Asaph Hall III was born in Goshen, Connecticut in 1820. His paternal grandfather had served as an officer in George Washington’s Continental Army during the American Revolution.
However, the younger Hall was forced into an early career decision after his father passed away at age 13. He became a carpenter’s apprentice at sixteen years of age and eventually went to college to study mathematics.
There, he soon developed a keen aptitude for predicting the orbits of the planets. He took a job at the Harvard Observatory in 1856, and married the same year. He became assistant astronomer at the U.S. Naval Observatory in 1862, and became a professor the next year.
Neptune, the eighth planet, had been discovered only ten years previous and as such, when Hall was given charge of the Naval Obervatory’s telescope, he naturally hoped to find something in the heavens that had not yet been seen by human eyes.
In August 1877 Hall did it not once, but twice. After encouragement by his wife to search when Mars made its closest approach to Earth, Hall discovered one of its moons — soon to be known as Deimos — on this date. Six days later, he discovered the other Martian moon, Phobos.
He later wrote that simple patience did the trick.
“I repeated the examination in the early part of the night of 11th [August 1877], and again found nothing, but trying again some hours later I found a faint object on the following side and a little north of the planet. I had barely time to secure an observation of its position when fog from the River stopped the work. This was at half past two o’clock on the night of the 11th. Cloudy weather intervened for several days.
“On 15 August the weather looking more promising, I slept at the Observatory. The sky cleared off with a thunderstorm at 11 o’clock and the search was resumed. The atmosphere however was in a very bad condition and Mars was so blazing and unsteady that nothing could be seen of the object, which we now know was at that time so near the planet as to be invisible.
“On 16 August the object was found again on the following side of the planet, and the observations of that night showed that it was moving with the planet, and if a satellite, was near one of its elongations. Until this time I had said nothing to anyone at the Observatory of my search for a satellite of Mars, but on leaving the observatory after these observations of the 16th, at about three o’clock in the morning, I told my assistant, George Anderson, to whom I had shown the object, that I thought I had discovered a satellite of Mars. I told him also to keep quiet as I did not wish anything said until the matter was beyond doubt. He said nothing, but the thing was too good to keep and I let it out myself. On 17 August between one and two o’clock, while I was reducing my observations, Professor Newcomb came into my room to eat his lunch and I showed him my measures of the faint object near Mars which proved that it was moving with the planet.
“On 17 August while waiting and watching for the outer moon, the inner one was discovered. The observations of the 17th and 18th put beyond doubt the character of these objects and the discovery was publicly announced by Admiral Rodgers.”
Hall also discovered a white spot on Saturn which allowed him to track the gas giant’s orbital period, but he became more famous for discovering the moons of Mars.
Once discovered, the moons needed names. Of course, Mars was known as the Roman god of war, but in Greek his name was Ares. In classic work The Iliad, Ares is able to summon both fear and dread to his service, named Phobos and Deimos respectively.
Deimos may have started life in the solar system as an asteroid — it’s about 1/12 the width of Earth’s moon and as such appears as little more than star-sized in its near-circular orbit of Mars, and is as bright from Mars as Venus appears to be from Earth.
It orbits Mars at a speed quite close to the planet’s rotation, so it appears in the sky for about two and a half Earth days from rise to set. Its cousin, Phobos, orbits much faster than the planet rotates so it appears to have retrograde motion in the sky — rising in the west and setting in the east — while Deimos does the opposite.
No missions have ever reached the Martian moons. The Russians tried three times — Phobos 1 was lost on the way to Mars, Phobos 2 failed shortly before landing, and the Phobos-Grunt mission never left low Earth orbit.
However, there are numerous proposals on the table to launch to Phobos and Deimos. The closest to fruition may be the Japanese government’s MMX proposal, which aims to launch to Mars in 2024 and return with samples from the soil of Phobos.
It’s also speculated that the moons may eventually be usable as staging areas for manned missions to Mars. Spacecraft could load up Martian soil in extremely low gravity, and carry it to Mars, releasing the soil as a thickener of the atmosphere in front of a descending spacecraft.
Happy Sunday and enjoy today’s open thread!