Phobos and Deimos

They have likewise discovered two lesser stars, or satellites, which revolve around Mars, whereof the innermost is distant from the centre of the primary exactly three of his diameters, and the outermost five: the former revolves in the space of ten hours, and the latter in twenty-one and a half.”

So wrote Jonathon Swift in Gulliver’s Travels when Gulliver travels on from Lilliput to floating island of Laputa, a land inhabited by mathematicians and astronomers. Swift was writing in 1726, a century and a half before the two small Martian moons were actually discovered.

Swift’s description is surprisingly accurate. The innermost moon, Phobos, orbits at a mean distance of 9,376 km (2.76 Mars radii) from the Martian centre; the outermost, Deimos, orbits at a mean distance of 23,463 km (6.92 Mars radii) from the Martian centre. The orbital period of Phobos is 7 hrs 39 mins; that of Deimos is 30 hrs 18 min.

Inevitably, there has been speculation that Swift learned about the moons from visiting Martians. In fact, there is nothing particularly mysterious about the ‘discovery’. At the time, Jupiter was known to have four moons; Earth has one, and Mars could therefore have two. Any Martian moons had to be small and close to the planet, or they would already have been observed. Swift would have used Kepler’s laws of planetary motion to calculate the orbital periods. Voltaire, writing in 1752, also mentions two Martian moons. It is presumed that he was influenced by Swift.

The actual discovery came in August 1877. Asaph Hall was an astronomer at the United States National Observatory in Washington, DC. In 1875, he was put in charge of the Observatory’s 26-inch (66 cm) refracting telescope, then the largest refractor in the world (it would be surpassed by the 28-inch refractor at the Royal Greenwich Observatory in 1893). In 1877, Mars made a close approach to Earth, and Hall’s wife, mathematician Angeline Stickney, encouraged him to look for Martian moons. Hall himself had believed that the chance of finding any moons was so small that without Angeline’s encouragement he might have given up.

On 12 August, Hall sighted Deimos, but soon lost it due to fog rising from the Potomac River. Not until the 17th were weather condition again favourable, and he recovered Deimos on the other side of Mars to where he had first seen it. On the 18th, while waiting for Deimos to come into view, he found Phobos. Further observations confirmed the existence of the two satellites, and the discovery was  announced by the USNO Superintendent, Admiral John Rogers the next day.

Hall named the moons Phobos (fear) and Deimos (terror) at the suggestion of Henry Madan, Science Master of Eton. Madan was inspired by Book XV of Homer’s Iliad in which Ares summons Fear and Fright.

Phobos has an apparent magnitude of +11.80 and Deimos +12.45, within the range of a good amateur telescope of 25 cm (10 inch) or more.

Although by no means the smallest moons in the Solar System, Phobos and Deimos are tiny. Phobos measures 27 x 22 x 18 km (17 x 14 x 11 miles) mean diameter 22.2 km (13.8 miles) and its mass is 1.08×1016 kg, and Deimos is 15 x 12 x 11 km (9 x 7.5 x 7 miles) mean diameter 12.6 km (7.8 miles) and a mass of 2.0×1015 kg.  The surface gravity of Phobos is 0.0057 ms-2 or 5.8 x 10-4 times that of Earth and the escape velocity is 11.39 ms-1 or 41 km/hr (25 mph); for Deimos the surface gravity is 0.0030 ms-2 or 3.0 x 10-4 times that of Earth and the escape velocity is 5.56 ms-1 or 20 km/hr (12.5 mph). A high jump athlete could just about jump into space from Deimos, though not from Phobos.

Phobos orbits just  6,000 km (3,700 miles) above the Martian surface, closer to its primary than any other Solar System body, and it is only slightly further from Mars than London is from New York. It is so close to Mars that it is not visible south of 70.4°S or north of 70.4°N. The orbital period is far shorter than the Martian day of 24 hrs 37 mins, so as seen from the surface of Mars it rises in the west, moves across the sky in 4 hours and 15 minutes, and sets in the east.

The orbit of Phobos is decaying at a rate of 1.8 cm per year, meaning that it will eventually collide with Mars or be pulled apart by tidal forces. This could happen in 30 to 50 million years from now. In 1958, the Russian astrophysicist Iosif Samuilovich Shklovsky suggested that based on the braking effect of the Martian upper atmosphere and the observed rate of orbital decay, Phobos would have to be hollow – possibly a sphere with a diameter of 16 km (10 miles) but a thickness of only 6 cm (2.5 inch). The suggestion that Phobos was an alien space station cropped up in the science fiction of the time, for example Mission to the Heart Stars, by James Blish. Shklovsky was assuming a decay rate of 5 cm per years, which was later shown to be an overestimate. In fact, purely tidal effects can account for the orbital decay; because Phobos is orbiting faster than Mars rotates, these effects are pulling it down rather than pushing it further away, as is the case for Deimos. Both moons are tidally locked, keeping the same face to Mars at all times.

Phobos is heavily cratered. The largest crater, the 9 km (5.6 mile) diameter Stickney, is named for Asaph Hall’s wife Angeline Stickney. The crater takes up a substantial portion of the surface area of Phobos, and the impact that created it must have nearly shattered the moon. Hall has had to make do with a much smaller crater. Two other features, Laputa Regio and Lagado Planitia are named after places in Gulliver’s Travels. The surface also bears many grooves and streaks, typically less than 30 meters (98 ft) deep, 100 to 200 meters (330 to 660 ft) wide, and up to 20 km (12 miles) in length. The grooves were once thought to have been caused by the impact that formed Stickney, but they appear to be of different ages. One possibility is that they are ‘stretch marks’ caused by the tidal deformation of Phobos, but these are too weak to deform a solid body. The suggestion, therefore, is that Phobos is a ‘rubble pile’ surrounded by a layer of powdery regolith (loose material) about 100 m (330 ft) deep. If so, it will break up when it falls to within a distance of 2.1 Mars radii (6,800 km; 4,225 miles) of the centre at which point its feeble gravity will be overwhelmed by that of Mars. At all events, the density of Phobos is too low for it to be composed of solid rock.

Deimos is less heavily cratered than Phobos. Only two features have been given names: the craters Swift and Voltaire.

Phobos and Deimos both appear to be composed of C-type rock, similar to blackish carbonaceous chondrite asteroids. The traditional view is that they are captured asteroids, but the low eccentricity and inclination of their orbits argues against this. One possibility is that they were formed from ejecta produced a large asteroid collided with Mars.

Very little was known about the physical condition of either satellite prior to the space age. The first photographs were taken by the Mariner 7 fly-by probe in August 1969; two years later the first closeups were obtained by the Mariner 9 orbiter. The satellites have been extensively photographed since. Both have also been photographed by rovers on the Martian surface. Due to its low orbital inclination, Phobos regularly causes annular eclipses of the Sun, but as its apparent diameter from the Martian surface is only a third that of the moon, it is too small to cause a total eclipse. The eclipses last around thirty seconds.

No successful landings have yet been made on either, although the Russians have made two attempts to land probes on Phobos. Phobos 1 and Phobos 2 were launched in 1988. Phobos 1 was lost en route to Mars after a technician accidentally shut down the probe’s attitude thrusters. Phobos 2 reached Mars orbit successfully, and it returned images of both Mars and Phobos. It was then supposed to approach to within 50 m of Phobos and deploy a pair of landers, but during this phase a computer malfunction caused the probe to lose contact with Earth.

In November 2011, the Russians tried again. Fobos-Grunt (‘Phobos Ground’) was supposed to be a sample-return mission, but the spacecraft failed to leave orbit and eventually fell back to Earth. Since the failure of this mission, there have been a number of proposals for a sample return mission to Phobos, but none are likely to launch in the next few years.

Many proposals for human exploration of Mars call for landings on Phobos and Deimos as a first stage.  Human missions to the Martian moons would result in the development and operation of new technologies, many of which would be required for an eventual landing on Mars, but without the attendant complexities and risks.

One giant leap for mankind: now for Mars

Forty years ago today, on 20 July 1969, Neil Armstrong and Buzz Aldrin became the first men to land on the Moon.

In Houston, the time was 15:17:40 CDT; in the UK 21:17:40 BST. Even aged 14, watching with my family, I was aware of how historic the moment was. I was an avid space enthusiast, my interest (like I suspect many boys of my age) having been sparked by Gerry Anderson’s TV shows such as Fireball XL5 and Thunderbirds. With us that evening was my grandfather, Robert “Pop” Mitchell, who was born in October 1892. He had just turned 11 when the Wright Brothers first flew at Kitty Hawk in 1903, a few years younger than I was in 1969. He was 19 when the Titanic sank and in his early 20s when he fought in the trenches of World War I, where he was seriously wounded in action.

As we now know, the mission came close to failure as the Eagle’s primitive computer, already overloaded, began to take the LM down towards an area strewn with boulders. Neil Armstrong was forced to take control and brought the spacecraft down safely with just 25 seconds of fuel remaining. But to those watching on TV and listening to the dialogue between Armstrong, Aldrin and CAPCOM Charlie Duke (who later went to the Moon himself), there was little hint of trouble:

102:44:24 Aldrin: 200 feet, 4 1/2 down.

102:44:26 Aldrin: 5 1/2 down.

102:44:31 Aldrin: 160 feet, 6 1/2 down.

102:44:33 Aldrin: 5 1/2 down, 9 forward. You’re looking good.

102:44:40 Aldrin: 120 feet.

102:44:45 Aldrin: 100 feet, 3 1/2 down, 9 forward. Five percent. Quantity light.

102:44:54 Aldrin: Okay. 75 feet. And it’s looking good. Down a half, 6 forward.

102:45:02 Duke: 60 seconds [at this point Eagle is down to her last 60 seconds of fuel].

102:45:04 Aldrin: Light’s on.

102:45:08 Aldrin: 60 feet, down 2 1/2. 2 forward. 2 forward.

102:45:17 Aldrin: 40 feet, down 2 1/2. Picking up some dust.

102:45:21 Aldrin: 30 feet, 2 1/2 down.

102:45:25 Aldrin: 4 forward. 4 forward. Drifting to the right a little. 20 feet, down a half.

102:45:31 Duke: 30 seconds [of fuel remaining].

102:45:32 Aldrin: Drifting forward just a little bit; that’s good.

102:45:40 Aldrin: Contact Light [these were actually the first words spoken from the Moon, not as is commonly thought, Armstrong’s famous change of call sign to “Tranquillity Base”].

102:45:43 Armstrong: Shutdown.

102:45:44 Aldrin: Okay. Engine Stop.

102:45:45 Aldrin: ACA out of Detent.

102:45:46 Armstrong: Out of Detent. Auto.

102:45:47 Aldrin: Mode Control, both Auto. Descent engine command override, off. Engine arm, off. 413 is in.

102:45:57 Duke: We copy you down, Eagle.

102:45:58 Armstrong: Houston, Tranquillity Base here. The Eagle has landed.

102:46:06 Duke: Roger, Tranquillity. We copy you on the ground. You got a bunch of guys about to turn blue. We’re breathing again. Thanks a lot.

102:46:16 Aldrin: Thank you.

The Moon walk wasn’t actually scheduled until around 07:00 BST next day, with NASA having scheduled a sleep period first, but Armstrong and Aldrin were understandably anxious to get on with the job and having just landed on the Moon I’d imagine sleep was the last thing on their minds. So shortly before four o’clock I dragged my brother (a few days short of his ninth birthday) out of bed and together we watched as Neil Armstrong became the first man to walk on the surface of the Moon and fluff his lines at the same time:

109:23:38 Armstrong: I’m at the foot of the ladder. The LM footpads are only depressed in the surface about 1 or 2 inches, although the surface appears to be very, very fine grained, as you get close to it. It’s almost like a powder. Ground mass is very fine.

109:24:13 Armstrong: I’m going to step off the LM now.

109:24:48 Armstrong: That’s one small step for [a] man; one giant leap for mankind.

About 20 minutes later, Aldrin joined Armstrong on the lunar surface:
109:43:08 Aldrin: That’s a good step.
109:43:10 Armstrong: Yeah. About a 3-footer.
109:43:16 Aldrin: Beautiful view!
109:43:18 Armstrong: Isn’t that something! Magnificent sight out here.
109:43:24 Aldrin: Magnificent desolation.

That first lunar EVA lasted just over 2½ hours. In addition to collecting contingency, bulk and documented lunar samples, Armstrong and Aldrin deployed a seismometer to detect moon quakes and a retro-reflector array to reflect laser beams back to Earth and so determine the Earth-Moon distance very accurately. Also left behind was a US flag; an Apollo 1 mission patch commemorating Ed White, Gus Grissom and Roger Chaffee; Soviet medals commemorating Yuri Gagarin and Soyuz 1 cosmonaut Vladimir Komarov; a gold olive branch; and a plaque mounted on the LM Descent Stage ladder bearing drawings of Earth’s Western and Eastern Hemispheres with an inscription reading “Here Men From The Planet Earth First Set Foot Upon the Moon, July 1969 A.D. We Came in Peace For All Mankind” together with signatures of the Apollo XI crew and President Nixon.

Finally there was a silicon disk containing goodwill statements by US Presidents Eisenhower, Kennedy, Johnson and Nixon and 73 other world leaders and heads of state. The latter detail makes interesting reading. The signatories include such notorious dictators as Nicolae Ceausescu, Ferdinand Marcos, the Shah of Iran, Chiang Kai-Shek, Park Chung-hee and Anastasio Somoza. Others perhaps more positively remembered include Queen Juliana, Archbishop Makarios, Indira Gandhi and Eamon de Valera. The only signatory still remaining in office is HM the Queen. France is conspicuous by its absence; so is the USSR and indeed all but a handful of communist countries; China was represented by the Republic of China in Taiwan.

For months afterwards the story was doing the rounds that the Chinese people had still not been told about the landing and in those pre-internet times it might have been true. By contrast, Soviet television gave extensive coverage to the event.

Before beginning preparations for blasting off from the Moon, Armstrong and Aldrin took their sleep period and, following their example, my brother and I went back to bed. By the time I woke my father was waking my grandfather and telling him about the moonwalk. Six months after the landing, my grandfather passed away, aged 77. His life thus spanned the entire history of human powered flight, from Kitty Hawk to the Sea of Tranquillity.

On 24 July 1969, Apollo XI returned to Earth safely and, after three weeks in quarantine, its crew emerged to a heroes’ reception. But astonishingly, the public almost immediately lost interest. Six more manned missions were sent to the Moon, but only the incredible drama of Apollo XIII made the headlines (and, a quarter of a century later, an excellent if not entirely accurate Hollywood movie). Since December 1972, not a single manned spacecraft has left Earth’s orbit.

In 2002, a moon landing hoax conspiracy theorist confronted Buzz Aldrin outside a Beverly Hills hotel and called him “a coward, a liar, and a thief.” Aldrin – then aged 72 – punched him in the face. Beverly Hills police and the city’s prosecutor refused to file charges.

It is a fact that thanks to unmanned space probes, we now have better maps of the surface of Mars than we do of the Moon; though NASA’s Lunar Reconnaissance Orbiter will redress the balance. Already it has returned images of abandoned Apollo hardware, unseen through all these years. The photographs from the Apollo XIV site are particularly good and show footprints left by Alan Shepard and Edgar Mitchell on the Moon’s surface; finally burying for good the ridiculous conspiracy theory that the Moon landings were faked.

As a boy, my grandfather could hardly have expected to see men land on the Moon in his lifetime, but I never doubted I’d live to see a Mars landing, assuming then that it would happen in the 1980s. If the will had been there, it would have done, but NASA was sidetracked by the space shuttle for decades before returning to the original Apollo concept in an updated form, Project Orion. Very tentatively NASA is now talking about an expedition to Mars in 2037. I’ll be 82 that year – I might just make it.

© Christopher Seddon 2009