Mission Briefing: How Fast Must an Orbiter Go?

Inside the Mission Briefing

1Every spring, the space agency's mission center outside Harlow Flats invites one eighth-grade class to sit in on a real planning briefing. This year Noemi Takeda's class watched engineers plan a future orbiter, a robot spacecraft that would circle a planet and map it for several years. The team has not chosen a planet yet, so flight engineer Dr. Halvard Mensah walked the class through the numbers for six possible targets.

2Dr. Mensah explained that an orbiter fires its engine only to reach its orbit and to make small corrections. Most of the time it coasts with the engine off. Its speed carries it sideways, while the planet's gravity pulls it toward the planet's center the whole time, so its path keeps bending around the planet. If the orbiter moves too slowly for its distance, its path bends inward and drops toward the planet. If it moves too fast, it swings far out and may never come back.

3To compare the targets fairly, the team ran a computer model. In the model, an orbiter circles each planet at 100,000 km from the planet's center, and the model ignores air, rings and moons. Table 1 lists each planet's mass compared with Earth's, the speed the orbiter needs to stay on that circle and the time it takes to go around once.

4Next, Dr. Mensah showed a second computer model, one that scientists use to study how planet systems form. It starts with a huge, slowly spinning cloud of gas and dust about 10,000 AU wide. (One AU is Earth's average distance from the Sun, about 150 million km.) In Run A, every bit of the cloud pulls on every other bit by gravity, as in nature. Table 2 in Part 2 shows snapshots of Run A. As the cloud shrank, the material that did not fall into the center flattened into a thin disk that kept spinning around the center clump, all in one direction. Small clumps in the disk pulled in nearby gas and dust and grew. In Run B, the team switched gravity off. The cloud slowly spread out, and no center clump formed.

5Our own solar system looks a lot like the end of Run A. The Sun holds about 99.8% of all the mass in the solar system. The eight planets all orbit the Sun in the same direction, and their orbits lie close to one flat plane.