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NASA Science News for June 18, 2003

Approaching Mars

Earth and Mars are converging for a close encounter in August. The
red planet is already an appealing target for sky watchers. 

June 18, 2003: Count slowly: one one-thousand, two one-thousand,
three one-thousand.... You just got about 30 km closer to the planet
Mars. 

Earth and Mars are rapidly converging. On August 27, 2003--the date
of closest approach--the two worlds will be 56 million km apart.
That's a long way by Earth standards, but only a short distance on
the scale of the solar system. NASA, the European Space Agency and
Japan are all sending spacecraft to Mars this year. It's a good time
to go. 

Between now and August, Mars will brighten until it "blazes forth
against the dark background of space with a splendor that outshines
Sirius and rivals the giant Jupiter himself." Astronomer Percival
Lowell, who famously mapped the canals of Mars, wrote those words to
describe the planet during a similar close encounter in the 19th
century. 

Already Mars is eye-catching. You can see it this month in the
morning sky--bright, steady and remarkably red. Only Venus near the
sun is brighter. 

Amateur astronomers looking through backyard telescopes have reported
in recent days great views of Mars's south polar cap. Made of frozen
water and carbon dioxide ("dry ice"), it reflects sunlight well. "I
can see the polar ice vividly using my 8-inch telescope," says Ron
Wayman of Tampa, Florida. He's also spotted "some faint darker-shaded
areas on the surface." 

Such markings will become clearer in the weeks ahead. On June 1st
Mars was 12.5 arcseconds across and it glowed like a -1st magnitude
star. On August 27th it will be twice as wide (25 arcseconds) and six
times brighter (magnitude -2.9). 

Much has been made of the fact that the August 27th encounter with
Mars is the closest in some 60,000 years. Neanderthals were the last
to observe Mars so favorably placed. This is true. It's also a bit of
hype. Mars and Earth have been almost this close many times in recent
history. 

Some examples: Aug. 23, 1924; Aug. 18, 1845; Aug. 13, 1766. In each
case Mars and Earth were approximately 56 million km apart. 

Astronomers call these close encounters "perihelic oppositions."
Perihelic means Mars is near perihelion--its closest approach to the
sun. (The orbit of Mars, like that of all planets, is an ellipse, so
the distance between the sun and Mars varies.) Opposition means that
the sun, Earth and Mars are in a straight line with Earth in the
middle. Mars and the sun are on opposite sides of the sky. When Mars
is at opposition and at perihelion--at the same time--it is very
close to Earth. 

August 27th is indeed the best perihelic opposition since the days of
the Neanderthals, but it scarcely differs from other more recent
ones. That's fine because all perihelic oppositions of Mars are
spectacular. 

Mars is a morning planet now. You have to wake up early to see it.
Soon, though, it will be more conveniently placed. By mid-July Mars
will rise in the east around 11 p.m. local time. In late August it
will appear as soon as the sun sets. It won't be long before everyone
can see Mars at a civilized hour. 

We'll be telling more stories about Mars in the weeks ahead. This
one, though, is finished. Did you make it to the end?
Congratulations! You're now 2000 km closer to Mars. 

Credits & Contacts
Author: Dr. Tony Phillips 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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