Expert: Courtney Seligman - 11/6/2008

Question
What would happen if an object the size of Pluto rammed into earth at the speed of sound?

Would this annihilate the earth or what?

Answer
When any object hits the Earth, it does so with at least the escape velocity of the Earth, which is 25,000 miles per hour, or nearly 40 times the speed of sound. (Even if it weren't moving relative to the Earth before running into us, our gravity would pull it into the Earth with at least that speed.) This would give it at least 1500 times as much energy per pound of material as if it were going at the much slower speed of sound.

As a result, even an object a few dozen miles in diameter would deliver such a blow to the Earth that all life on Earth would be destroyed by (a) the impact, (b) the shock waves from the impact (earthquake waves in the Earth, blast/sound waves in the atmosphere), and/or (c) the catastrophic changes in the atmosphere caused by the melting and vaporization of the object, and the best part of a thousand times as much of the Earth.

However, even an object a hundred times the mass and size of Pluto would be too small to actually 'destroy' the Earth. The Earth would be shattered, shaken, partially melted and vaporized by the impact, but practically every bit of the incoming object and the Earth would, even if thrown out into space, fall back onto the Earth, to form a new version of the planet. In fact, an impact perhaps twice that large is thought to have occurred around 4480 million years ago, resulting in the ejection of enough bits and pieces to form the Moon (after a century or so of collisions between those pieces), but the Earth was still here afterwards -- just bigger and more massive than ever, thanks to the addition of the material from the impacting object.

Courtney Seligman
Professor of Astronomy
Long Beach City College