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What is the Distance from Earth to Mars?
by Fraser Cain on December 5, 2013
http://www.universetoday.com/14824/distance-from-earth-to-mars/
Sending spacecraft to Mars is all about precision. It’s about blasting off from Earth with a controlled explosion,
launching a robot into space in the direction of the Red Planet, navigating the intervening distance between our
two planets, and landing with incredible precision. This intricate and complicated maneuver means knowing the
exact distance from Earth to Mars. And unfortunately, this distance is always changing.
The first person to ever calculate the distance to Mars was the astronomer Giovanni Cassini, famous for his
observations of Saturn. Giovanni made observations of Mars in 1672 from Paris, while his colleague, Jean
Richer made the same observation from Cayenne, French Guiana. They used the parallax method to calculate
the distance to Mars with surprising accuracy. However, astronomers now calculate the distance to objects in
the Solar System using the speed of light. They measure the time it takes for signals to reach spacecraft orbiting
other planets. They can bounce powerful radar off planets and measure the time it takes for signals to return.
This allows them to measure the distance to planets, like Mars, with incredible accuracy.
How Far is Mars From Earth?
So, how far away is Mars? The answer to that question changes from moment to moment because Earth and
Mars are orbiting the Sun. Both Earth and Mars are following elliptical orbits around the Sun, like two cars
travelling at different speeds on two different racetracks. Sometimes the planets are close together, and other
times they’re on opposite sides of the Sun. And although they get close and far apart, those points depend on
where the planets are on their particular orbits. So, the Earth Mars distance is changing from minute to minute.
A Quick Explainer on Orbital Mechanics
The planets don’t follow circular orbits around the Sun, they’re actually travelling in ellipses. Sometimes
they’re at the closest point to the Sun (called perihelion), and other times they’re at the furthest point from the
Sun (known as aphelion). To get the closest point between Earth and Mars, you need to imagine a situation
where Earth and Mars are located on the same side of the Sun. Furthermore, you want a situation where Earth is
at aphelion, at its most distant point from the Sun, and Mars is at perihelion, the closest point to the Sun.
When Earth and Mars are closest
When Earth and Mars reach their closest point, this is known as opposition. It’s the time that Mars appears as a
bright red star of the sky; one of the brightest objects, rivalling the brightness of Venus or Jupiter. There’s no
question Mars is bright and close, you can see it with your own eyes.
And theoretically at this point, Mars and Earth will be only 54.6 million kilometers from each other. But here’s
the thing, this is just theoretical, since the two planets haven’t been this close to one another in recorded history.
The last known closest approach was back in 2003, when Earth and Mars were only 56 million kilometers
apart. And this was the closest they’d been in 50,000 years.
Need that in miles? The closest possible distance from Earth to Mars in miles is 33.9 million miles.
Here’s a list of Mars Oppositions from 2007-2020 (source)
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Dec. 24, 2007 – 88.2 million km (54.8 million miles)
Jan. 29, 2010 – 99.3 million km (61.7 million miles)
Mar. 03, 2012 – 100.7 million km (62.6 million miles)
Apr. 08, 2014 – 92.4 million km (57.4 million miles)
May. 22, 2016 – 75.3 million km (46.8 million miles)
Jul. 27. 2018 – 57.6 million km (35.8 million miles)
Oct. 13, 2020 – 62.1 million km (38.6 million miles)
2018 should be a very good year, with a Mars looking particularly bright and red in the sky.
When Earth and Mars are furthest apart
On the opposite end of the scale, Mars and Earth can be 401 million km apart (249 million miles) when they
are in opposition and both are at aphelion. The average distance between the two is 225 million km.
When Mars is Close, You’re Go For Launch
Mars and Earth reach this closest point to one another approximately every two years. And this is the perfect
time to launch a mission to the Red Planet. If you look back at the history of launches to Mars, you’ll notice
they tend to launch about every two years.
Here’s an example of recent Missions to Mars, and the years they launched:
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MER-A Spirit – 2003
MER-B Opportunity – 2003
Mars Reconnaissance Orbiter – 2005
Phoenix – 2007
Fobos-Grunt – 2011
MSL Curiosity – 2011
See the trend? Every two years. They’re launching spacecraft when Earth and Mars reach their closest point.
Spacecraft don’t launch directly at Mars; that would use up too much fuel. Instead, spacecraft launch towards
the point that Mars is going to be in the future. They start at Earth’s orbit, and then raise their orbit until they
intersect the orbit of Mars; right when Mars is at that point. The spacecraft can then land on Mars or go into
orbit around it. This journey takes about 250 days.
Communicating with Mars
With these incredible distances between Earth and Mars, scientists can’t communicate with their spacecraft in
real time. Instead, they need to wait for the amount of time it takes for transmissions to travel from Earth to
Mars and back again.
When Earth and Mars are at their theoretically closest point of 54.6 million km, it would take a signal from
Earth about 3 minutes to make the journey, and then another 3 minutes for the signals to get back to Earth. But
when they’re at their most distant point, it takes more like 21 minutes to send a signal to Mars, and then another
21 minutes to receive a return message.
This is why the spacecraft sent to Mars are highly autonomous. They have computer systems on board that
allow them to study their environment and avoid dangerous obstacles completely automatically, without human
intervention.
The distance from Earth to Mars is the main reason that there has never been a manned flight to the Red Planet.
Scientists around the world are working on ways to shorten the trip with the goal of sending a human into
Martian orbit within the next decade.