Communicating with spacecraft at Mars always involves a wait. Depending on how far apart the planets are, it can take up to 21 minutes to get a signal from Earth to the Red Planet, resulting in a round-trip time of more than 40 minutes. The lag can be agonizing for an engineer trying to steer a surface probe or debug a software problem. On Wednesday, when Mars is closer to Earth than ever in human history, the one-way travel time of light and radio signals will be just 3 minutes and 6 seconds.
ASTRONOMERS LOVE TO measure cosmic distances in light-years. In this case, you can think of the distance between the two planets as being 186 light-seconds.
Earthlings have an historic chance to see Mars at its brightest, and to see the Red Planet at any given instant as it existed just 186 seconds earlier in time.
Just look in the east-southeast sky on any clear evening soon after darkness falls and you’ll see a fiery yellow-orange “star” blazing brilliantly.
Named for the Roman god of war, Mars is often called the Red Planet. But anyone who takes even a casual glance will see that it’s more like a yellowish orange — the color of a dry desert under a high sun. Mars is much like a desert, in fact, dry and covered in sandlike dust. So a desert under the sun is exactly what you’re looking at, since Mars is visible because of the sunlight it reflects our way.
From now into September, Mars shines with a topaz glow that is brighter than any other object in its region of the sky, except on those nights when the moon is in the general vicinity.
As with any unusually close approach of Mars to Earth, this one makes Mars appear exceptionally brilliant and indeed, from now into the early fall, Mars will easily outshine Sirius (the brightest of all stars) and even Jupiter (the planet normally second in brightness only to Venus).
Skywatchers should note, however, that to the naked eye Mars still appears as a point of light, not anything near as impressive as the moon. Telescopes are required to glimpse views of any features on the surface of Mars.
THE SETUP, BY THE NUMBERS
On Thursday, Mars will reach “opposition,” the moment when the sun, Earth and Mars form a straight line in space, with Earth and Mars on the same side of the all-important star.
When a planet reaches opposition, it lies exactly opposite the sun in our sky: It rises at sunset, reaches its highest point in the sky at midnight, and sets at sunrise. To imagine this from above, envision the solar system as a giant racetrack. Earth is moving in the inner lane. Mars comes to opposition when the faster-moving Earth overtakes and passes the outer planet.
Mars comes to opposition about every 26 months. But because Earth and Mars both have elliptical orbits — not quite circular — no two oppositions are created equal.
This year’s opposition is superior to all the others in nearly 60,000 years because Mars will be very near its closest point to the sun (perihelion) when coming to opposition. Such “perihelic oppositions” of Mars are rather infrequent, occurring about every 15 to 17 years.
Recent perihelic oppositions saw Mars approach Earth to within 35.1 million miles in September 1956, 34.9 million miles in August 1971 and 36.5 million miles in September 1988.
Because Earth and Mars follow elliptical orbits around the sun, Mars’ closest approach to Earth usually occurs several days before or after opposition. This year Mars arrives at perihelion just a scant 42 hours after its opposition.
So on Wednesday, according to astronomer Myles Standish at NASA’s Jet Propulsion Laboratory, Mars will come within 34,646,418 miles of Earth at 5:51 a.m. ET, which is just about as close as it can possibly come.
At that moment, it will then take a light beam 3 minutes and 6 seconds to cross the interplanetary gulf between Earth and Mars.
Things won’t appear much different in the nights and mornings immediately surrounding that historic moment, so great observing can be done without worrying about the exact moment.
Opposition comes on the following day. The next opposition to bring Mars this close — closer, actually — will not occur until Aug. 29, 2287. The distance will be 34,603,170 miles.
THE SHOW GOES ON
Even after the main event this year, Mars’ inevitable fade-down will initially be very slow and gradual.
In fact, it will still continue to shine at its absolute brightest magnitude of -2.9 through Sept. 2. (Astronomers use negative numbers on their scale of magnitude to denote the brightest objects.) Mars will still outshine Jupiter through Oct. 8. And it will continue to rival Sirius, the brightest of all stars, until Oct. 28.
The next opposition of Mars will come in early November 2005, but because Mars will then be more than 8 million miles farther from Earth than it is this month, it will appear to shine with just two-thirds of its peak 2003 radiance.
Interestingly, there is a long-term cycle of 79 years where the circumstances of any particular Mars opposition will replicate almost exactly.
On Aug. 22, 1924, for instance, Mars made nearly as close an approach to Earth as this year. In fact, at its closest in 1924 it was just 11,764 miles farther from Earth than it will be on Wednesday. And 79 years from now, on Aug. 30, 2082, Mars will again make another exceptionally close approach to Earth, though again falling just short of matching this year’s proximity, by 78,487 miles.
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York. This is an updated version of an article that originally appeared Aug. 8.
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