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Only two of four terrestrial planets have satellites:

The Moons of Mars


Phobos by Mariner 9
Phobos by Mariner 9
Deimos by Viking 2
Deimos by Viking 2
Among the nine planets in our Solar System, the four nearest the Sun are referred to as terrestrial. Earth is third from the Sun and Mars is fourth. Among the four terrestrial planets, only Earth and Mars have natural satellites. Phobos and Deimos were detected first by American astronomer Asaph Hall at the U.S. Naval Observatory in Washington, D.C. Peering through his 26-inch telescope, Hall spotted what he called a "Mars star" moving against the black background of space near the Red Planet. He scribbled in his log that Phobos was "faint and difficult to observe."

More than a century later, we know what Phobos looks like. Interplanetary probes have beamed photos of it to Earth. Deimos appears smooth in some photos, but is covered with craters. It is less than nine miles from end to end.

What is it? Phobos is the larger of Mars' two natural satellites, but is it actually a Martian moon? Or is it an asteroid captured by the gravity of Mars? Scientists have been asking about that since Hall discovered Phobos in 1877. Answers have been hard to come by, even after robots were sent to explore the tiny body.

Some scientists think Phobos is not a natural satellite, but an asteroid grasped by Martian gravity. If so, Phobos eventually could give visiting scientists a chance to analyze matter unchanged in 4.6 billion years ago since the infancy of the Solar System.

The former Soviet Union had hoped to clear up the moon question when it launched a pair of spacecraft, named Phobos 1 and Phobos 2, in 1988. Unfortunately, the first was lost when a ground controller sent the craft a wrong command that turned its electricity-generating solar panels away from the Sun. The second studied Phobos for two months before ground controllers lost contact with it.

MARS MOON PHOBOS IS HIP DEEP IN POWDER

PHOBOS AND DEIMOS IMAGES:     JPL     JPL MARS     GSFC     PHOTOJOURNAL     MSSS     ARIZONA STATE UNIV


Learn more:
Human Exploration of Mars:
There have been three stages of exploration so far
Sand dunes: Dust Storms: Air: Carbon Dioxide: Outflow Channels: Valley Networks: Rift Valley: Ice: Ice caps: Frost: Water: Artesian Water: Mars Weather: Mars Photo Galleries: Planet features: Canals: Rocks: Mountains: Dating and aging: Seasons:
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Exploring Mars
Mars Probes
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Mars Orbiter 2005
Mars Scout 2007
NASA MARS HISTORY:
Rover Spirit 2003
Rover Opportunity 2003
Express 2003
Odyssey 2001
Polar Lander 1999
Climate Orbiter 1998
Deep Space 2 1999
Global Surveyor 1996
Pathfinder Lander 1996
Rover Sojourner 1996
Pathfinder Mission 1996
Viking-1 Lander 1975
Viking-2 Orbiter 1975
Viking-1 Lander 1975
Viking-1 Orbiter 1975
Mariner 9 Orbiter 1971
Mars 3 Lander 1971
Mariner 4 Flyby 1964
Viking Mission 1975
Mars Meteorites - JPL
EXPLORATIONS PLANNED:
2003 & Beyond - Goddard
2005 & Beyond - JPL
Mars Exploration - JPL
Plans to Explore Planets


SOLAR SYSTEM:
Solar System - JPL
Welcome to the Planets - JPL
Planetary Photojournal - JPL
Mars - Athena - NASA Ames
Solar System Tour - BBC
Mars - New York Times
Windows...Universe - UMich
Mars - Apollo Society
Planetary Society
Mars Society
The Nine Planets
Planet Mars Company
Solar System - STO
Solar System Tour
Artist conception of Mars with water four billion years ago
NASA CONCEPTION OF MARS WITH
WATER FOUR BILLIONS YEAR AGO
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