# Eli5: How do astronomers determine the orbits of planets and asteroids?

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How do you figure out how fast something is moving, what direction it is moving in, and where exactly it will be at any given time in the future when all you have to go on is pictures? I’m especially wondering about exoplanets and asteroids, the best pictures of those we can get are practically pixels. I don’t understand how we can measure mass, velocity, and orbits from that.

In: Physics

They do it by taking as many pictures as they can over time and feed the data into computers that are programmed to calculate orbital trajectories. Asteroids aren’t that hard if you can get a few good looks at them. If you get an image of the same asteroid every two weeks for six months, you have plenty of data to calculate an orbit. Exoplanets are a bit different – transit times across the face of distant stars coupled with a star’s wobble provide data points to calculate certain features of the exoplanet. Also, while something may be blurred out in a visual image, it may be crisper in an X-ray observation (or other non-visual imaging system), allowing more detail to be known.

First, a bit of a nitpick. We’re way past the point of just using a visual telescope image to analyze something in space. We have telescopes that can analyze the light spectrum passing through an atmosphere in order to determine its contents. We can look out using X-ray or radio wave telescopes in order to increase what we can see outside of the visible spectrum.

However, even just using the visible light telescopes of the past, we can glean a lot of info about anything we can see.

Lets start with asteroids. If we’re looking for an asteroid, its going to be in our solar system. Once we find something moving, we begin tracking it, and measuring how far and in what direction it is moving. Once we have that info, calculating its orbit is just a matter of math that we’ve had since Newton’s work on gravity. Once we calculate all the parameters of a rocks orbit, we can figure out its mass based on its size and speed.

Exoplanets, which are planets around other stars, are trickier. We have to actually watch the star, and measure how much it’s light dims when the exoplanet passes in front of it, or we measure how much the star wobbles as the planet orbits it. If we can get a sample of light from the planet, we can run it through a spectrometer to see what light was absorbed by the planets atmosphere, which will tell us what its made of.