Mars' Moons Unveiled: Hall's Historic 1877 Discovery

On August 11th in Science History, a significant event took place in 1877 when American astronomer Asaph Hall discovered the two moons of Mars, Phobos and Deimos, at the U.S. Naval Observatory in Washington, D.C.

Hall had been observing Mars for some time, hoping to find any satellites orbiting the red planet. He was using the observatory's 26-inch refracting telescope, which was one of the largest and most powerful telescopes of its time. On the night of August 11th, Hall noticed a faint object near Mars and realized that it was moving. He had discovered the first moon of Mars, which he later named Phobos, meaning "fear" in Greek.

Six days later, on August 17th, Hall discovered a second, even fainter moon orbiting Mars, which he named Deimos, meaning "dread" in Greek. Both names were inspired by the horses that pulled the chariot of Ares, the Greek god of war, which was fitting as Mars was named after the Roman god of war.

The discovery of Phobos and Deimos was a significant milestone in the field of astronomy. It provided further evidence that other planets in our solar system could have moons, just like Earth. The discovery also sparked interest in the study of Mars and its potential for harboring life.

Since their discovery, numerous missions have been sent to study Phobos and Deimos in greater detail. In 1969, NASA's Mariner 7 spacecraft provided the first close-up images of the moons. Later missions, such as the Soviet Phobos 2 probe and NASA's Mars Global Surveyor, have revealed that the moons are heavily cratered and have a very low density, suggesting that they may be composed largely of rock and ice.

One of the most intriguing findings about Phobos is that it orbits Mars at a distance of only about 9,000 kilometers, which is closer than any other known moon orbits its planet. This close proximity means that Phobos is slowly being torn apart by Mars' gravitational forces. Scientists predict that within the next 50 million years, Phobos will either collide with Mars or break apart and form a ring around the planet.

The discovery of Phobos and Deimos by Asaph Hall on August 11th, 1877, remains a remarkable achievement in the history of astronomy. It paved the way for future explorations of Mars and its moons, and continues to inspire scientists to this day as they seek to unravel the mysteries of our solar system.


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Episode Transcript

Speaker 1 (00:00):
Welcome to This Moment in Science History podcast on August eleven.
In science history, a significant event took place in eighteen
seventy seven when American astronomer Asath Hall discovered the two
moons of Mars, Phobos and Daimos, at the U. S.
Naval Observatory in Washington, d C. Hall had been observing

(00:21):
Mars for some time, hoping to find any satellites orbiting
the red planet. He was using the observatory's twenty six
inch refracting telescope, which was one of the largest and
most powerful telescopes of its time. On the night of
August eleven, Hall noticed a faint object near Mars and
realized that it was moving. He had discovered the first

(00:42):
moon of Mars, which he later named Phobos, meaning fear
in Greek. Six days later, on August seventeen, Hall discovered
a second, even fainter moon orbiting Mars, which he named Demos,
meaning dread in Greek. Both names were inspired by the
horses that pulled the chariot of Ares, the Greek god
of war, which was fitting as Mars was named after

(01:03):
the Roman god of war. The discovery of Phobos and
Demos was a significant milestone in the field of astronomy.
It provided further evidence that other planets in our Solar
System could have moons just like Earth. The discovery also
sparked interest in the study of Mars and its potential
for harboring life. Since their discovery, numerous missions have been

(01:24):
sent to study Phobos and Damos in greater detail. In
nineteen sixty nine, NASA's marin Or seven spacecraft provided the
first close up images of the moons. Later missions, such
as the Soviet Phobos two probe and NASA's Mars Global
Surveyor have revealed that the moons are heavily cratered and
have a very low density, suggesting that they may be
composed largely of rock and ice. One of the most

(01:48):
intriguing findings about Phobos is that it orbits Mars at
a distance of only about nine thousand kilometers, which is
closer than any other known moon orbits its planet. This
close proximity means that Phobos is slowly being torn apart
by Mars's gravitational forces. Scientists predict that within the next
fifty million years, Phobos will either collide with Mars or

(02:10):
break apart and form a ring around the planet. The
discovery of Phobos and Demos by Assov Hall on August eleventh,
eighteen seventy seven, remains a remarkable achievement in the history
of astronomy. It paved the way for future explorations of
Mars and its moons, and continues to inspire scientists to
this day as they seek to unravel the mysteries of
our Solar system. And that wraps it up for another episode.

(02:34):
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for listening.