The Dawn mission and orbital mechanics

The Dawn mission launched in 2007 reached its second goal, the asteroid Ceres, in Feburary 2015. Ceres is the largest object in the asteroid belt, large enough to be clasifed as a dwarf planet. So it’s of interest to scientists. For example how come it hasn’t been demolished by impacts with other asteroids over the ages. To get to Ceres, the Dawn spacecraft had to take a long an roundabout path, which is standard in space motion.

Motion is space around planets and suns are very different from motion on Earth that we’re used to. This tends to thrown students of orbital mechanics for a loop. For one, there is not straight line motion in space, all movement is on a curve, or more correctly an ellipse. Orbital motion is based on “orbits” around a central gravitational body like a planet, star, or large asteroid. The closer you are to a large body, the stronger its gravitational pull on you. The pull turns a stright path into a curved one around the large body.

In a solar system, the largest gravitational body is the star, like our Sun, followed by large gas giants like Jupiter. The Sun is dominat thus all the planets orbit around the Sun. But then why do the moons of the planets orbit around their planet instead of directly around the Sun? The reason is due to something called the sphere of influence (SOI). The SOI is a sphere around a large object that defined where it’s gravity is dominant. So moons are inside their planet’s SOI, thus orbit the planet as opposed to the Sun. The size of the SOI is based on the mass of the main body. So Jupiter has a larger SOI than Earth which has a larger SOI than Ceres. Thus in order to “get into the orbit” of a large body, you have to get close enough to enter its SOI. This means you have to catch up to whatever body you’re looking for, like Ceres.

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