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)