New Horizons hurdling towards where we hope to find Pluto
For the last nine and a half years, a satellite called New Horizons has been racing towards Pluto at the breakneck speed of nearly nine miles per second. This pace was necessary in order to make the journey across the immense distance from Earth to Pluto in any way practical. It also means that, as the satellite nears the dwarf planet, there is very little room for error in plotting the trajectory. Even if all goes according to plan, New Horizons is only hoping to get one shot at photos and measurements before passing Pluto by.
Hitting a (strangely) moving target
Unfortunately for the planners of this mission, the story only gets more complicated. Pluto was only discovered in 1930, and thanks to having a 248-year orbit, we have yet to witness it complete a trip around the Sun. This means that we have very little data about what Pluto’s relative positioning should look like compared to other planets. We do know that it has a strange orbit though, being constantly pulled on by its largest moon, Charon. Pluto’s small size compared to its moons mean the gravitational influence of these moons more than what you’d find on say, Neptune. The fact that the moons are described as “tumbling in absolute chaos” could be the icing on the cake.
Assuming New Horizons is continuing to the right patch of sky, and it appears to be doing just that, we don’t actually want to get the satellite too close to Pluto. In addition to the moons, there’s a chance of collision with undiscovered rocks and debris, any of which could destroy our probe. So to keep a safe distance, and conveniently narrow the optimal target zone even further, New Horizons should fly by Pluto at a distance of around 7,767 miles.
Measure twice, pilot once
So how is any of this accomplished? The distances involved mean that instructions to the onboard computer take nine hours to be transmitted, and another nine hours before feedback can be received. Despite the speed of the satellite itself, changes in course must be plotted more like a chess game than a Formula 1 race. To help minimize error, New Horizons’ course is actually being plotted by two independent teams. If their calculations match, a move is probably correct. If the teams come to differing conclusions, more scrutiny can be applied before mission managers at NASA make the final call.
Or rather, it could have been, up until last weekend. The last day to safely make a significant change to the satellite’s course was July 4th. At this point, the course is basically set, and the remaining questions are now concerning when to start recording data, and for how much longer we need to keep our fingers crossed.
Source: Pluto-bound probe faces its toughest task: finding Pluto by Alexandra Witze, Nature