Mars is the fourth planet from the sun. It has been a favorite object of skywatchers for centuries and of space explorers for decades. The distance to Mars depends on when you measure it, and which vantage point you’re considering. We’ll assume first you are wondering this:

How far is Mars from Earth?

Mars and Earth orbit the sun at different speeds: Earth has an inside track and gets around the sun more quickly. Plus, both have elliptical orbits, rather than perfect circles. So the distance to Mars from Earth is constantly changing. In theory, the closest the planets could come together would be when Mars is at its closest point to the sun (perihelion) and Earth is at its farthest point (aphelion). In that situation, the planets would be 33.9 million miles (54.6 million kilometers) from each other. But that has never happened in recorded history. The closest known approach was 34.8 million miles (56 million km) in 2003.

At the other end of the scale, when both planets are at aphelion — their farthest distance from the sun — and the two worlds are on opposite sides of the sun, they can be 250 million miles (401 million km) apart — a whopping 7.37 times the closest possible separation.

On average, the distance to Mars from Earth is 140 million miles (225 million km).

In their race around the sun, Earth on its inside track laps Mars every 26 months. This close approach provides an opportunity — a launch window — to send spacecraft to the red planet. Rather than pointing the spacecraft at Mars, engineers aim it in a wide orbit around the sun. The sun's gravity gives the spacecraft a boost — called a gravity assist or slingshot effect —saving time and fuel. The spacecraft's orbit then intersects with Mars.

How far is Mars from the sun?

Mars has a very eccentric orbit; that is, it deviates from a perfect circle more than any other planet's orbit (Pluto, no longer considered a planet, has an even more eccentric orbit). At its farthest distance (aphelion), Mars is 154 million miles (249 million km) from the sun. At its closest (perihelion), Mars is 128 million miles (206 million km) distant. On average, the distance to Mars from the sun is 141 million miles (228 million km). Mars revolves around the sun in 687 Earth days, which represents a Martian year.

Measuring the distance

The distance to Mars from Earth was first determined by Giovanni Cassini in 1672 using the parallax method. He sent a colleague, Jean Richer, to French Guiana while he stayed in Paris. They took measurements of the position of Mars, and triangulated those measurements with the known distance between Paris and French Guiana. Their calculations were only 7 percent off from today's more precise instruments.

— Tim Sharp, Reference Editor

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