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From the Wikipedia article, List of artificial objects leaving the Solar System:

Although other probes were launched first, Voyager 1 was able to achieve a higher speed and has overtaken all others. Voyager 1 overtook Voyager 2 a few months after launch, on 19 December 1977. It overtook Pioneer 11 some time in the late 1980s, and then Pioneer 10—becoming the probe farthest from Earth—on February 17, 1998.

Voyager 2 is also moving faster than the probes launched before it, and thus overtook Pioneer 11 in 1997 and will overtake Pioneer 10 sometime in 2021.

New Horizons will also likely pass the Pioneer probes, but will need many years to do so. It will not overtake Pioneer 11 until ~2082 and will not overtake Pioneer 10 until ~2130. Barring some unforeseen gravity assist it will never overtake the Voyagers.

My query is why Voyager 1 is faster than all the other probes? Even New Horizons would never be able to overtake it, even with its more modern technology.

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  • $\begingroup$ It's a good question. There is probably an answer here already, so don't feel bad if your question is closed as a duplicate. You can always ask a follow-up! $\endgroup$
    – uhoh
    Commented Aug 12, 2017 at 16:48
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    $\begingroup$ @uhoh I also thought this question was previously asked but I was unable to find it, so I asked it myself. $\endgroup$
    – user8277998
    Commented Aug 12, 2017 at 18:07
  • $\begingroup$ Those are the fun questions :) $\endgroup$
    – uhoh
    Commented Aug 12, 2017 at 18:09
  • $\begingroup$ Related: space.stackexchange.com/questions/10195/… $\endgroup$
    – Hobbes
    Commented Aug 12, 2017 at 19:16
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    $\begingroup$ Keep in mind that speed isn't actually a good thing in space probes. High speed generally means you get where you're going faster, but it also means you're in contact with your target for less time, so you have less time to take photos and readings. More modern technology lets us pack more science into a brief flyby window, but we don't strive to move probes as fast as we possibly can. "Fastest space probe" is just a function of the needs of its particular flight path, not something done intentionally because we really wanted to huck something out of the solar system super hard. $\endgroup$
    – Darth Pseudonym
    Commented Jul 14, 2023 at 19:35

3 Answers 3

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I can add one interesting bit about why Voyager 1 is going faster than Voyager 2. Voyager 1 had assists from Jupiter and Saturn, but Voyager 2 had assists from those and Uranus and Neptune. So you might think that Voyager 2 should have been accelerated more by the assists, and so should be going faster. Yet it's not.

The reason is that in order to get a close flyby of Neptune's moon Triton, Voyager 2 had to fly by the leading side of Neptune, which reduced its heliocentric energy instead of increasing it. After its Uranus flyby, Voyager 2 had a higher heliocentric energy than Voyager 1. Had they left it at that, Voyager 2 would now be going faster than Voyager 1. But Neptune slowed it down.

Here are plots of the heliocentric specific energy of the Voyagers 1 and 2:

Voyager 1 Voyager 2

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Why Voyager 1 is faster than everybody else ?

Five vehicles have been sent on a solar system escape trajectory: Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and New Horizons. These five missions were launched on different dates, had different objectives, and hence had trajectories and different levels of gravity assist. One of the five was bound to be moving the fastest. This turned out to be Voyager 1.

None of these five vehicles was sent on a trajectory with the goal of making it the fastest. Each of the escaping vehicles instead had mission goals. The mission goals of Voyager 1 (flybys of Jupiter, Saturn, and Titan) happened to make it the vehicle of those five with the greatest hyperbolic excess speed.

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    $\begingroup$ You can do better than this! At least link to an answer that explains the Grand Tour and the opportunity to get so far out and visit so many planets so quickly. The Voyagers are so fast not only because something has to be the fastest. $\endgroup$
    – uhoh
    Commented Aug 12, 2017 at 22:59
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    $\begingroup$ @uhoh Actually, unless all five were planned to have the exact same speed, one must be the fastest, and one must be the slowest. Not by planned for speed, per se, just because in any group of moving objects without matched speeds some are faster than others, and one is the fasted of the group. On the other point, the answer could have been simply "Voyager 1's path collected greater gravity assist than the others," which would be complete and correct. Conversely, the answer could have given the complete math for all five trajectories, in 30,000 characters of incomprehensible formula $\endgroup$
    – user20122
    Commented Aug 13, 2017 at 3:20
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    $\begingroup$ @GypsySpellweaver SE is more than the pursuit of the shortest possible answer that is not wrong; a good answer explains and highlights implications. The speeds are the result of the interplay of the era of the launch, funding, and the relative positions of the planets. A good answer will include reference to this, and lucky alignment that made the Voyager missions possible. Formulae are not necessary, and they would be very comprehensible if they were. I know that because I learned about them here in space exploration stackexchange by reading great answers by David Hammen and others! :) $\endgroup$
    – uhoh
    Commented Aug 13, 2017 at 4:56
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    $\begingroup$ @uhoh Had the OP asked for detailed explanation in some fashion, I'm sure the answer would have had more details. "Why" received a decently detailed answer anyway (flybys of Jupiter, Saturn, and Titan). OP can learn more, as you have, by reading other posts, or by asking further questions. Also, sometimes, like at the end of a trying week late in the day, some people (volunteering their time) might not want to repeat what they've said half a dozen times already, and still give a good answer to a simple question. Of course, if so inclined, you can give a more detailed answer instead. $\endgroup$
    – user20122
    Commented Aug 13, 2017 at 5:16
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    $\begingroup$ @uhoh We each do our part to make this a better resource for others. Of course, we learn best by explaining, so the answer can do as much good for you as the OP. =) $\endgroup$
    – user20122
    Commented Aug 13, 2017 at 6:43
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There are two factors at play here. The dominant factor is the gravity assists, see Mark Adler's answer.

The other factor is the launch speed:

  • New Horizons was launched with a heliocentric speed of 16 km/s. It had one gravity assist, at Jupiter. That would have added in the region of 10-12 km/s to its speed.
  • the Pioneers were launched at 14.47 km/s. Had gravity assists at Jupiter, not sure about any speed gained at Saturn.

I haven't found launch speeds for the Voyagers. Voyager 1 had 2 gravity assists, Voyager 2 had 4 assists, but one of those reduced Voyager's speed to below that of Voyager 1.

The speed graph after launch looks like this. Speed drops off as the craft gets further away from the Sun (=has to climb out of the Sun's gravitational field). Lots of speed can be added by gravity assists, if you approach the planet the right way round (pass behind the planet). If you pass in front of the planet, you're slowed down.

enter image description here

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