Mars is the fourth planet from the sun, and the second closest to Earth (Venus is the closest). But the distance between the two planets is constantly changing as they travel around the sun.
In theory, the closest that Earth and Mars would approach each other would be when Mars is at its closest point to the sun (perihelion) and Earth is at its farthest (aphelion). This would put the planets only 33.9 million miles (54.6 million kilometers) apart. However, this has never happened in recorded history. The closest approach of the two planets occurred in 2003, when they were only 34.8 million miles (56 million km) apart.
The two planets are farthest apart when they are both at their farthest from the sun, on opposite sides of the star. At this point, they can be 250 million miles (401 million km) apart.
The average distance between the two planets is 140 million miles (225 million km).
The speed of light
Light travels at approximately 186,282 miles per second (299,792 km per second). Therefore, a light shining from the surface of Mars would take the following amount of time to reach Earth (or vice versa):
- Closest approach: 182 seconds, or just over 3 minutes
- Farthest approach: 1,342 seconds, or just over 22 minutes
- On average: 751 seconds, or just over 12.5 minutes
Fastest spacecraft so far
The fastest spacecraft launched from Earth was NASA’s New Horizons mission, which is en route to Pluto. In January 2006, the probe left Earth at 36,000 mph (58,000 kph). The time it would take such a probe to get to Mars would be:
- Closest approach: 942 hours (39 days)
- Farthest approach: 6,944 hours (289 days)
- On average: 3,888 hours (162 days
But then things get complicated …
Of course, the problem with the previous calculations is that they measure distance between the two planets as a straight line. Traveling through the farthest passing of Earth and Mars would involve a trip directly through the sun, while spacecraft must of necessity move in orbit around the solar system’s star.
Although this isn’t a problem for the closest approach, when the planets are on the same side of the sun, another problem exists. The numbers also assume that the two planets remain at a constant distance; that is, when a probe is launched from Earth while the two planets are at the closest approach, Mars would remain the same distance away over the course of the 39 days it took the probe to travel. [Countdown: The Boldest Mars Missions in History]
In reality, however, the planets are continuously moving in their orbits around the sun. Engineers must calculate the ideal orbits for sending a spacecraft from Earth to Mars. Their numbers factor in not only distance but fuel efficiency. Like throwing a dart at a moving target, they must calculate where the planet will be when the spacecraft arrives, not where it is when it leaves Earth. Spaceships must also decelerate to enter orbit around a new planet to avoid overshooting it.
How long it takes to reach Mars depends on where in their orbits the two planets lie when a mission is launched. It also depends on the technological developments of propulsion systems.
Here is a list of how long it took several historical missions to reach the red planet. Their launch dates are included for perspective.
- Mariner 4, the first spacecraft to go to Mars (1964 flyby): 228 days
- Mariner 6 (1969 flyby): 155 days
- Mariner 7 (1969 flyby): 128 days
- Mariner 9, the first spacecraft to orbit Mars (1971): 168 days
- Viking 1, the first U.S. craft to land on Mars (1975): 304 days
- Viking 2 Orbiter/Lander (1975): 333 days
- Mars Global Surveyor (1996): 308 days
- Mars Pathfinder (1996): 212 days
- Mars Odyssey (2001): 200 days
- Mars Express Orbiter (2003): 201 days
- Mars Reconnaissance Orbiter (2005): 210 days
- Mars Science Laboratory (2011): 254 days