Sunday, April 22, 2001
This summer is shaping up to be the summer of Mars.
The 2001 Mars Odyssey mission won't reach the red planet until October, but the work of the Mars Global Surveyor is continuing and, on June 13, Mars will be at opposition, putting it in prime viewing location for us eager watchers here on Earth.
When a planet is at opposition, it is directly opposite the sun with regards to Earth. It's easier to visualize this if you make a quick sketch. Draw two circles, one inside the other. Make a dot to represent the sun right in the middle. The circle closest to the sun is Earth's orbit, so make a dot somewhere on that circle to represent our planet. Connect the "dots" of the sun and Earth and extend this line through the outer circle, which is Mars' orbit. Place Mars where the line crosses the circle and voila! You have opposition.
Readily seen: You can see from your drawing that the nighttime Earth -- the side facing away from the sun -- is facing directly toward Mars, and that Mars is being fully lit by the sun. This is a great time to observe our neighbor planet because it's "up" all night.
Mars reaches opposition about once every 780 days. To picture this, pretend that your circles are racetracks and the Earth and Mars are tiny speedsters. Earth has the inside track and takes about 365 days to circle the track once. Mars has the outside track and takes about 687 days to make one circle.
Traveling at the same speed, then, Earth will "lap" Mars almost two times before they line up again.
You'll notice that the ambitious schedule of Mars missions announced by NASA calls for a launch about once every two years. Now that schedule makes sense -- the launches are timed to take advantage of the close proximity of Mars to Earth when the red planet is at opposition.
Because Mars' orbit is not a perfect circle, sometimes it is closer to the sun at opposition than other times. This is called a "favorable opposition," and Mars watchers look forward to this times.
Good viewing: The opposition this summer will be fairly close, and Mars' imperfect circle will bring it even closer in 2005, when Mars will reach perihelion -- its closest approach to the sun.
Because we've been throwing astronomy terms around, it doesn't seem right to give you just half of them. For every opposition there's a conjunction -- when the planet is directly opposite the Earth on the other side of the sun. To picture this, go back to your orbit sketch. Take the line that connects the dots of the Earth, sun, and Mars, and extend it out to the other side. Where this line crosses the orbit of Mars is the planet's conjunction point.
It's easy to visualize why this is a terrible time to observe Mars. And, if perihelion is the closest approach to the sun, then there must be a word for the point when it is farthest away. It's aphelion. (I remember the difference between the two by thinking aphelion/apart.)
The opposition of Mars in 1995 took place when the planet was at aphelion, making the disc of Mars through our telescopes much smaller than usual.
The 2001 Mars Odyssey was launched to take advantage of this summer's opposition. Notice that the mission's April 7 launch was several months before opposition and its arrival in December is several months after.
Why not launch on June 13, the date of opposition?
Far apart: When the tiny race car of the Earth "laps" the race car of Mars, they're not right next to each other. Even though drivers on a terrestrial racetrack may be close enough to touch each other when they pass, the planets speeding around the solar system's track are millions of miles away from each other.
Earth's average distance from the sun is 93 million miles, which is also called one astronomical unit. Mars' average distance is 1.524 a.u., which is about 142 millions miles away. Subtract the two, and even at opposition the two planets are 49 million miles away. That's a mighty long reach.
To send a spacecraft from Earth to Mars, it makes sense to take advantage of the "slingshot" effect that Earth provides. The mathematicians who deal with orbital mechanics and trajectories figure in the thrust provided by Earth's rotation and its orbit and launch craft so the Earth gives them an extra "push" on their way -- like releasing a stone from a slingshot.
Because of the large distances involved, even if a stone was flung from a slingshot at thousands of miles an hour when the planets were directly aligned, Mars would be long gone by the time the stone reached the spot where the planet was when it was launched.
Figuring everything out, mathematicians need to know the speed of the spacecraft to send a probe on the shortest path possible between Earth and Mars, and this means launching it to a spot where Mars will be when the spacecraft gets there.