In August this year, the New Horizons spacecraft will cross into the region of space beyond Neptune’s orbit. It won’t be the first human object to go this far: the two Voyager space probes have already done that, and then Pioneer 10 with them. What will be special about New Horizons is that it’ll be the only one with enough power to receive commands from Earth, perform observations, and relay its findings back. Unlike the Voyager and Pioneer probes, New Horizons will not be a symbolic, space-born artifact but the first fully functional scientific experiment to travel that far. To be fair, Voyager 1 at the cusp of the interstellar medium still has its ears open for out-of-the-ordinary stuff but it doesn’t have enough juice to turn its head.
From its new perch, New Horizons will be privy to the lives of a belt of bodies named for the astronomer Gerard Kuiper, who speculated on them in the 1950s. The Kuiper belt, like the asteroid belt between Mars and Jupiter, bears signatures of the formative days of the Solar System, which were quite tumultuous. Various studies of asteroids, Kuiper belt objects (KBOs) and satellite systems of the gas giants Jupiter and Saturn have shown that after the planets formed, they moved around quite a bit before settling in their current orbits. One interesting way we know this is because of some similar properties between the asteroid belt and the KBOs. Even though they’re so far apart (~4.2 billion km between them), how could they have had a shared history?
Look to Jupiter. According to one of the models of planetary formation, called the Grand Tack Model, Jupiter once came as close to the Sun as Mars is today, adulterating the asteroid belt with objects from the Kuiper belt its prodigious gravitational pull would’ve tugged along, before moving back. Then, according to the Nice Model, Jupiter pulled in more KBOs into orbit around itself – explaining why many moons of the ice- and gas-giants in that part of the Solar System look and feel like large KBOs. However, as compelling as these models seem, they’re far from being known to be absolute true. Astronomers need to make more observations.
That’s why it’s exciting that New Horizons is entering the vicinity of the Kuiper belt. Its findings would be both seminal and extremely important in understanding how the Solar System was born, why it has an anomalous constitution of planets, and how the ice giants Uranus and Neptune came to be.