What would happen if a massive comet crashed into the sun?

Most comets that brush past the sun end their lives in a whimper. But according to new calculations, a big enough comet that plunges into the sun should go out with a bang.

For the past few years, NASA’s Solar and Heliospheric Observatory has discovered comets at a rate of three or more a week as they pass very close to the sun. The smaller of these sungrazers don’t usually make it far. “If you’re a loosely packed snowball, then going to the sun is something you should probably try to avoid,” says Karl Battams, NASA’s sungrazer expert at the US Naval Research Laboratory in Washington DC.

It isn’t the sun’s outer layer, or corona, that melts these comets as they graze its edge. The corona, though very hot, is too thin to transfer much heat. Instead, the intense glare of solar radiation sublimates ices into gas that escapes into space or causes the comets to crack apart.

But recently observed comets have made it closer to the sun’s surface than ever before. In 2011, comet Lovejoy actually passed through the solar corona, emerging much worse for wear but still loosely together. Comet ISON barely survived a similar trip in 2014.

So what would occur if a comet hit the sun head on, slamming into its lower atmosphere? “There’s no reason for it not to happen,” Battams says. “The sun is a pretty big target, and there’s enough stuff around flying in the solar system.”
Supersonic snowballs in hell

A team led by John Brown, Astronomer Royal for Scotland, has calculated the answer. “I give talks about these and I call them supersonic snowballs in hell,” Brown says.

To reach the sun’s lower atmosphere, a comet would need a mass of at least 109 kilograms – a lower limit roughly a hundred times smaller than comets ISON and Lovejoy.

If a comet is big enough and passes close enough, the steep fall into the sun’s gravity would accelerate it to more than 600 kilometres per second. At that speed, drag from the sun’s lower atmosphere would flatten the comet into a pancake right before it exploded in an airburst, releasing ultraviolet radiation and X-rays that we could see with modern instruments.

The crash would unleash as much energy as a magnetic flare or coronal mass ejection, but over a much smaller area. “It’s like a bomb being released in the sun’s atmosphere,” Brown says. The momentum propelled by the comet could even make the sun ring like a bell with subsequent sun-quakes echoing through the solar atmosphere.

Brown acknowledges that the work is speculative – both in the sense that a sun-plunging comet hasn’t yet been seen and in the physics that would determine its fate. One issue that could make a big difference is the poorly understood propensity of comets to break up under stress.

A true impactor is likely to be a one-off event that might happen once a century. But thinking ahead in case of a sun-striking comet is a worthy exercise for a phenomenon that has almost certainly happened in the solar system’s past and will happen again in the future, Brown says. In 1994, the impact of comet Shoemaker-Levy 9 on Jupiter was a surprise to planetary scientists who doubted violent events like that could happen on human timescales.

The calculations may also apply to other solar systems, where young stars are bombarded with far more comets than the sun has to face.