Episode Notes: Balls and Potatoes in Space
1The science podcast club at Corbin Valley Middle School records a short episode every two weeks to answer a question from a listener. This month the question came from a sixth grader named Ilse: "In photos from space probes, big moons look like smooth balls, but small moons and asteroids look like lumpy potatoes. Why is that?" Eighth graders Tamsin Okeke and Rasmus Aguilar took on the question with help from their science advisor, Ms. Beatrix Nwosu.
2Tamsin started with data that space probes have gathered over the past fifty years. For six bodies in the solar system, she listed the width, the mass and the pull of gravity on a 1 kg rock resting on the surface. She also noted what each body orbits and how it looks in probe photos. Ms. Nwosu pointed out that rock and ice seem hard and stiff, but under a strong, steady squeeze that lasts millions of years they can slowly sag and flow, a little like very thick putty.
3Rasmus looked for clues about loose material on small bodies. Close-up photos of the asteroid Itokawa, which is only about 0.5 km long, show that loose gravel has collected in its low, smooth areas, while big boulders cover its higher parts. A probe that visited the asteroid Bennu, also about 0.5 km wide, found that it is a loose pile of boulders and gravel with empty gaps between the pieces. Scientists think that nothing glues most of these pieces to each other.
4Ms. Nwosu added one more fact. On any one body, a rock with more mass is pulled harder. On Ceres, for example, the pull on a 1 kg rock is about 0.28 N, and the pull on a 2 kg rock is about 0.56 N.
5Before recording, the two hosts agreed on a rule for their episode: every part of their answer to Ilse had to rest on evidence from the data or the probe photos, not on a guess. Tamsin planned to open the episode with the table, and Rasmus wanted to close it with the asteroid photos. They also hoped to answer a second question that listeners often send in: does gravity really act on something as small as an asteroid?