Reading Astronomy News: This Interstellar Asteroid is Accelerating
05/24/2019
By Stacy Palen
As I was working on the first draft of Chapter 3 for the fourth edition of Understanding Our Universe, I ran across this wonderful summary article about ‘Oumuamua by Steven Spence of GotScience Magazine. You probably remember that ‘Oumuamua is the first interstellar object that we’ve observed in our Solar System. This article compactly lays out the case for how we know it comes from another star, how fast it’s moving, and why its acceleration as it leaves the Solar System is “weird." It’s practically chapter 3 in a nutshell!
To read This Interstellar Asteroid, by Steven Spence, click here.

In the discussion of where ‘Oumuamua came from, the author states that it is “moving on an open hyperbolic trajectory.” Make a sketch of what such a trajectory would look like. How is it different than a path followed by an orbiting object?
Answer: They should draw a hyperbola; many will have to look this up. A hyperbola is not closed, like an ellipse is closed.

‘Oumuamua’s trajectory is described as having an eccentricity of 1.23. But the maximum eccentricity for an ellipse is 1.00. What is the resolution of this apparent contradiction?
Answer: Only objects that are orbiting, and bound to the system by gravity, travel on ellipses. Objects that are unbound can travel on trajectories that don’t meet the requirements for an ellipse.

In the article, ‘Oumuamua’s speed is described as fast enough to cover the distance from Earth to the Moon in 73 minutes. The New Horizons space craft covered the same distance in 8 hours. Approximately how many times faster is ‘Oumuamua traveling than New Horizons?
Answer: In “cowboy” math, 73 minutes is about an hour. So ‘Oumuamua must be traveling about 8 times faster than New Horizons, to cover the same distance in an 8^{th} of the time.

‘Oumuamua reached the orbit of Jupiter about an hour earlier than expected. Why does this imply that the object has accelerated?
Answer: In order to cover the distance in less time than expected, the object must be traveling faster than expected. This means there must be an unexpected increase, or lessthanexpected decrease, in the speed of the object.

The best idea, currently, for how ‘Oumuamua is accelerating is that it is “cometlike outgassing.” Outgassing occurs when a jet of gas shoots out from the object. Use Newton’s third law to explain how an object “outgassing” can cause an object like ‘Oumuamua to accelerate.
Answer: This is exactly how a rocket accelerates! Because it is a closed system, if some of the mass accelerates in one direction, there must have been a force that pushed that mass. By Newton’s third law, there must be an equal and opposite force pushing the rest of the mass in the opposite direction.
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