In space there isn’t much to get in the way of something. So a comet will maintain its speed (more or less) indefinitely until it collides with something else. So we can look at the direction of the comet and its speed and calculate how long it would take for it to make one full orbit back to where it is when we look at it now.

There are several methods that scientists use to date space material such as comets. One common method is radiometric dating, which involves measuring the relative abundances of certain radioactive isotopes and their decay products in a sample. This can be used to determine the age of rocks and other materials on Earth, as well as objects in space such as comets and asteroids. Another method is called crater counting, which involves counting the number of impact craters on a surface and using this information to estimate the age of the surface. This method is commonly used to date surfaces on planetary bodies such as the Moon and Mars, as well as on smaller objects like comets and asteroids.

Orbit paths follow some pretty simple math.

If you know an orbiting thing’s position, speed, and direction, you can calculate its orbit path using basic undergrad physics using the equations for gravity and momentum. Space is great because the “assume no friction or air resistance” thing actually works.

Once you know the orbit path and speed, it’s easy to work out where the object was at any point in the past, and where it will be in the future, including the last time it would have been near Earth.

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