Asaph Hall was about to give up his frustrating search
for a Martian moon one August night in 1877, but his wife Angelina urged
him on. He discovered Deimos
the next night, and Phobos
six nights after that.
Ninety-four years later, NASA's Mariner 9
spacecraft got a much better look at the two moons from its orbit around Mars
The dominant feature on Phobos, it found, was a crater 10 km (6 miles) wide
- nearly half the width of the moon itself. It was given Angelina's maiden
Hall named the moons for the mythological sons of
Ares, the Greek counterpart of the Roman god, Mars. Phobos means fear or
panic (think "phobia"), and Deimos means flight (as in running after an overwhelming
defeat). Fitting names for the sons of a war god.
Mars' moons are among the smallest in the solar system. Phobos is a bit
larger than Deimos, and orbits only 6,000 km (3,700 miles) above the Martian
surface. No known moon orbits closer to its planet. It whips around Mars
three times a day, while the more distant Deimos takes 30 hours for each
orbit. Phobos is gradually spiraling inward, drawing about 1.8 meters closer
to the planet each century. Within 50 million years, it will either crash
into Mars or break up and form a ring around the planet.
To someone standing on the Mars-facing side of Phobos, Mars would a large
part of the sky. And people may one day do just that. Scientists have discussed
the possibility of using one of the Martian moons as a base from which astronauts
could observe the Red Planet and launch robots to its surface, while shielded
by miles of rock from cosmic rays and solar radiation for nearly two-thirds
of every orbit.
Like Earth's Moon
Phobos and Deimos always present the same face to their planet. Both are
lumpy, heavily-cratered and covered in dust and loose rocks. They are among
the darker objects in the solar system. The moons appear to be made of carbon-rich
rock mixed with ice and may be captured asteroids
Phobos has only 1/1000th as much gravitational pull as Earth
. A 150-pound (68 kg) person would weigh two ounces (68 grams) there. Yet NASA's Mars Global Surveyor
has shown evidence of landslides, and of boulders and dust that fell back
down to the surface after being blasted off the moon by meteorites.