Feature

Text Size

Earthlings Will Get a Better View of Mars
10.20.05
 
The armies of ancient Rome, before going into battle, trained outside the city walls on a field named for Mars. Processions honoring victorious military commanders began on the "Campus Martius" and ended inside the city. Mars was the god of war. In the arms race of the Roman Empire, Mars was the celebrated winner.

In the celestial race around the Sun, however, Earth vanquishes Mars. Like a chariot racing around an oval track, Earth has been passing Mars every two years and two months since long before the rise and fall of Rome. Each time it does this, the two planets come closer than at any other time during their orbits. The next close encounter will occur at 8:19 p.m. Pacific Daylight Saving Time (11:19 p.m. Eastern Daylight Saving Time) Oct. 29, 2005.

This image shows four artist's renderings arranged in a square. On the top are two side-by-side views of Mars, Earth, and the Sun against an inky black background of interplanetary space studded with pinpricks of light that represent stars. In the foreground, taking up the bottom third of both views, is the top portion of the sphere of Mars. Mars faces away from the viewer toward the sun and is mostly in shadow except for a white, light-reflecting, icy polar cap and reddish-brown circumference that reflects some sunlight. Toward the upper left-hand corner in both images is a white-hot sun spewing rays of yellow light in all directions. In the left-hand frame, Earth is a shadowy circle almost directly between Mars and the Sun. In the right-hand frame, Earth has moved farther to the right relative to Mars, and Earth's blue-and-white left half is visible where it reflects the light of the sun. The bottom pair of artist's renderings shows two views of a rover on Mars, with the camera on top of the rover's mast at the bottom front edge of the image and a flat, reddish, martian horizon beyond it. On the left-hand side, corresponding to the orbital artist's rendering above it, is a pink martian sky. On the right-hand side, corresponding to the orbital artist's rendering above it, is half of a whitish orb representing the Earth in the martian sky. Image to right: Earth Viewed from Mars
Though Earthlings will be able to get a good look at Mars during close approach, NASA's two rovers on Mars will not be able to see Earth. That's because at the moment, Earth is on the daytime side of Mars. As shown in the top half of this artist's rendering, when it's nighttime on Earth, it's daytime on Mars. After Earth passes Mars, the rovers will be able to see the sunlit side of Earth again just before dawn. The bottom half of this illustration shows what the rovers would see if they looked toward Earth. Image credit: NASA/JPL-Caltech
+ Click for larger image


Blazing in the Eastern Sky

Not all close encounters are equal. Some, like this one, are more spectacular than others because the distance between the two planets is always changing.

"The orbits of the planets are ellipses," explains Moriba Jah at NASA's Jet Propulsion Laboratory, a navigator on the Mars Reconnaissance Orbiter, a mission now en route to the red planet. "They're not perfect circles. Gravitational effects from the Sun and planets pull on the planets' orbits."

This photo shows the chest, shoulders, and face of a smiling, dark-skinned man with dark eyes, a pug nose, round, rosy cheeks, a hint of a moustache above plump lips, and a mop of black hair woven into dozens of tight braids. He is wearing a long-sleeved blue dress shirt, open at the collar, with vertical pinstripes of orange, white, and light blue. Image to left: Space Navigator Moriba Jah
Moriba Jah, a navigator at NASA's Jet Propulsion Laboratory, works full time getting spacecraft to their destinations. To do this, navigators need to know the positions of the planets and the Sun, as shown in this animation of the orbits of Earth and Mars from 2003 through 2008. Image credit: NASA/JPL-Caltech
+ Click for larger image


During October and November of 2005, Mars will look like a blazing, orange-yellow star in the night sky. Usually, Mars is less obvious. The Red Planet comes close enough for such exceptional viewing only once or twice every 15 or 17 years, says geologist Tim Parker, a member of the Mars Exploration Rover science team.

Not that Mars is close. Even during this encounter, 43 million miles (69 million kilometers) will separate Earth from Mars. That's an enormous distance. But it's only about one-third of the average 140 million miles (225 million kilometers) between the two. In 2003, Mars came closer to Earth than in the previous 60,000 years. That time, Mars was within 35 million miles (56 million kilometers) of Earth, within a day of reaching the closest point possible between the two planets.

"The very closest approach – the most that you can get – is when both planets are lined up at the time when Earth is at its farthest point from the Sun and Mars is at its closest point," says Jah.

A Bright but Indecisive Warrior

Mars is already fairly conspicuous in the eastern sky. On a clear night, if you gaze at the red planet through a small telescope, you may be able to see light and dark markings on the surface or perhaps even the ice cap at the south pole. Of course, the bigger the telescope, the more detail you can see. Another similar encounter will not occur until 2018.

Bathed in red, the color of blood, Mars reminded the ancients of an angry warrior whose behavior was erratic and unpredictable. As the planet moves across the night sky, it appears to make a hairpin turn and head in the opposite direction. A couple of months later, Mars makes another hairpin turn and resumes its earlier heading.

This apparent change in direction is an optical illusion. It's like the impression you get if you look out the window of a train just as the train on the next track pulls ahead. For a moment, it seems like you're going backward. (What's the matter with this train, anyway? Oh, wait a minute. The slower train only looks like it's moving backward relative to the motion of the faster train.) After they've passed, the two vehicles look like they're moving in the same direction again.

The same thing happens when Earth overtakes Mars. Other planets appear to change direction, too, but they're so much farther away that the effect is not as noticeable to the naked eye.

A Little Help from the Sun

Even though Mars doesn't always come close enough for astronomers to get a good look at it when Earth speeds past, navigators have an opportunity to launch a spacecraft to Mars every 26 months.

Unlike vehicles in science fiction movies, spaceships don't just take off in any direction. Escaping Earth's gravitational pull requires a tremendous amount of energy. Even then, without an extra push, a spacecraft would never reach the orbit of Mars.

This vertical image shows a tiny pinpoint of light in a sea of dark, pinkish-brown sky that represents the Earth as viewed from Mars. Image to right: Mars Rover Snapshot of Earth
NASA's Mars Exploration Rover Opportunity took this picture of Earth last May when Earth was orbiting the sun almost 90 degrees away from Mars just after sunset on Mars. Image credit: NASA/JPL-Caltech
+ Click for larger image


To get a spacecraft to Mars, navigators time the launch so the spacecraft reaches Mars when the red planet is on the opposite side of the Sun from Earth. This way, they take advantage of solar gravity, which causes a spacecraft to "fall" toward the sun, traveling in an arc around it. This motion follows physical laws described by Isaac Newton and Johannes Kepler in the 17th century.

"What you want to do is launch the spacecraft so it goes around the Sun to meet Mars," says Jah. "We give it the least amount of energy possible and let the Sun's gravity do as much as it can for us."

As a result, like a warrior in a Roman chariot race, the spacecraft comes up on Mars from behind. But instead of sailing past, it slows down and lands.

More at Mars Exploration Program .