Astronomer goes viral after checking astronaut’s orbital math that seemed too weird to be true

An aerospace engineer and astronomer caused a stir on social media after he fact-checked an astronaut’s math that seemed too good to be true.

Max Fagin, who has worked for NASA and SpaceX and also served as commander of the Mars Desert Research Station, wrote on X that an astronaut casually told him a counterintuitive statement last week. The statement was that “it takes 2 hours to orbit the surface of a rocky object.”

This sounded a bit strange to Fagin, but when he looked skeptical, the astronaut just smiled and told him to “check it out for yourself” – which he did. At first he thought of the low orbits of (mostly) rocky objects that we know: the Earth, Mars and the Moon.

All of these orbits take about 2 hours, with the period of the low Earth orbit being the fastest. Fagin did the math and realized that the radius and mass of an object are not important when calculating low orbits. What is important is the density.

The astronaut was right, it takes about 2 hours to orbit an object made of rock. When he looked into it further, he realized that the same physics keeps asteroids above a certain size from rotating faster than once every two hours.

“Most asteroids larger than 200 meters [656 feet] spinning at a speed less than that which would cause an object at its equator to experience weightlessness,” an article on the mechanics of moving asteroids explains“This equates to rotation rates of less than 12 revolutions per day (2 hours/revolution), with most rotation periods being 4 hours or longer.”

As Fagin explains, asteroids grow by collisions of smaller rocks and are held together (often very loosely) by their own gravity. Any of these objects spinning faster than once every two hours would throw material into orbit until it shrinks or slows its rotation to the 2-hour limit defined by its density. And that, I’m sure you’ll agree, is pretty cool to know.

Leave a Comment