Tip:
Highlight text to annotate it
X
What is the difference between prograde and retrograde motion, and how does it relate
to asteroids? Join the 321science team as we explore two types of asteroid motion.
Astronomers use the terms "prograde" and "retrograde" to describe the way that Solar System objects
rotate or spin while they orbit. Objects such as asteroids have two primary
types of motion: orbital motion around the Sun, and rotational motion, or spin, around
an axis or "pole". You can use a simple trick with your right
hand to figure out whether or not these motions are prograde or in the same direction.
First, curve your fingers in the direction of orbital motion, and observe which direction
your thumb points -- in this case, "up". Next, curve your fingers in the direction
of the asteroid's spin. If your thumb points in the same direction as it did for the orbital
motion, the spin is prograde. An asteroid has a retrograde spin if the direction
of its spin is opposite that of its orbit, and your thumb points in the opposite direction.
Just like asteroids, planets can have either prograde or retrograde spins - Earth spins
prograde; Venus spins retrograde. The direction that an asteroid spins can determine
how its orbit will change under the influence of the Yarkovsky effect.
If the asteroid is spinning in the prograde sense, its orbit will expand. If the asteroid
is spinning in the retrograde sense, like the asteroid Bennu, the target of the OSIRIS-REx
mission, its orbit will shrink. Knowing the spin direction is critical for
mission planners to design maneuvers around the asteroid.
Click the links to learn more about Bennu and about about how the Yarkovsky effect works.
This video is an OSIRIS-REx production. OSIRIS-REx is a partnership of the University of Arizona,
NASA Goddard Space Flight Center and Lockheed Martin.