Though New Horizons is armoured with Kevlar to protect it against high-velocity impacts with dust grains and other small particles, the main concern is that it may hit a tennis-ball sized object as it approaches Pluto. There could be plenty. In 2011, a tiny fourth moon named Kerberos (a guard dog from the underworld) and only 30 kilometres wide, was discovered by researchers using the Hubble Space Telescope. That resulted in an even more intense search for hazards and the 2012 discovery of another moon, now known as Styx, which has a diameter of about 20 kilometres.
Suddenly, Pluto had evolved from a wannabe planet to an entire system. The big concern was that if Pluto has many moons, it might also have rings like those of Saturn, Jupiter, Uranus and Neptune which are composed of tiny particles of rock or ice, but still big enough to puncture New Horizons’ armour.
Nobody had worried about rings in New Horizons’ path, but neither had anyone expected Nix, Hydra, Kerberos and Styx. If there are rings, it would be best to avoid them.
The potential hazard means the mission’s route past Pluto remains undecided. As it closes in on its target, New Horizons’ own instruments should soon be able to pick out obstacles. If they are discovered, it’s not too late to make a course correction.
The final stages of the flyby will be under automatic pilot. And for the 24 hours of the most intimate encounter between the spacecraft and Pluto, Earth will lose contact with New Horizons altogether. That’s because the spacecraft’s antenna is in a fixed position. It is aimed by “pointing” the entire vessel in the desired direction – a cost-saving measure that will doubtless cause anxiety on the crucial day.
Even if all goes well, it will take 16 months to beam all the data back to Earth. This longed-for information will come back in dribbles – rather like letters from sea captains in the days of the tall ships.
Two of the larger Kuiper Belt objects have been designated as potential post-Pluto bonus targets. Billions of kilometres beyond Pluto, they are reachable by course corrections – presuming New Horizons doesn’t have to waste too much fuel on a collision-avoidance manoeuvre.
And New Horizons’ ultimate fate? This last great explorer of our generation, it will continue transmitting data back to Earth, and will eventually move beyond the confines of our Solar System to join the two Voyager spacecraft in the depths of outer space. But for now, the Voyagers are still within the outer limits of the Solar System, and call home regularly. They each carry a greeting card to aliens in the form of golden phonograph records containing sounds and pictures of Earth. New Horizons offers other souvenirs: an American flag, a 1991 US postage stamp bearing the image of Pluto and the words “Not yet Explored” – and a tiny vial of Clyde Tombaugh’s ashes, the man who saw Pluto’s flickering light from an observatory in 1930.
