An aerospace engineer and astronomer has caused a stir on social media after verifying an astronaut’s accounts that seemed too good to be true.
Max Fagenwho has worked for NASA and SpaceX, as well as serving as commander of the Mars Desert Research Station, Written on X An astronaut unexpectedly told him a counterintuitive statement last week: “Any object made of rock takes two hours to orbit its own surface.”
This seemed a bit strange to Fagin, but when he looked skeptical, the astronaut smiled at him and said, “Check it out for yourself” – which he did. First, consider the low orbits of the (mostly) rocky bodies we know of: Earth, Mars, and the Moon.
All of these orbits take about two hours, with the period of low Earth orbit being the fastest. Fagin did the math and realized that the radius and mass of the object were not important when calculating low orbits. What mattered was the density.
The astronaut was right: a rocky body takes about two hours to rotate. After looking more closely, he realized that the same physics makes asteroids larger than a certain size rotate no faster than once every two hours.
Most asteroids larger than 200 meters in diameter [656 feet] “Rotation at a rate less than that which would cause a body on its equator to experience weightlessness,” Research Paper on the Mechanics of Moving Asteroids It is clear“This translates to rotation rates of less than 12 cycles per day (2 hours/cycle) with most having rotation periods of 4 hours or more.”
As Fagen explains, asteroids grow by colliding smaller rocks, and are held together (often very loosely) by their own gravity. Any such object that orbits faster than once every two hours will fling material into its orbit, until its rotation shrinks or slows to the two-hour limit determined by its density. We’re sure you’ll agree that this is pretty cool to know.
