Heavy launch a large enough spacecraft to go into orbit around Pluto and do useful science?  How long would such a mission take (assuming it could use Jupiter to do a gravity assist).

    You could certainly do a Pluto orbiter mission with a Falcon Heavy.  Its payload for a direct launch to Pluto (without gravity assist by Jupiter or in the inner solar system) is quoted as 3500 kg, which is sufficient for a simple orbiter.  (The Galileo spacecraft which orbited Jupiter had a mass of 2562 kg, and that was considered a “battlestar” class mission built with 1980s technology.)

    That said, mission design is an extremely complicated art, as everything is a trade-off and each choice you make interacts with everything else.  For example, a gravity assist at Jupiter will usually reduce transit time to Pluto by around three years, but in order to take advantage of it Jupiter must be in the right position in relation to Pluto, and that alignment is only in effect for around three years of Jupiter’s orbit of about twelve years.  That means that in order to exploit a Jupiter gravity assist your funding and mission development must be aligned with that window and, if you miss it, you’ll have to wait another twelve years.  (New Horizons was deliberately designed so that if it missed the window it could be de-scoped for a longer transit to Pluto with less propellant on board to do science in the fly-by.)

    An orbiter, unlike a fly-by, needs to stop and go into orbit when it arrives at the destination.  This poses a painful trade-off among transit time, spacecraft mass, and propellant mass fraction.  The faster you get to Pluto, the more velocity you need to kill to go into orbit.  New Horizons arrived at Pluto travelling 13.78 km/sec, which is around twice the velocity of a satellite in low Earth orbit.  Pluto has no atmosphere thick enough to help slow down the spacecraft, so killing all of that velocity has to be done by propulsion, and because Pluto is so small, it doesn’t help you much with a gravity assist.  This means that the faster you get to Pluto, the harder it is to stop when you arrive.

    In October 2018, the Southwest Research Institute, a key participant in New Horizons, published a mission design for a spacecraft to be launched between 2025 and 2040 which, using a Jupiter gravity assist and electric (ion) propulsion, proved on the Dawn asteroid mission, and gravity assists by Pluto’s moon Charon, could explore all of Pluto, Charon, and Pluto’s other moons, and then depart Pluto and explore multiple Kuiper belt objects in a twenty-five year mission.  Here is a less technical description of this mission plan.

  5. Clarification: In the original post, I showed the data rate of the downlink from New Horizons to the Madrid Deep Space Network station as 501 bits per second.  This is the data rate in the three-axis stabilised configuration used during the fly-by, in which only one of the two redundant 12-watt travelling-wave tube amplifiers is being used to transmit data to Earth.  (The precise data rate depends upon which antenna is tracking the spacecraft and the weather at the antenna site.)

    Once the encounter is complete and New Horizons transitions to pure downlink mode, the spacecraft is spun up so that its antenna remains pointed at Earth without need for the guidance system and the scientific instruments are powered down.  This makes sufficient electrical power available to power up the redundant 12-watt transmitter, which can simultaneously send data with a different polarisation than the main transmitter, almost doubling the downlink data rate.  The estimate of twenty months to downlink all of the data collected during the fly-by assumes both transmitters are being used.

    Every January, New Horizons, as seen from the Earth, passes through a solar conjunction: the Sun passes between Earth and the spacecraft.  The Sun generates so much radio noise that it is impossible to receive the signal from the spacecraft.  By coincidence, the solar conjunction this year will be from January 4 through 7, shortly after the fly-by, so bulk data download will not begin until after the conjunction is over.

  6. Bryan G. Stephenssays:
    #11 2018-12-30 at 14:02 UTC  [Quote]

    The rocket equation is a harsh mistress.

    Indeed, it is. However, check this out.


    The Stardrive Report
    Published on Dec 29, 2018



    This video is about new physics discoveries on how UFOs like the “Tic Tac’ are able to fly using very small amounts of energy. We discuss the political and military ramifications of this disruptive technological surprise in the light of Trump’s replacement of General Mattis by Patrick Shanahan, the former student of his uncle, MIT Professor John Trump. Shanahan is allegedly the man who released the information we discuss and he has also championed the creation of the US Space Force.



  7. This page:

    LORRI Images from the Ultima Thule Flyby

    is posting raw, unprocessed JPEG images from the New Horizons Long Range Reconnaissance Imager (LORRI) shortly after they are received on Earth.  LORRI is a visual light telescope with a 20.8 cm aperture and 1024×1024 pixel monochrome CCD image sensor with 12 bits per pixel dynamic range (this is reduced to 8 bit per pixel when images are posted in JPEG format).

    The images posted at this writing (2018-12-31 14:45 UTC) are all navigation images where groups of four by four pixels are binned into a single average pixel, reducing the resolution to 256×256 pixels for faster transmission to Earth.  In all of these images so far, Ultima Thule has just been a single pixel.  The images have not been subjected to contrast stretching or another other image processing, and most contain cosmic ray hits which may be mistaken for stars.  You will probably need to load the images into an image processing program and stretch the contrast to see Ultima Thule.  As the spacecraft gets closer, more detail should become visible.

    Just so you don’t get too excited, here is the most recent image before I posted this comment, from 2018-12-30 01:46:05 UTC, which I have image processed to bring out detail.  The original is even darker.  And no, I have no idea why they posted these images in JPEG rather than PNG, which would have avoided all the nasty compression artefacts and, for mostly black images, hardly been any larger.

    New Horizons LORRI image KELR_MU69_NAV-CRIT_L4_2018364

  8. Here is a higher-resolution rendering of the first image showing more than one pixel from this document.

    New Horizons: Ultima Thule quick look image

    The image description is:

    Just over 24 hours before its closest approach to Kuiper Belt object Ultima Thule, the New Horizons spacecraft has sent back the first images that begin to reveal Ultima’s shape. The original images have a pixel size of 6 miles (10 kilometers), not much smaller than Ultima’s estimated size of 20 miles (30 kilometers), so Ultima is only about 3 pixels across (left panel). However, image-sharpening techniques combining multiple images show that it is elongated, perhaps twice as long as it is wide (right panel). This shape roughly matches the outline of Ultima’s shadow that was seen in observations of the object passing in front of a star made from Argentina in 2017 and Senegal in 2018.

    Much more detailed images, and other kinds of data, will be gathered today and tomorrow as New Horizons speeds towards its closest approach to Ultima at 12:33 a.m. EST on Jan. 1. A small sample of these images and other data will be returned to Earth in the next few days, though it will take about 20 months to downlink the full data set.

    New Horizons was approximately 1.2 million miles (1.9 million kilometers) from Ultima when this image was taken on Dec. 30, 2018.

  9. Last night, there was a live stream broadcast of events associated with the fly-by of Ultima Thule.  The live stream was a bit of a mess, with missing audio at times, and inability to link to portions while it was still in progress, which went on, and on, and on.

    Finally, here is a recording of the first part of the event.  I’ve set the start point to the pre-encounter press briefing by the New Horizons team, followed by a question and answer session.  It was during the press briefing that the first image hinting at the shape of Ultima Thule, shown above in comments #18 and #20, was revealed.  After the question and answer session, there is some “outreach” stuff, including some guy with a guitar.

    There is a 21 minute gap between the end of the press briefing and the start of the Q&A with just the title card displayed; The Q&A starts at the 2:39:00 mark in the video.

  10. This is video of the event commemorating the moment of closest fly-by.  Since there was no downlink from the spacecraft during the encounter, this was done purely by the clock, and there is no new information in this video.  There’s a long dead air gap between 04:19 and 13:00—skip ahead to save time.

  11. Here is a recording of the “Evening Program” leading up to the fly-by.  There are panel discussions, speakers, and videos about small solar system bodies, the New Horizons mission, and the instruments on board the spacecraft.

  12. Events scheduled for later today (2019-01-01) are:

    • Signal acquisition: Coverage begins at 14:45 UTC , with start of the “Phone home” transmission expected at 15:28 UTC and the end of transmission at 15:45 UTC.
    • Post-Fly-by Press conference:  Starts at 16:30 UTC.  It is likely the second “failsafe” image taken during the approach (if successful) will be released at this time.

    You should be able to view these events on either the NASA Live YouTube channel (player below) or on the Johns Hopkins University Applied Physics Laboratory YouTube channel (events should appear as they occur in the “Uploads” section).

    The downlink of the first medium-resolution image taken during the fly-by (nicknamed “NYT 1”, because it is intended for media such as the New York Times) will begin at 20:15 UTC and will run through 23:35 UTC.  This will be a monochrome LORRI image at 300 metres per pixel, yielding an image around 100 pixels across.  There is no scheduled time for release of this image to the public.


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