Mars 2003: The View from Earth
Every twenty-six months, the Earth passes between Mars and the sun. Astronomers call this moment "opposition" because, as seen from Earth, Mars lies opposite the sun in the sky. This is the time when Mars lies closest to us, shines at its brightest and appears largest in a telescope.
We have been racing toward the Red Planet all year. Our closest approach occurs on August 27, at 9:51 Universal Time (UT), when we pass just over 55.7 million kilometers or 34.6 million miles from Mars. At that moment, Mars will appear five times larger and fifty-eight times brighter than it did on January 1. The actual moment of opposition occurs thirty-two hours later. Mars will shine all night long, gleaming like a brilliant ember set among the faint stars of the constellation Aquarius.
By any measure, 2003 will be the best year for Mars observing this decade. Because Mars follows a more eccentric orbit around the sun than the Earth, all Mars oppositions are not created equal. Those that occur when Mars lies nearest the sun, at perihelion, bring it almost twice as close to us as oppositions that occur when Mars lies farthest from the sun, or at aphelion.
The 2005, 2007 and 2010 oppositions take Mars to ever farther parts of its orbit. (More information about upcoming Mars oppositions, including sky maps, can be found in Celestial Delights.) The 2003 opposition, which occurs three days before perihelion, brings the Red Planet 1.8 times closer to us than the 2012 opposition, which happens two weeks after aphelion.
This year's opposition is exceptional in another sense, too. Last year, Belgian astronomer Jean Meeus published the results of his study of Mars close approaches in his book More Mathematical Astronomy Morsels (Willmann-Bell, Inc., 2002). Using analytical theories developed at the Bureau des Longitudes in France, Meeus showed that Mars will be closer to us on August 27 than at any time in the past 73,000 years.
Looking for a more precise value required moving away from analytical theories, which by necessity include approximations, and into the more accurate -- but more exacting -- numerical integration of the equations of motion. In the spring of 2002, Meeus asked Aldo Vitagliano of the Universitá di Napoli Federico II in Naples, Italy, to check his result using numerical integration.
Vitagliano found that the last Mars opposition closer than this year's occurred on September 12, 57,617 B.C., or 59,619 years ago.
"Vitagliano tested his result by experimenting with the most uncertain factors," says Myles Standish, an expert in solar system dynamics at the Jet Propulsion Laboratory (JPL) in Pasadena, California. For example, he changed the initial conditions and constants of his calculation by starting with positions calculated by different versions of the JPL ephemeris generator (DE200 and DE405/6).
But the greatest source of uncertainty in the position of Mars over time turns out to be our knowledge of asteroid masses. "As small as they might be, the perturbations due to the asteroids must have some long-term effect on the orbits of planets, especially Mars," Vitagliano told me.
So he performed his computation without taking into account the effects of the three largest asteroids: Ceres, Pallas, and Vesta. He found essentially the same date of closest approach and the same value for the minimum distance (the difference was just one hour in the date and 480 kilometers in the distance). "It is a nice surprise that the effect is so small that the general properties of the orbit are essentially unaffected after 60,000 years," Vitagliano says.
Looking backward in time, it's possible that a couple of the larger asteroids could have approached one another close enough that their gravitational interaction caused a substantial orbital change. That would make their subsequent motions quite uncertain. "If one of them then happened to approach Mars, it could alter Mars' orbit to a measurable degree," Standish says. "This, however, is reasonably unlikely, so we can have pretty good faith in Vitagliano's results."
Within the limits of our ability to account for its whereabouts, Mars will be closer to Earth on August 27 than it has been in nearly 60,000 years. That's an interval ten times longer than all of recorded history. The last time Mars came this close, Neanderthals flourished and humans had not yet occupied Australia.
To be honest, though, the event has greater intellectual than observational significance. The Red Planet passes only 1.9 million miles (3.0 million kilometers) closer than it did in 1988, the last year in which a Mars opposition coincided with the planet's perihelion. And the 1924 opposition brought Earth and Mars to within 12,000 miles (20,000 kilometers) of their minimum distance this year.
Still, a record is a record.
The poster and animation linked here illustrate this year's dramatic martian changes. In each, a sequence of Mars maps shows the planet's phase and angular size every fourteen days throughout the year. The poster shows martian surface features as they appear at 0 UT on each date. See the table below for more information about Mars on the dates illustrated.
Because the rotational rates of Mars and Earth are almost the same, we see the same martian features for several successive nights. They rotate about 224 degrees in the fourteen days between poster images. This means that adjacent Mars maps show nearly opposite hemispheres. We can watch this effect with two well known features: the four volcanoes of the Tharsis region in the north and the Hellas impact basin in the south. For example, the Mars image for August 13 shows the Tharsis volcanoes in the upper left. Viewing at the same time on August 27, we instead find a large oval area -- the Hellas basin -- turned our way.
The maps are based on the U.S. Geological Survey's digital Mars mosaic, which is derived from data gathered during the Viking missions of the 1970s. As a result, they better represent an astronaut's view of Mars than that seen through a telescope on Earth.
Mars in 2003
The Red Planet's changing aspect over the year. Values are computed for a geocentric observer. Each date corresponds to a Mars image on the poster or in the animation.
Date Mag. Percent Angular Distance Constellation