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Electives

On Aug. 28, 2003, Mars will be at "opposition," the moment when the Sun, Earth and Mars will form a straight line; since we are closer to the Sun than Mars, this is also when we are overtaking Mars in our respective orbits .

Mars comes to opposition about every 26 months. But because of the elliptical orbits of Earth and Mars, not all oppositions are created equal. The 2003 opposition will be superior to all the others because Mars will be very near to its closest point to the Sun, called perihelion, when it arrives at opposition.

Such "perihelic oppositions" of Mars are in themselves, rather infrequent, occurring about every 15 to 17 years. The most recent one took place in September 1988, when Mars passed to within 36.5 million miles (58.7 million kilometers) of Earth.

Though Mars' opposition comes comes on Aug. 28, it will be closest to Earth on Aug. 27.

At the close approach, the Red Planet will be brighter than Jupiter and all the stars in the night sky, outshone only by Venus and the Moon

The result, in late August, will be a proximity that hasn't occurred for 73,000 years, according to a calculation made by Jean Meeus and first reported by SPACE.com in November. Interestingly, even closer passes are in store for our descendents.

How it works

The orbital dalliance is complex, but it's not so hard to envision when broken down into parts and sprinkled with sports analogies. We turned to Myles Standish, an expert on planetary orbits at NASA's Jet Propulsion Laboratory, for just that.

The main thing that alters the distance between the two planets is the fact that Earth takes a path closer to the Sun. Physics dictates that inner planets must zip around more quickly than outer planets. Earth requires 365 days to go around the Sun once. Mars needs 687 Earth-days to fill up a calendar.

Right now, Earth is catching up with Mars each day, like a race car driver heading into Turn 1 on the inside, preparing to lap a sputtering competitor for the umpteenth time. [Animation]

When the pass occurs, Earth and Mars will be on the same side of the Sun, as seen from above, all three objects lined up in a row. This happens about every 26 months. When this configuration occurs, astronomers say Mars is at opposition.

But the separations during these oppositions vary wildly. On Aug. 27 this year, Mars will be 34.65 million miles (55.76 million kilometers) away. The last opposition, in 2001, involved a separation of more than 41 million miles (67 million kilometers). In 1995, the distance between the two worlds was nearly double what it will be later this year.

Part of what's behind the seemingly erratic behavior is that a number of things have to line up at the same time.

Lining up

The measurements involved in the close approaches every 26 months vary because during a year, the Earth's distance from the Sun varies from its average, greater and smaller, by nearly 2 percent, and Mars' distance varies from its average by more than 9 percent, greater and smaller.

The trick is to get the furthest part of Earth's orbit from the Sun (its aphelion) to line up with the closest part of Mars' orbit to the Sun (its perihelion).

These orbital points slowly change their directions in space over thousands of years. They are presently evolving toward alignment, Standish says. So, when they do get nearly aligned and when Mars and Earth pass near the key points at the same time, then a truly close approach occurs.

Standish suggests envisioning the orbits as a bit like one football inside another. Both are slowly revolving end over end at different rates, constantly changing the relationship between the two. All the while, the planets are travelling along their respective football shapes with comparative speed.

The ultimate differences in proximity due to the changes that occur over millennia are not great, however.

The difference

Prior to the 1988 close pass, the two planets were even closer in 1971, just 34.9 million miles (56.2 million kilometers) apart. The 2003 approach is less than 1 percent closer than the one in 1971, Standish points out.

"So it's not like you're going to see something gigantic in the sky," he said. "It’s not like Mars is going to look like the Moon or anything."

Mars will appear strikingly brilliant, however. It will be about as bright as Jupiter ever gets. It will shine like a beacon in its characteristic red or orange, in stark contrast to most of the other planets and stars, which exhibit little color.

Officially, Mars will reach magnitude minus-2.9 on a scale used by astronomers to denote brightness. Lower numbers indicate brighter objects, and negative numbers are reserved for the very brightest.

The Red Planet will present a large enough disk for backyard astronomers with good-sized telescopes to discern some of the planet's features, such as the polar ice cap, dark surface features and perhaps even storm clouds.

The big picture

Professional astronomers won't pay the event much attention. Mars is so well studied from probes orbiting the planet and even some that have landed, that there is little scientists still need to know that can be examined from Earth any better now than last year or next year.

One possible exception involves radar studies. Radar works best at close range. But Standish said there probably wouldn't be much if any extra effort in this area, either, because radar has gotten so sophisticated that it works even on targets far away.

One might think this would be a great time to launch a spacecraft to Mars, to take advantage of a shorter travel plan. But that business has to do with another layer of complexity that renders the extra-close approach moot. Space probes are actually launched on trajectories very similar to Earth's orbit, by necessity. They travel arcing paths that only gradually make their way outward, intersecting Mars about eight months after launch.

Quarterbacks know all about this (at least those who don't throw a lot of interceptions to slow defensive backs). You have to lead the target, because it takes time for the ball to get there.

"Imagine it as a couple of race cars going around a track," Standish suggests. "It's a very wide track, and one of them is on the inside and one is on the outside. If you're going to throw a ball from the inside to the outside, you don't throw it to where the guy is now, you throw it ahead and it intercepts him as he's going along."

He said the slightly reduced distance of this opposition is no advantage compared to the normal launch windows, which do, however, take advantage of the 26-month cycle for Earth and Mars being on the same side of the Sun.

And then, even more is to happen...

There are even closer approaches on the horizon because, to continue Standish's analogy, the concentric footballs are not yet fully aligned.

"Earth's aphelion and Mars' perihelion are still not in the same direction, but they're getting closer during this present era," he said. "The alignment motion is so slow -- thousands of years -- that the bodies will pass through the right spots a number of times [during their regular orbits] while the alignment is quite close.

A handful of even closer oppositions, seven to be exact, will occur between the August 2003 event and the year 3000, according to new calculations by Standish recently provided to SPACE.com. The next one that will be closer than this year's occurs in 2287.

The closest of all comes in 2729, when Mars will be about 34,580,000 miles (55,651,000 kilometers) from Earth