Who Pulls Hardest? Gravity, Asteroids and a Museum Meteorite

The Meteorite Hall

1On a geology field trip, the eighth graders at Copper Ridge Middle School spent the morning collecting rock samples in a dry streambed and the afternoon at the county natural history museum. The last stop was the meteorite hall. Kenji Halloran held a fist-sized iron meteorite that was much heavier than a rock of the same size. The museum educator, Dr. Marisol Vantongeren, explained that most meteorites are broken pieces of asteroids. Most asteroids orbit the Sun in the asteroid belt, about 2.2 to 3.3 AU from the Sun, between Mars and Jupiter. (1 AU is Earth's average distance from the Sun, about 150 million km.)

2A gravity calculator in the hall shows how hard different bodies pull on a 1 kg rock placed anywhere in the solar system. It gives each pull in micronewtons (µN), or millionths of a newton. Kenji first placed the rock at different distances from the Sun (Table 1). Then he compared the Sun's, Jupiter's and Earth's pulls at two spots: the middle of the asteroid belt, and the orbit of Callisto, a moon that circles Jupiter about 1.88 million km from the planet's center (Table 2). A panel beside the screen says the Sun has about 1,050 times the mass of Jupiter, and Jupiter has about 318 times the mass of Earth.

3A wall display traces how one meteorite reached Earth. For billions of years, its parent asteroid circled the Sun in the belt. Then another asteroid smashed into it, and the blow sent a broken piece onto a new orbit that crossed Earth's path. Years later the piece passed close to Earth and fell toward it, moving faster and faster. As it plunged through the air at high speed, the air in front of it was squeezed and heated, and the rock's outside grew hot enough to glow. Most of the rock burned away, but a small part landed in a farm field.

4The last panel in the hall looks far beyond the solar system. It shows a globular cluster, a ball of several hundred thousand old stars that travels around the center of the Milky Way. The stars inside the cluster keep moving around within it, yet the cluster has stayed in one piece for billions of years.