The Satellites Watching the Storm

Eyes Above Gull Harbor

1Every August, the coastal town of Gull Harbor holds a storm preparation week before the peak of hurricane season. This year the town's emergency manager, Ms. Renata Albescu, visited the eighth grade science class at Gull Harbor Middle School. She showed the class the satellite images her office watches when a storm forms far out over the ocean. Some images come from weather satellites that pass over the North and South Poles about every 100 minutes. Others come from a weather satellite that seems to hang above the same spot over the equator, so it can watch a storm grow hour by hour. Last fall, those satellite pictures gave the town three days of warning before a strong storm reached the coast.

2Eighth grader Ezra Kowalczyk asked how the satellites stay up without engines running all the time. Ms. Albescu explained that a satellite is launched to a high speed sideways to Earth's surface. Earth's gravity pulls it toward Earth's center the whole time, so its path keeps curving around the planet instead of going straight off into space. Near Earth, a satellite needs a speed of several kilometers per second to keep curving around the planet without falling back into the air below. The class then looked up data on several objects that orbit Earth, including the Moon, in Table 1.

3Next, the class looked at how strong Earth's pull is at different heights. Table 2 in Part 2 shows the force of Earth's gravity on 1 kg of mass at each height. The class noticed that astronauts on a space station float inside it even though the station is not very far above Earth.

4Finally, their teacher, Mr. Tobin Rasmussen, set up a model. Four students held the corners of a stretchy cloth tight, and he set a heavy metal ball in the center, which made a dip. When a student rolled a marble straight across the flat part of the cloth, it rolled in a nearly straight line. When she rolled it sideways near the dip, it curved around the ball several times. After a few laps it slowly spiraled in and hit the ball. With a lighter ball in the center, the dip was shallower, and the marbles curved around it less sharply.