It may be a model of long-term stability, but our solar neighbourhood’s future holds some cataclysmic events and spectacular changes

Reported by Giles Sparrow

Ever since it settled down from a period of early turbulence about 4 billion years ago, our Solar System has provided a more or less stable home to Earth and the other planets and bodies that orbit the Sun. But how long will it stay that way? It’s certain that the Sun is ultimately doomed to brighten and swell enormously in size, becoming a red giant whose bloated outer atmosphere will threaten to engulf the Solar System’s inner worlds 7 billion years from now - but long before then there are sure to be other changes that alter our planet and others, perhaps beyond recognition.

In the relatively near future, it is tidal forces between planets and their satellites that are likely to have the most impressive effects. These arise as angular momentum is transferred between the planet and moon due to tides on the planet and in response to the moon’s gravity, resulting in a consistent ‘tug’ on the moon’s orbit. For most moons in the Solar System the result is that the satellite slowly spirals away from the planet, while the planet slows its rotation. This is what is happening in our own planetary system - the Moon gets an average of 3.8 centimetres (1.5 inches) farther away from Earth each year, while Earth’s rotation slows by 1.7 milliseconds per century. Over millions of years the orbits of most moons in the Solar System will get wider.

But for a couple of moons, the opposite is happening - they’re slowly spiralling towards their planets. One of these is Phobos, the inner of Mars’ two rocky moons - but why is it different? “The inward migration of Phobos is due to the fact that its orbital period is slightly shorter than the rotational period of Mars,” explains Professor Ben Black of the City University of New York. “The Martian tidal bulge tugs at Phobos like a dog-walker pulling on a leash.” Alongside graduate student Tushar Mittal, Black recently looked at the Martian moon’s future evolution, suggesting that in about 20 to 40 million years from now Phobos may reach a crisis point when it gets too close to Mars.

Previous calculations had revealed how long it would take for the 22-kilometre (14-mile) moon to migrate inwards, but Black and Mittal’s work suggests Phobos might not make it that far. “We predict that Phobos won’t make it all the way to Mars, but instead will break apart sooner than that,” says Black. “We find that the key to Phobos’ fate is its strength. If Phobos is sufficiently strong, tidal stresses won’t be able to tear it apart, and it will crash into Mars intact. But the evidence points to Phobos being very weak.”