Earth and Mars Are About to Get Very Cozy

https://www.yahoo.com/news/earth-mars-very-cozy-160012171.html?nhp=1

Few things in space are as sloppy as a solar system—and few solar systems are as sloppy as ours. That can be a very good thing if you like cosmic sky shows, because now and again, the familiar nighttime heavens can change in dramatic ways. Monday will be one of those times, when Mars makes an unusually close pass by Earth, drawing nearer—and looming larger—than at any time since 2005.

Here’s what’s behind the cosmic coziness:

The orbit any planet makes around its parent star is fixed and knowable. Earth takes 365.256 days to make a single lap around the sun. (The 365 part is how we measure our year, and the .256 is why we tack on an extra day at the end of February every four years.) For Mars, a year is 686.93 days. For Neptune—well, a single Neptunian year takes 164.79 Earth years.

Those different orbital speeds mean that the distance between any two planets is always changing. As Earth zips around in the solar system’s No. 3 lane, for example, it sometimes finds itself on the complete opposite side of the sun from Mars, putting the two planets as much as 249 million miles (401 million km) apart. Every other year, however, Earth laps Mars, overtaking its pokier sister and bringing the two planets briefly as close as 33.9 million miles (54.6 million km). It’s during those biannual windows that NASA typically launches its missions to Mars, keeping the travel time from one planet to the other to a minimum.

Not all close encounters between Earth and Mars are equal, however. Few planets in any solar system orbit their suns in a perfect circle. Instead they follow a slightly egg-shaped path, which means that each orbit has a perihelion (closest approach to the parent star) and an apehelion (farthest approach). Mars’ closest approach to the sun is 128.4 million miles (206.6 million km) and its furthest is 154.8 million mi. (249.2 million km). For Earth, the perihelion is 91.4 million miles (147.1 million km) and the apehelion is 94.5 million miles (152.1 million km).