Our Solar System is vast. We on Earth cruise around the Sun in an orbit with a radius of about 150 million km (93 million miles), but the most distant planet, Neptune, is about thirty times further from the Sun. Enormous distances and the limitations of rocketry means reaching the outer planets and other distant worlds is always a slow process, but early in the Space Age we learned to exploit nature to shave years off the journeys. This is the principle of gravitational assists (sometimes called slingshot manoeuvres). New Horizon’s voyage to the dwarf planet Pluto is a great example of a gravitational assist in action.
New Horizons was launched on 19 January 2006 (back when Pluto was still a planet) and, thanks to its relatively small mass of about 1 tonne being boosted by a mighty Atlas V launch vehicle, is still the fastest moving spacecraft to leave Earth to date. When the rocket motor in the last stage of the launch vehicle shut down New Horizons was travelling at 16.21 km/s (58 000km/h or 36 000 mph) with respect to Earth, so fast that New Horizons already exceeded solar escape velocity (when it was discarded, the final STAR-48 stage of the rocket was also traveling at the same speed and heading out of the Solar System in the same general direction- but it missed Pluto by hundreds of millions of kilometres) . Thanks to this impressive speed, it crossed the orbit of the Moon nine hours after launch- the Apollo missions carrying people to the Moon took three days to cover this distance.
However, fast or not, the probe did not travel directly to its final destination, Pluto and its family of moons. Instead New Horizons spent its first year in space taking a longer and indirect route arcing across the Solar System towards Jupiter. As it neared the giant planet, New Horizons began to speed up as Jupiter’s gravitational influence increased, at the same time its course began to change as Jupiter pulled the probe towards it. Seen from above, the probe’s path would have developed a distinct kink, as Jupiter eased it into a new trajectory. On 28 February 2007 the tiny probe made its closest approach about 2 million kilometres from Jupiter then continued on its way. As it left it was traveling about 4 km/s (14 400 km/h or 8950 mph) faster than before the encounter with Jupiter.
This may sound outrageous. Where did this increase in speed come from? It was essentially stolen from Jupiter. The giant planet is moving in its own orbit at about 13 km/s (46 800 km/h or 29 000mph) with respect to the Sun. New Horizons was essentially being dragged along by Jupiter’s gravitational field as it approached the planet. As a result of the encounter New Horizons sped up but to balance the books Jupiter was slowed down in its orbit around the Sun. Jupiter lost as much kinetic energy as New Horizons gained, so the giant planet is not orbiting the Sun quite as fast as it was before the encounter, so thanks to New Horizons, Jupiter’s year has slightly increased, but not by much. The scale of Jupiter’s speed loss is in proportion to the size difference between the giant planet and the spacecraft so Jupiter was slowed by about a million trillionth of a millimetre per second.
Thanks to this ingenious manoeuver at a stroke four years were cut from New Horizon’s journey time to Pluto. There was a serious reason to make the journey as fast as possible; scientists wanted New Horizons to reach Pluto before its thin atmosphere disappeared, freezing as the planet moved further away from the Sun. Less happily this great speed means it was impossible for the probe to attain an orbit around Pluto. New Horizons screamed past Pluto without stopping, actually passing between the dwarf planet and its largest moon Charon. No available propulsion system could have reduced the probe’s velocity enough for it to enter orbit around Pluto.
New Horizons also received a gravitational assist of about 14mph (about 0.006 km/s) from its encounter with Pluto as described in the video below.
After its rendezvous with Pluto New Horizons will be redirected to encounter the Kuiper Belt Object 2014 MU69 in January 2019. Once its mission is over New Horizons will fly on into interstellar space, destined to probably outlive the Sun as it spends eternity wandering the stars.
(Article by Colin Johnston, Science Education Director. Article updated on 19 July 2016)
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Jakke · June 15, 2018 at 23:17
Serious question – How is NH powered?
A · January 8, 2019 at 22:30
Sorry this is so late but if you’re still curious, it’s powered by a radioisotope thermoelectric generator, which uses the heat produced from the radioactive decay of plutonium to generate electricity.
Rob · June 1, 2017 at 01:05
Regards to taxpayers money unmanned missions are probably cheaper and its fuel cost is bought and paid for. Not to mention. We should see a lot more than Pluto in the future?
Alex · July 5, 2016 at 23:47
Still says 2019
“(Article by Colin Johnston, Science Education Director. Article updated on 4 September 2019)”
admin · July 18, 2016 at 07:15
Dear Alex, thank you, I have just returned from annual leave and have fixed this typo.
Aex · July 4, 2016 at 04:07
Updated by Colin from the year 2019, my my !
Is this a typo, or have you sent it elsewhere, and this is a pre-packaged builtin to keep the sheep happy.
admin · July 18, 2016 at 07:13
Dear Aex, thank you for bringing this typo to my attention, I’ve fixed it now.
TRS · December 3, 2015 at 12:39
How did New Horizons not get sucked into the orbit of Gas Giants ! i believe it was close enough to Jupiter at least to use its gravitational pull to increase its speed!!
one other thing, the probe used the gravitational pull to get increase speed, what about escaping the gravitational pull which the planet would exert while the probe is moving away?
Alex · July 4, 2016 at 04:24
Astrophysics is not your forte or you do not follow star-trek.
I’m just an interested bystander but certainly as NH speeds into the gravity well it reaches escape velocity and the planet has less time to pull it back.
A carefully plotted path is mathematically able to sling out with a net increase of speed even after the pull back you worry about 🙂
chuck · October 3, 2015 at 00:42
a true waste of taxpayers money. who gives a rats ass what pluto looks like? the speed at which this thing was travelling past pluto could not have provided any worthwhile information that would be useful here on earth.
Kyle T · November 5, 2015 at 00:48
I can think of many many greater true wastes of taxpayer money happening everyday.
At least this has a coolness factor beyond anything we’ve accomplished yet as a species, let alone as a nation.
Not to mention…bringing back the, “because we could.”
Lee · January 5, 2017 at 21:19
I care what Pluto looks like.
Randi · September 8, 2015 at 06:14
Crazy to think something can get all the way to the moon faster than you can get to many places on Earth! Thank you for this interesting article.
ishaan gupta · July 24, 2015 at 20:50
Now advancement in researches on moon…
As journy to moon in just 9 hrs…. Amazing….
Rachel · July 15, 2015 at 12:01
Never mind. Just read it again. You’re saying its speed had increased by 4 km/s after leaving Jupiter. Not that its speed was 4km/s at that time.
admin · July 15, 2015 at 13:33
Dear Rachel, no problem, I’ve tweaked the wording slightly to make it more clear.
Rachel · July 15, 2015 at 11:59
How could it have been going faster after it left Earth (16.21 km/s) than after it left Jupiter (4 km/s), according to your article above? Surely if anything it should have been going even faster than 16.21 km/s after its gravitational assist?
Lee · January 5, 2017 at 21:17
Don’t forget about the gravitational effect of the Sun.
New Horizons slowed down by an ever decreasing rate all the way and it’s speed is still decreasing right now due to the Sun, albeit at an ever decreasing rate.
Guest 47 · July 14, 2015 at 14:37
Great article! What I could not find from the internet is why New Horizons was able to reach the orbit of Saturn in only 2.5 years with a single gravitation assist from Jupiter, while Cassini took 7 years and required it to go to Venus twice, Earth, Mars, and Jupiter for gravitation assist. I suppose the fact that Cassini is 5 times heavier than New Horizons could be the answer. But would be nice to get a confirmation. In future, it might be better to just send a series of smaller crafts than one large one. They could reassemble at the destination, perhaps.
admin · July 17, 2015 at 11:48
Dear Guest, you are correct in saying that Cassini (at 6 tonnes) was too heavy to get to Saturn with one gravity assist, there are more details in the NASA document VVEJGA Trajectory (link).
Lee · January 5, 2017 at 21:27
The key factor here is deceleration. Cassini took longer to get there because its arrival speed at Saturn needed to be slower than New Horizons, so it didn’t need to use as much thrust (and hence as much (heavy) fuel) to get into orbit. If Cassini had traveled as fast as New Horizons it would not have had enough fuel to slow down and enter orbit.
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