Every two years, Earth catches up to Mars’ orbit and aligns with the planet and the sun in a straight line. While Earth takes 365 days to orbit the sun, Mars takes 687 Earth days. As a result, the opposition occurs about every 26 months.
The close approach, however, varies between 35 million and 63 million miles, according to NASA, since Mars is on an elliptical orbit around the sun. Mars will be even closer in 2018, coming within 35.8 million miles of Earth, according to NASA.
While Mars will be closer than it has been in a long time, viewing details of the Red Planet may require a powerful telescope. For those wanting to check out Mars' close approach, online observatory Slooh will host a live look at the Red Planet beginning at 9 p.m. EDT.