StarDate

The tiny moons of Mars are hard to explain. Scientists aren’t sure how they formed and how they wound up in their current orbits. But a Japanese mission that launched this month could settle the conundrum.

The mission is MMX – Mars Moons Exploration. It will scan both Phobos and Deimos – chunks of rock the size of a small city. And it will land on Phobos, gathering samples for return to Earth.

Both moons are dark, like a class of asteroids. In fact, astronomers long suspected they were captured asteroids. In this scenario, rocky “leftovers” from the birth of the planets flew close to Mars. The planet was surrounded by a cloud of gas and dust. As Phobos and Deimos passed through it, the cloud acted as a brake, slowing them enough for Mars to grab them.

But it’s not easy for a planet to capture an asteroid. And the orbits of the moons aren’t like those of the captured asteroids of the giant outer planets.

So scientists came up with another model: a giant impact. A massive space rock could have slammed into Mars, blasting out material that formed a ring around the equator. The debris coalesced to form several small moons. Phobos and Deimos are the only survivors.

In yet another scenario, Mars captured a single large asteroid. It was sheared apart by Martian gravity, creating a ring that gave birth to Phobos and Deimos.
MMX will arrive at Mars next year – a big step toward solving the mystery of the Martian moons.



Script by Damond Benningfield

In Gulliver’s Travels, which was published 300 years ago today, Jonathan Swift wrote that the people of Laputa had made an important discovery: “two lesser stars, or satellites, which revolve about Mars.” The fictional moons were small and close to the surface, orbiting the Red Planet in a matter of hours.

Earthly astronomers took a little longer to find the real moons – they weren’t discovered until 1877. They were named Phobos and Deimos – fear and panic – after attendants of the god of war.

Phobos is the larger moon. It’s lumpy, with an average diameter of about 14 miles. It orbits just 3700 miles from the surface of Mars. It rises in the west, then sets in the east less than four-and-a-half hours later.

Deimos is about eight miles in diameter. It’s about three times farther from Mars, so it takes about two-and-a-half days to cross the sky, from east to west. It’s shaped a bit like a potato with a big dimple on one side.

Several spacecraft have flown past the moons. But they’ve raised a lot of questions about them. Now, 300 years after Gulliver’s Travels, scientists may take a big step toward finding some answers. Japan launched the Mars Moons Exploration mission this month. The craft will study both moons from close range. And it will touch down on Phobos, hoovering up a few grams of pebbles and dust for return to Earth.

More about that tomorrow.

Script by Damond Benningfield

The Moon is crossing the constellation Taurus. Tonight, it’s quite close to the bull’s shoulder – the Pleiades star cluster. The bull’s bright eye, the star Aldebaran, is well below the Moon as they climb into view. It’ll be about the same distance to the right of the Moon tomorrow night.

A dead “starship” is headed in Aldebaran’s direction. It won’t get there for more than two million years. But just in case anyone finds it, it carries a message.

Pioneer 10 was launched in 1972. It was the first spacecraft to fly through the asteroid belt, and the first to explore Jupiter, the Sun’s largest planet. After that, it raced out of the solar system and into interstellar space. NASA last heard from it in early 2003. Today, it’s more than 13 billion miles away.

The “message” is a plaque that’s made of aluminum and coated with gold. It measures nine by six inches. It includes the outlines of a naked man and woman. They’re standing in front of an outline of Pioneer to indicate their size. It also includes a chart of the solar system that shows Pioneer’s home planet. Other diagrams help pinpoint Earth’s location in the galaxy.

No one expects Pioneer to ever be found. It won’t get all that close to Aldebaran, and the star’s only known planet isn’t a good home for life. After Aldebaran, Pioneer 10 will continue sailing through the cosmos – an extinct emissary from Planet Earth.

Script by Damond Benningfield

If you overlook the fact that you’d be instantly vaporized, there’d be one small advantage to standing at the surface of the Sun: You’d age more slowly than your friends and family back on Earth.

The effect is called time dilation. It comes from Relativity – Albert Einstein’s theories of motion and gravity. They tell us that clocks at different locations tick at different rates. Keep in mind, though, that they tick at different rates as seen by observers moving at different speeds and in different gravitational fields.

According to Special Relativity, as you go faster your clock slows down. General Relativity says your clock slows down as the pull of gravity gets stronger.

On Earth, the effect has been confirmed by placing atomic clocks in high-flying jet aircraft. And GPS satellites are programmed to compensate for the effect. If they weren’t, the whole system would fail within days.

Time dilation is more pronounced at the surface of the Sun because of the Sun’s great mass. A clock there would slow down by about 67 seconds per year compared to one on Earth. And when the Sun expires, its “corpse” will have even stronger gravity, slowing a clock by more than an hour per year.

Time dilation is strongest just outside a black hole. An outside observer would see a clock at the cusp move almost infinitely slowly – never ticking off the final second before plunging into the black hole.

Script by Damond Benningfield

Few of us have to hunt wild game to survive the long, cold winter these days. But that hasn’t always been the case. Native tribes and European settlers alike had to stock up as the weather grew cooler. And they relied on an autumn full Moon for help: the Hunter’s Moon.

And that Moon is coming up tonight. It’s the full Moon after the Harvest Moon, which appeared in September.

Like the Harvest Moon, the Hunter’s Moon helped people prepare for winter. Its light made it easier to see deer and other animals. And at high northern latitudes, the Moon rises only about a half-hour later each night around full Moon – a much smaller gap than average. So the big, bright Moon was in view by the time evening twilight faded – allowing hunters to stay in the fields long into the night.

Despite what you might think, the Harvest and Hunter’s Moons aren’t any bigger and brighter than any other full Moons. The Moon’s orbit and its cycle of phases aren’t synchronized. So some years the Hunter’s Moon is closer than average. But in other years, it’s farther than average. This year we’re in luck, because it’s on the closer side. At the moment of full Moon, it’ll be about 230,000 miles away – a few thousand miles closer than average.

That helps make this year’s Hunter’s Moon an especially beautiful sight, lighting the autumn fields for hunters – and everyone else, too.

Tomorrow: stretching the year.

Script by Damond Benningfield