The Mars E-mail
Every August since 2003, we've gotten questions from people who receive a particular curious e-mail message. The subject line is usually something like this:
Subject: Fwd: FW: FW: Fw: Fwd: Mark your calendar in August (You don't want to delete this)
In earlier years, people got a simple e-mail with a picture included. More recently, a Power Point show (.pps) has been attached. Opening with "The Red Planet is about to be spectacular!" the message seems to say that the planet Mars is going to do something unusual in August -- possibly appearing as big as a full moon. After a bit of astronomical information, the message ends: "Share this with your children and grandchildren. NO ONE ALIVE TODAY WILL EVER SEE THIS AGAIN!"
Real or bogus?
Here's what's really happening:
- In August 2003 -- that's seven years ago -- Earth came unusually close to Mars. On August 27, 2003, by a fraction of a percent, we were closer than any time in the last 60,000 years or so. Mars looked very bright in the sky for several weeks around that date. Around the world, planetariums were swamped with visitors who mistakenly believed you had to look on the night of August 27, 2003 or you'd miss whatever there was to see.
- This close approach had been predicted many years before. Everyone interested in astronomy knew it was coming. Professional and amateur astronomers prepared to get better-than-average views of Mars through their telescopes. The twin Mars rovers called Spirit and Opportunity were launched to take advantage of the short travel time to the "red planet." And, somewhere, sometime in that spring or summer, someone wrote the e-mail and sent it to some friends. The e-mail was quickly passed from person to person around the world.
- The e-mail had confusing wording. It was trying to say that if you magnify Mars through a telescope you can make it look as big in the telescope as the full moon does without a telescope. That's true. However, many people interpreted it to mean that you could go outside and see Mars as big as the moon, or two moons in the sky, or something like that. A side-by-side picture of the moon and Mars, doctored to look the same size, reinforced this idea.
False. Mars never looks larger than a star unless you magnify it with a telescope.
Actually, Earth and Mars come near each other about every 26 months. The last time this happened was in late January 2010. The next time will be in March 2012. (I am writing this in August 2010.) Some of these approaches are closer than others. Approaches to Mars that are exceptionally close occur about every 15 years. The next exceptionally close approach will be in July 2018, when Mars will be almost as close as it was in August 2003.
- The e-mail mentioned August and early September ("this month and next"), but did not mention a year.
- So, every year in August the e-mail goes around again, and planetariums all over the world - including ours - get questions about it.
You can check this yourself, thanks to websites and astronomy software that can show you planet positions for any date almost instantly. Let's look at some diagrams generated by the free "Solar System Live" viewer at http://www.fourmilab.ch/solar. We'll use dates calculated by Jean Meeus, probably the world's foremost authority on this type of orbital calculation, from his book "Astronomical Tables of the Sun, Moon and Planets."
Here's the inner solar system on August 27, 2003.
The sun and planets are represented by big colorful dots. Their sizes are exaggerated but their positions are correct. The blue dot with the plus sign is Earth; the red dot with polar caps and the traditional sword-and-shield symbol is Mars. Also shown are the planets' orbits. Blue parts of the orbits are even with or slightly north of the plane of Earth's orbit, green parts slightly south of it.
Notice that the orbit of Mars looks off-center. In the lower-right part of the picture it's almost twice as close to Earth's orbit as in the upper-left part. That's correct. This so-called eccentricity was first calculated by the astronomer Johannes Kepler in the early 1600's. (Notice that the orbit of Mercury is even more eccentric.)
On August 27, 2003, Earth and Mars happened to come near each other almost exactly at the point where their orbits are closest. The actual distance was roughly 35 million miles.
Times when Earth comes near Mars are often called "oppositions," because Mars appears in almost exactly the opposite direction in the sky from the Sun. However, because of the slight tilt of Mars's orbit compared to Earth's, the moment of closest approach may differ from the moment of opposition by as much as 8 1/2 days. So, for the present, I'll talk about "close approaches."
Now, let's go forward in time. The planets go around the sun counterclockwise as seen from this viewpoint. Earth takes about 365 days to go once around the sun; Mars takes almost twice as long, about 687 days. When we get to October 30, 2005, Earth has caught up with Mars again.
Earth has made two trips around the sun plus a little extra, Mars one trip plus a little extra. So the location of the meet-up has shifted slightly around the solar system, to a place where the orbits are not so close. Distance from Earth to Mars is about 43 million miles this time.
Twenty-six months later, on December 18, 2007, the planets meet again, but the place of the encounter has shifted more, and Earth and Mars are still farther apart:
We can continue in the same way, to January 27, 2010:
March 5, 2012:
April 14, 2014:
Notice that the place of the encounter is now moving around to the area where the orbits get closer again. The next two approaches will be closer.
May 30, 2016:
And July 31, 2018. This is the next unusually close approach between Earth and Mars, with Mars only about 3 percent more distant than in 2003:
So, approaches between Earth and Mars occur about every 26 months. If we continued with this exercise we'd see that exceptionally close approaches, down in that lower-right corner of our picture, occur about every 15 years. That lower-right part of the picture is where the Earth travels every year in July, August and September, so the exceptionally close approaches occur in those months.
But these time intervals are not exact. So some unusually close approaches are closer than others. The Mars e-mail mentions an exceptionally close approach in 2287. Here it is:
Wait another four and a half centuries, and we get closer still. Mr. Meeus calculates that the closest approach between the years zero AD and 3000AD will occur on September 8, 2729:
Earth to Mars distance will be about two tenths of one percent closer than in August 2003, about 3 percent closer than in 2018.
You can see Mars, with some effort, in August 2010 in the early evening sky. It appears faint and low in the west at nightfall, not far from the almost unbelievably bright planet Venus. At the Planetarium our star shows reproduce these arrangements so you can preview them conveniently in the Star Theater.
So that's some context for the curious Mars e-mail that circulates every August. In astronomy circles, this e-mail is sometimes called "The Mars Hoax," but I'd disagree. A hoax involves a deliberate attempt to deceive, and I don't think that's what is going on here. I think some well-meaning person who understood almost all the science involved wanted to share their excitement. Whoever it was had a hidden talent for what is now called viral marketing. As far as we can tell, no one promotes this e-mail. It goes around and around because people like it.