Page 29 - (DK) Eyewitness - Mars

Discoverer of the moons

In August 1877, astronomer Asaph Hall studied the
Red Planet night after night, searching for moons.
Hall used the most powerful telescope of the time, but
bad weather often blocked his view. When the weary
Hall wanted to give up, his wife, Angeline Stickney
Hall, urged him to keep trying. The weather finally
cleared, and Hall was rewarded when he discovered Arago Medal of the French
Deimos, and then Phobos. Academie des Sciences
HONORS FOR DISCOVERY
A LOVE OF GEOMETRY Hall received several medals for his
Asaph Hall was born in Goshen, work, including France’s Legion of
Connecticut, in 1829. His passion for Honor, the Arago Medal of the French
geometry and algebra won him a place Academie des Sciences, and the Gold
at the Harvard Observatory, Medal of the Royal Astronomical Society
Cambridge, Massachusetts, then at of Great Britain. In 1998, Hall’s family
the U.S. Naval Observatory. Legion of Honor donated his medals to the U.S. Naval
Observatory in Washington, D.C.
Dimensions and orbits The satellites of Mars

Phobos is about 16 miles (27 km) on its long axis, while
Deimos is 10 miles (16 km) long. Phobos orbits at more than This table shows the radius of each moon’s orbit around Mars. It
also shows the time each orbit takes, measured in Martian days,
3,700 miles (6,100 km) above Mars. Deimos orbits at more and the orbit speed. The moons’ dimensions (length, width, and
than twice that distance away. Phobos orbits about three height) and surface area are also listed.
times a Martian day, while Deimos orbits once every 1.26
days. Scientists believe Phobos is slowly dropping and in PROPERTY DEIMOS PHOBOS
50 million years will crash onto Mars. Orbital radius 14,577 mi 5,827 mi
23,459 km 9,378 km
MOONSHADOW Orbital period 30 hrs 18 min 7 hrs 39 min
The shadow of Phobos was captured Mean orbital velocity .87 mi/sec 1.3 mi/sec
by the Mars Orbiter Camera in 1999. 1.4 km/sec 2.1 km/sec
The shadow is caused by Phobos’s
passing between Mars and the Sun, Dimensions (miles) 10 x 7.5 x 6 16.1 x 13.7 x 11.5
which often happens because the (kilometers) 16 x 12.5 x 10 26.8 x 22.8 x 19
moon orbits Mars about every eight Area 250 sq mi 625 sq mi
hours. This image covers an area 400 sq km 1,000 sq km
155 miles (250 km) across.

Theoretical orbit of
Deimos man-made satellite

MARTIAN LUNAR SYSTEM
In this chart, the orbits of the outer moon,
Deimos, and inner moon, Phobos, bracket a
Mars third orbit. This additional orbit marks the
course of a man-made satellite that would
orbit at the same speed as Mars revolves on
its axis. Seen from Mars, this satellite would
always seem to stay in the same place.

Phobos

ESCAPE VELOCITY
Future astronauts could jump
off Deimos into space. By just
leaping upwards, they would
reach an “escape velocity”—the
speed needed to “escape” a
gravitational field—of 18.7 feet
per second (5.7 m/sec). Larger
Phobos with its stronger gravity
requires a leap of 33.8 feet
per second (10.3 m/sec).

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