What to Expect When New Horizons Visits 2014 MU69, Ultima Thule

Update 28 December: NASA administrator Jim Bridenstine has announced that despite the U.S. government shutdown, NASA TV will be available for the flyby. I have updated the links below again.

Check this blog entry for MU69 photos taken and returned by New Horizons before 1 January.

New Horizons is rapidly approaching its New Year’s encounter with the most distant world ever visited, 2014 MU69. Closest approach will be at a distance of 3,500 kilometers at about 05:33 on 1 January UTC, and it’ll happen at a zippy 14.16 kilometers per second. Space fans can’t wait to see pictures of this distant, tiny world, an ancient relic of the formation of our solar system, which the mission and NASA have nicknamed “Ultima Thule,” until it is formally named. (I think we should all get to give it our own nicknames. I'm partial to "Moo," or maybe "Peanut.")

We don’t have much idea what MU69 is going to look like. What we know about it so far is pretty limited. It’s probably about 20 to 35 kilometers across, roughly 10 times the diameter of Rosetta’s comet 67P/Churyumov-Gerasimenko. It seems to have a very irregular shape. It might be a binary (two objects orbiting close to each other) or bilobate (like 67P). Its surface is reddish. It might be a fragment of a bigger object, in which case it might have heated up a little after it formed, or it might actually have accreted to its current size, in which case it could show layers and agglomerations like 67P.

As happened at Pluto, New Horizons will not be communicating with Earth during closest approach, because it will be focused on gathering all the science it can during the high-speed flyby. Whenever it does turn back to point at Earth to transmit data it will take more than 6 hours for its data to traverse the distance between us. So when are we going to get to find out what MU69 looks like?

A few precious data downlinks in the days before and after the flyby will contain some small photos from New Horizons’ Long Range Reconnaissance Imager (LORRI), which takes grayscale snapshots. Lower-resolution color comes from the Multicolor Visible Imaging Camera (MVIC), a component of the Ralph instrument. The first images that we get back from New Horizons will not be the best images that New Horizons has taken. The reasons for that are complicated; I’ll explain below. Before I do that, I’ll show you what to expect in the days around closest approach:

One important caveat: the times reported above are when the images will be downlinked. This is not the same as when they will be published. New Horizons (unlike Curiosity, Opportunity, InSight, solar missions, and formerly Cassini) doesn't push images straight to the Web once they land on Earth. The mission will process them, and the team will write captions, and then NASA will have to vet the captions, and then NASA will publish the images at a time of day that'll maximize news coverage, all of which means it could be up to a day or so after downlink that these images get released.

Here are some of the best ways to keep up with the mission:

• My Twitter, of course, though I will not be at the Applied Physics Laboratory (APL) for the flyby
• The mission official Twitter and principal investigator Alan Stern
• Planetary Society Board member Heidi Hammel will be onsite at APL, as will Planetary Radio host Mat Kaplan
• Mission webpage at APL and mission webpage at NASA
• Forum threads at unmannedspaceflight.com and nasaspaceflight.com
• “The New Horizons Kuiper Belt Extended Mission,” article by Stern et al., on arXiv
• “Great Expectations: Plans and Predictions for New Horizons Encounter with Kuiper Belt Object 2014 MU69 ('Ultima Thule'),” article by Jeffrey Moore et al., on arXiv

And now, a detailed look at all the plans for the mission to MU69.

Mission Design

In addition to the LORRI and MVIC cameras, New Horizons has several other science instruments. Remote sensing instruments include LEISA (which, together with MVIC, make up the Ralph instrument). LEISA takes spectra in infrared wavelengths. Alice is an ultraviolet spectrograph that can measure composition and also do stellar occultations to look for a coma around MU69. New Horizons also has three in-situ instruments for measuring fields and particles: the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), Solar Wind Around Pluto (SWAP); and the Student Dust Counter (SDC).

Mission planners have split the flyby into three main phases:

• Currently, we are in the Approach phase, which began on 16 August and runs through 24 December (7 days before closest approach). During approach, New Horizons has been taking images for optical navigation and searching for potentially dust-generating hazards like rings or moons. (So far, it has not spotted any.) The in-situ instruments have been busy gathering data on the fields and particles in interplanetary space.
• The Core phase will last 10 days, from 24 December to 2 January. Nearly all of the science will happen in just a 2-day period around closest approach, with intense imaging and spectroscopy to map MU69, study its composition, figure out its rotation rate and pole, and search for dust, rings, and satellites.
• The Departure phase lasts one more week, from 3 through 8 January. After that, New Horizons will spend 20 months downlinking all the data, until September 2020.

A Challenging Encounter

Several things will make the MU69 flyby significantly more difficult than the Pluto flyby. MU69 is small, dark, and faint. New Horizons won’t see it as more than a dot until 3 days before closest approach. MU69 is darker, and the sunlight dimmer, than the LORRI camera was designed for. Also, because MU69 is so small, New Horizons has to pass much closer to it than it did at Pluto in order to see interesting detail on the surface, resulting in a rapidly changing viewpoint near closest approach.

As a result of all of these factors, LORRI’s images may be a little smeared and will be noisier than we saw at Pluto. MVIC is less affected by these challenges; the way that MVIC works, it’s possible for New Horizons to compensate for the low light and high speed by sweeping more slowly across MU69 to build up a stronger signal.

Another significant challenge arises from the fact that MU69 was only discovered in 2014. We don’t have a very long observational arc on its orbit around the Sun. So navigators don’t have as precise information as they’d like to have on its position. In particular, our knowledge of the position of MU69 along the line of sight from Earth is uncertain by several thousand kilometers. While New Horizons is approaching MU69 at a distance, it’s seeing the target at the same perspective that we do from Earth; New Horizons isn’t getting much parallax to refine our knowledge of exactly how far away it is along New Horizons’ line of sight. As New Horizons gets closer, the uncertainty in MU69’s position becomes a significant issue, because the real position of the body may be completely outside the field of view of a photo focused on the predicted position. In order to make sure it doesn’t miss the target, the spacecraft will have to scan its instruments across a region of space in which it is most likely to find the tiny world, taking a lot of data on empty space to make sure to get data on the object.

Core Phase

Have I mentioned yet that 2014 MU69 is small and faint? Even when New Horizons is just 3 days away from encounter, it will still only appear as one pixel to the sharpest-eyed camera, LORRI. So nearly all of New Horizons’ time through 28 December will be spent snapping photos for optical navigation, one set of images per day, and turning back to send them to Earth. It’s only in the final days before the flyby that New Horizons will see the world shift much against background stars, helping navigators to reduce the uncertainty in MU69’s position. Their final opportunity to update the pointings of LORRI images will be about 3 days before closest approach.

Late in the day on 29 December, they’ll start taking more science data. (By the way, these dates are all UTC according to the spacecraft’s clock. Because of the 6-hour one-way trip from New Horizons to Earth for radio signals, the time on the spacecraft’s clock matches the time of day in the U.S. Central time zone when we’ll be receiving the data, right in between Eastern and Pacific time -- so it’s “late in the day on 29 December” for those of us in the U.S., too. Handy for me!) Some long-exposure image sets will search for satellites. They’ll do “plasma rolls” to orient the in-situ instruments in all directions, characterizing the fields and particles as they get close. Shorter-exposure imaging will start gathering resolved images of MU69, but it will still only be barely larger than a LORRI pixel.

Toward the end of 29 December, science data gathering will really begin. A dense set of observations will gather a lightcurve on MU69, a measure of how its surface appears to change with rotation, over a period of about 6 hours. Prior imaging will have given hints of MU69’s rotation rate and pole direction, but these will be the first dense set of lightcurve data that should really help nail down those orbital characteristics. Is MU69 a fast rotator or slow rotator? Is it pointing a pole at the Sun, or are we seeing it sideways? We don’t know yet and we won’t really find out until those data land on Earth. Color images will start in order to see if MU69’s color varies with longitude, but MVIC won’t be able to resolve MU69 as more than a pixel yet.

Next, 30 December will see another 6-hour block of science, a mix of color imaging, deep satellite searches, and lightcurve data. The same happens early on 31 December, after which New Horizons enters the most intense, Inner Core phase of the flyby. New Horizons’ last communication phase with Earth will wrap up at 14:47 UTC according to the spacecraft clock on 31 December, or 20:55 UTC back here on Earth. The last images we receive will show MU69 as just a couple of pixels. Then, we wait.

Closest approach happens on 1 January at 05:33 according to the spacecraft’s clock, but it won’t be talking to us at the time.

Our first indication of how things went will arrive on Earth on 1 January at 15:28 UTC, or a very pleasant 10:28 in the morning in the Eastern time zone where the mission will be operating. There’ll be a quick “phone home” conveying spacecraft health, but no science data.

The first science data begins arriving later the same day, at 20:15 UTC (15:15 EST). The four-hour downlink should include a photo that is about 100 pixels across (see the simulation at the top of this post). Alan Stern calls these first downlinks the “New York Times” or “NYT” downlinks because he’s trying to bring down data that is valuable both scientifically and for public communication about the mission.

The second batch of science data will begin arriving on 2 January at 01:55 UTC (1 January at 20:55 EST). There might be a 200-pixel image in this downlink -- or maybe not. It depends where MU69 actually was. If it’s not in this downlink, it will hopefully be in a second image from the same observation that will hopefully arrive on Earth during the downlink that begins on 2 January 16:44 UTC (11:44 EST).

Here’s a detailed timeline of the above in several local time zones. SCET is "Spacecraft Event Time," the time according to the spacecraft's clock. All other times are "Earth Received Time," when we receive the signal from the spacecraft.

Departing MU69

On departure, the team will attempt to bounce a radio signal from the Deep Space Network off of MU69 and detect it with New Horizons. They’ll look at the Sun with Alice to see if they can detect any gases in the coma. As they did at Pluto, they’ll look for rings and dust in the sunlight around MU69. Very quickly, though, the mission will return to a fairly quiet cruise, beginning to digest all the wonderful data. Once all is said and done, the mission will have collected 50 gigabits of data from MU69, as compared to 55 at Pluto. As MU69 is substantially farther from Earth than Pluto was, the data downlink will be slower; it’s expected to take 20 months, conveniently wrapping up around the end of fiscal year 2020.

When will we get the much better images, the ones that’ll show MU69 in the greatest detail? You’re going to have to be patient: it’ll be long after the flyby. They are taking about 900 images in the highest-resolution observation, of which the target will only be in a few. Identifying which images contain the target before they are transferred to Earth will take a while. First they’ll use lower-resolution images to improve their knowledge of MU69’s position. They can use that to narrow down the subset of images that contain the target. They will also be downlinking metadata for all the images before they actually downlink the data. The metadata will contain information like the histogram of the pixels in the image -- and if there’s a Kuiper belt target, the median pixel value will be higher than it is for an image just containing space. They’ll generate a list of likely images and slog away at downlinking them, hopefully having the best ones on the ground by the end of February.

Best of luck to the whole New Horizons team for a fantastic flyby!