Last week New Horizons passed within 14,000 km of the center of Pluto. I assume this will have put New Horizons within the gravitational influence of Pluto. Pluto's average orbital speed is 4.7 km/s.

Will New Horizons have gained (or lost) any significant speed due to gravity-assist effects as it passed though Pluto's gravitational influence? If so, how much?

  • $\begingroup$ Jupiter gave NH a 4 km/s boost; its surface gravity is about 40 times that of Pluto, so I would guess the change in speed for a Pluto flyby would be on the order of 100 meters per second at most? $\endgroup$ Commented Jul 21, 2015 at 0:16
  • $\begingroup$ @RussellBorogove: Is surface gravity the best way to compare? Maybe comparing mass would be better. Compare Pluto's 1.3 × 10²² kg with Jupiter's 1.9 × 10²⁷ kg. $\endgroup$ Commented Jul 22, 2015 at 7:35
  • $\begingroup$ Neither one turned out to be a very good proxy; the difference was about a factor of 1000 instead of 40 or 100000. Curse you, calculus! $\endgroup$ Commented Jul 22, 2015 at 15:51

1 Answer 1


I've been trying to figure out where I read about this, and then realized that I didn't read it. I saw it in a video.

The NASA New Horizons channel on Youtube made some short videos called "Pluto in a Minute". In the video, How Did Pluto Accelerate New Horizons?, the speaker explains how a planetary body's sphere of influence works, and then mentions that as New Horizons flew past Pluto, it gained about 13 mph (approximately 21 km/h). Very insignificant, indeed.

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    $\begingroup$ I've telling myself I need to make the time to watch those little NASA videos. Now I know why :) $\endgroup$ Commented Jul 21, 2015 at 1:09
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    $\begingroup$ OMG they stole Pluto's Orbital Energy.... Surely there should be a bad Sci-Fi Move about this somewhere..... But, back in the real world, 13 mph is a pretty big boost. You just have to remember that "way out there" Pluto is the biggest boy on the block. Being that far from the Sun, the sun (and other things) "pull" on the probe much less then Pluto does. en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics) Pluto is freckin' huge compare to earth (in terms of SOI). Now back to launching Kerbals. $\endgroup$
    – coteyr
    Commented Jul 21, 2015 at 10:22
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    $\begingroup$ For non-Americans: 13 mph ≈ 21 km/h $\endgroup$ Commented Jul 24, 2015 at 5:06

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