The Moons of Mars Help

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By — McGraw-Hill Professional
Updated on Sep 16, 2011

The Moons of Mars—Deimos

Some astronomers think Phobos and Deimos are ex-asteroids that ventured too close to the planet and were captured by its gravitation, although there is reason to believe that one or both of them congealed from ejected material as the result of large asteroids or small protoplanets striking Mars long ago. Both moons are tiny compared with their parent planet, and both moons require large telescopes to be seen by Earthbound observers.


“We’ll be passing Deimos and then looking at Phobos from a distance as part of this tour,” says the first officer. “Deimos is the smaller of the two. It orbits the planet in about 30 Earth hours. There has been some talk of putting several large communications satellites on Deimos so that it can serve as a repeater for maintaining contact among exploration crews.”

“Landing on Deimos would be a problem, wouldn’t it, because of the low gravity?” you ask.

“Deimos is too small to have any gravitation to speak of, at least from a practical point of view. It is a chunk of rock only about 13 kilometers (8 miles) in diameter. Anyone who wants to rendezvous with Deimos will have to dock with it instead. One suggested scheme has been to harpoon it. A small rocket would be fired at Deimos, would crash-land there, and then burrow into the surface. However, no one has been able to figure out how to make sure the harpoon wouldn’t get pulled out and send construction workers scattering into Mars orbit. The escape velocity is less than 6 meters (about 10 feet) per second.”

“Aren’t regular communications satellites good enough?”

“Generally speaking, yes. But there would be room for gigantic storage batteries on a piece of rock like Deimos or Phobos, and these could serve several satellites. They could be charged with massive solar panels,” says the first officer.

“There’s something,” you say, pointing to an irregular object, half lit by the Sun, the other half eerily glowing with Mars-shine. Deimos is only about 20,000 kilometers (12,500 miles) above the Martian surface, and the Red Planet looms large.

“That is Deimos,” says the first officer. “It orbits Mars in a nearly perfect circle. That casts some doubt on the theory that Deimos is an asteroid that was thrown out of its original solar orbit by the gravitation of Jupiter. But that is a popular theory.”

“How much extra fuel did it cost us to see this piece of rock?” you ask.

“Not much,” says the first officer. “But it would cost too much to go right up to Phobos, the inner moon; we will have to be content to look at it through our telescopic cameras. Come with me.”


The first officer leads you into a dimly lit room with a huge screen on one wall. There is something strange about that screen; it is obviously there, but you can’t ascertain how far away it is. “Is that a holographic projection system?” you ask.

“Yes,” says the first officer. “And the first projection we’ll see is an animated rendition of Mars, Phobos, and Deimos (Fig. 6-4). The piece of rock you just saw is the smaller, higher, and slower of Mars’ two moons. Phobos orbits much closer to the planet. This illustration is to scale. Note that Phobos revolves around Mars much faster than Deimos. In fact, Phobos is inside what is called the synchronous-orbital radius . Phobos revolves around Mars faster than the planet rotates on its own axis. This means that an observer on Mars will see Phobos move across the sky from west to east.”

“It almost looks like an artificial satellite in this picture.”

Mars From Venus To Mars Phobos

Figure 6-4. Mars, Deimos, and Phobos, and the orbits of the two moons, drawn to scale and viewed from high above the Martian north pole.

“Phobos eventually will suffer the fate of most artificial satellites. It likely will spiral into Mars and crash. This will produce a significant impact. Phobos is not huge, but I wouldn’t want to be on Mars when it hits. The gravitation of Mars may break Phobos up before it can crash, and then Mars will have a ring system.”

The next image is of a dark, stonelike object that seems to be falling out from underneath a curved, inverted Martian horizon. “That is Phobos rising right now,” says the first officer. “Actually we are looking back at it. We are still at a higher altitude than Phobos and have slowed down in preparation for Mars orbit. Phobos is catching up to us. We will be passing its orbital level while it is safely on the opposite side of Mars.”

“It looks like a lump of coal,” you say.

“Phobos is made of material called carbonacious chondrite , similar to that of many meteoroids and asteroids,” says the first officer. “Its albedo is only 0.06. This means that it reflects only 6 percent of the light that strikes it.”

“How large is Phobos?”

“Slightly bigger than Deimos, about 20 kilometers (12.5 miles) in diameter, but elongated.”

“Right now it looks like a hand grenade with the top part taken off so that there’s a hole in the top,” you say.

“Wait a while and it’ll change,” says the first officer. He’s right; a short while later it looks almost spherical.

“Is the hole an impact crater?” you ask.

“Yes. It is called Stickney ,” says the first officer. “If Phobos was originally an asteroid, it struck a lot of other asteroids before it attained orbit around Mars. The impact that made Stickney might be the one that knocked Phobos out of the asteroid belt, if, that is, Phobos was indeed an asteroid at one time. I am not sure that this is true.”

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