New Horizons is en-route to 2014 MU₆₉, where it is planned to fly by 1 January 2019.

Considering fuel and other considerations, how close to 2014 MU₆₉ can it get?

(At the time of writing, October 2015, the IAU has not yet assigned a name to 2014 MU₆₉)

For the record: The object has become named as Arrokoth. It had, in fact been nicknamed earler as Ultima Thule, reflecting a Norse conception of the farthest reaches of the world. But that name has become cluttered with racist connotations, and scientists eventually chose to use an Algonqiun word for "sky".

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    $\begingroup$ In this universetoday hangout PI Alan Stern says that they can fly as close as they want, and will probably fly much closer than they did to Pluto where they considered many factors like occultation of the Sun and Earth and the position of Charon. To be decided. $\endgroup$
    – LocalFluff
    Oct 23, 2015 at 14:16
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    $\begingroup$ @LocalFluff That's an hour an a half long video, do you perhaps remember at which point this is mentioned? Also, if you can find that, could you please convert your comment into an answer? Cheers! $\endgroup$
    – TildalWave
    Oct 23, 2015 at 15:26
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    $\begingroup$ @TildalWave No. $\endgroup$
    – LocalFluff
    Oct 23, 2015 at 15:45

2 Answers 2


The flyby distance will depend on how accurately they can measure 2014 MU69's orbit. This KBO was discovered only 3 years ago, and because it's so faint we only have a few observations to go on.
For the Pluto encounter, the NH team went back to the Lowell observatory archives to measure the original plates made by Tombaugh when he discovered Pluto, in an effort to improve the accuracy of Pluto's position data. With far less data to go on, 2014 MU69's orbit will have some uncertainty.
This uncertainty will be mainly in the direction along NH's trajectory, so it's the timing of the flyby that will be difficult. As a result, there'll be an uncertainty in flyby distance as well.

As of January 2016, the science team is targeting a flyby distance of 2000-3000 km.

Edit: In December 2017, another course correction was made and the flyby distance is planned to be 3500 km.

The maneuver both refined the course toward and optimized the flyby arrival time at MU69, by setting closest approach to 12:33 a.m. EST (5:33 UTC) on Jan. 1, 2019. The prime flyby distance is set at 2,175 miles (3,500 kilometers);

There's a contingency plan to increase the flyby distance to 10,000 km if obstacles (moons, rings, etc.) are detected. The final decision will be made in December 2018, based on observations in the months before.

New Horizons timeline

  • $\begingroup$ I just realized that because of the very low rate that downlinked data can be transmitted at this distance, it's extremely important to keep New Horizons intact and healthy for at least several months after the flyby. There won't be "live video". $\endgroup$
    – uhoh
    Jan 28, 2018 at 10:49
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    $\begingroup$ Yes. After the Pluto encounter it took more than a year to download all the data, they're expecting a slightly shorter download period this time (I assume that's because MU69 is so much smaller, so there will be fewer photos). $\endgroup$
    – Hobbes
    Jan 28, 2018 at 16:20

As close as they think can safely be achieved. I expect that will be in the range of hundreds of km, but that's purely speculation. It should be a fair bit closer than Pluto, however, because there they do not have to worry about flying too close to debris orbiting 2014-MU69. It's Hill Sphere is only about 400,000 km, and it seems likely that there wouldn't be any object of significance much closer. It seems the team hopes to have it's closest approach be less than 1000 km, from social media posts.

They have plenty of fuel to exact target it (within the uncertainty of it's position and New Horizons), and in fact, with the surplus of fuel they have, I expect it will basically target it spot on. Incidentally, the first burns have already started to aim New Horizons at 2014-MU69, as described in today's New Horizons mission news article. The biggest difficulty, at least initially, is likely targeting it precisely, as there is significant uncertainty in it's orbit. No doubt further studies, including from New Horizons, will help to refine its location.

EDIT It seems like the planned distance to target is about 1900 miles, per American Space. That is 4 times closer than Pluto's flyby.

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    $\begingroup$ Why don't they have to worry about debris during the 2014-MU69 flyby? Is that because of already being on extended mission at that point and it wouldn't be such a big deal if a collision ended the mission, or because there can't be any debris for some reason? How about own moonlets? $\endgroup$
    – TildalWave
    Oct 23, 2015 at 15:23
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    $\begingroup$ They would need to worry about that, since they need to survive the flyby in order to return the data from the flyby. $\endgroup$
    – Mark Adler
    Oct 23, 2015 at 18:27
  • $\begingroup$ I just assumed there isn't likely to be much dust orbiting something so small, but I could well be wrong. $\endgroup$
    – PearsonArtPhoto
    Oct 23, 2015 at 18:32
  • $\begingroup$ @PearsonArtPhoto I thought that they selected 2014-MU69 for being a low albedo / dark body. So if there's moonlets, debris rings,... it's fair to assume they'd be dark too and remain undetectable to LORRI for a long time, which might not leave enough time for collision avoidance with comms delay as long as it will be. Hubble can't resolve faint features like that, they wouldn't be discernible in the noise (I made this animation of the best it can do). I'd hate to think that NH is on some sort of a kamikaze mission now. Surely they (APL) must have a plan? $\endgroup$
    – TildalWave
    Oct 23, 2015 at 18:39
  • $\begingroup$ @TildalWave: Added a bit more, I just don't think that's very likely based on it's size, but I posted a bit more information. I'm sure they'll figure out just how closely they want to target it over the course of the next few years. $\endgroup$
    – PearsonArtPhoto
    Oct 23, 2015 at 20:28

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