Universe Today

The Earth is so close to the Moon and yet the number of craters on the Moon are far more abundant. Have meteoroid and asteroid bombardments been selective? That is, have they been pounding the lunar surface but somehow sparing Earth's? Not really.

Earth is definitely receiving its fair share of meteoroid and asteroid assaults. In fact, meteoroids rain nonstop on both the Earth and the moon every minute. However, our planet is protected by its atmosphere. The moon, unfortunately, doesn't have one.

When small meteoroids pass through the atmosphere, friction causes them to ignite and disintegrate; the larger ones simply explode. The shooting stars and meteor showers that we see are the more visible ones. Surely you didn't think meteoroids only struck at night, did you?

On the moon, even the tiniest meteoroids strike the surface unhindered, forming the smaller-sized craters on the moon. These tiny craters even leave marks on moon rocks and are easily spotted on the rock samples brought home by the Apollo missions.

The tiny craters on the Apollo moon rocks are among the most substantial evidence that the Apollo landings actually took place. You can't hope to find rocks with the same features here on Earth.

So far, we've explained the presence of small craters on the Moon. How about the larger ones? The giant craters on the Moon were obviously formed by much larger astronomical objects, like asteroids or comets.

Bombardments from comets or asteroids are not common – both on Earth and on the Moon. However, why do we see more of them on the Moon's surface?

The reason for this is because of the absence of water (in liquid form) and climates on the lunar surface. These factors cause erosions, which subsequently conceal the evidence of such major bombardments. It would be safe to say that the large lunar craters fashioned by asteroid and comet impacts were formed millions of years ago.

This is consistent with the theory that the dinosaurs were wiped out by this kind of impact some 65 million years ago. While the evidence of such an impact, as well as even older impacts, are well preserved on the Moon, the same thing cannot be said for the Earth.

The Earth's high plate tectonic activity is also one major factor that has concealed large craters. Although we are not yet sure whether the Moon has any plate tectonic activity, it is believed that, if the Moon in fact has any such activity, the Earth's plate tectonic movement is still far more active.

An asteroid may not have wiped out the dinosaurs after all. We've got that story here in Universe Today. Want to know where the biggest crater is?

Come October 9, 2009, LCROSS will perform a lunar impact. Find out which crater NASA has chosen for the impact. If you want to know more about the largest crater on the Moon, NASA's got the right stuff.

There are some interesting episodes from Astronomy Cast that we'd like to recommend:

The Source of Atmospheres, the Vanishing Moon, and a Glow After Sunset

The Moon, Part 1

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The first attempt to send a rocket to the Moon via balloon hit a snag on Monday. The first test of the Aeronautics and Cosmonautics Romanian Association's (ARCA) balloon-launched rocket (or "rockoon") ended in failure when the "inflation arms" used to fill the balloon became entangled in the balloon itself. The arms had to be cut, and the operation – which required the use of a large naval frigate — was curtailed. ARCA hopes to compete in the Google Lunar X PRIZE, and intends on using their unusual rocket system to send an equally unique spherical lunar lander to win a $30 million prize.
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How far away is the Moon? The Moon is an average distance of 384,399 km, or 238,854 miles from the Earth.

That's an average, though. The Moon travels in an elliptical orbit around the Earth. Sometimes it's closer and sometimes it's more distant. From here on Earth, the size of the Moon can vary by more than 15% in size from when it's at its closest to when it's at the most distant point. When the Moon is at the closest, called perihelion, it's only 363,104 km from Earth. And then at its most distant point, called aphelion, the Moon is 405,696 km from Earth.

The brightest full Moons occur when the Moon is at the closest. A large, bright full Moon can be 30% brighter than when it's at its the most far away.

Astronomers know the exact distance to the Moon because there are small retroreflectors sitting on the Moon which bounce laser light. They were put there by the Apollo astronauts during their missions for just this purpose. Astronomers fire a laser pulse at the Moon and time how long it takes to make a return trip from Earth to the Moon and back. They can then calculate how far away the Moon is.

We've written many articles about the Moon for Universe Today. Here's an article about how LCROSS discovered buckets of water on the Moon, and here's an article about how long it takes to get to the Moon.

If you'd like more info on the Moon, check out NASA's World Book on the Moon, and here's a link to the Lunar and Planetary Science page on the Moon.

We have recorded several episodes of Astronomy Cast about the Moon. Here's a good one, Episode 113: The Moon, Part 1.

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Question: how far is it from the Earth to the Moon? The average distance from the Earth to the Moon is 384,399 km or 238,854 miles.

This is an average number, though. The Moon actually takes an elliptical path around the Earth. Sometimes it's closer, getting as close as 363,104 km. And it can also be further, getting as far as 405,696 km.

When the Moon can look 15% larger when it's at the closest point of its orbit. And every few years the closest Moon happens at the same time that the Moon is full. When this happens, it can be 30% brighter when we see a full Moon when it's at the furthest point of its orbit.

Scientists actually know the distance to the Moon within a few centimeters. When the astronauts of the Apollo missions were on the Moon, they placed small reflective mirrors on the surface. Scientists on Earth can shoot a laser beam and calculate how long it takes to bounce off these reflectors and return to Earth. By knowing the speed of light, they can calculate the distance light traveled to get to the Moon and back.

We have written many articles about the Moon for Universe Today. Here's an article about looking at the Moon in a telescope, and here's an article about the Moon landing hoax.

If you'd like more info on the Moon, check out this article that explains the retroflectors that scientists use to determine the distance to the Moon.

We've also recorded many episodes of Astronomy Cast about the Moon. Here's a good one, Episode 113: The Moon, Part 1.

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Water has long been suspected to exist in the permanently shadowed polar craters on the Moon, and now the LCROSS impact has allowed scientists to make a direct and definitive finding of this precious resource in a place NASA and other space agencies are considering exploring with human expeditions. Many say this could be a game-changing discovery for the future of lunar science and exploration. Unlike the previous announcement in September of water on the Moon, where water exists diffusely across the moon as hydroxyl or water molecules adhering to the surface in low concentrations, this new discovery could mean underground reservoirs of water ice. "There is too much water to be just absorbed in the soil," said Anthony Colaprete of the LCROSS mission at Friday's press conference. "There has to be real solid ice there. You could melt it and drink it."
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Are you wondering how far is the Moon from Earth? Well, the exact number is an average of 384,399 km, or 238,854 miles.

This number is an average because the Moon takes an elliptical orbit around the Earth. Sometimes it's closer than this average number, and other times it's more distant. At its closest point, called perihelion, the Moon measures 363,104 km from the Earth. And then at the most distant point of its orbit, known as aphelion, the Moon is 405,696 km from Earth. This is a big difference: 42,592 km.

Because of this huge different in difference, the size and brightness of the Moon can change dramatically when the full Moon matches up with a close Moon. It can be 15% larger and 30% brighter than an average Moon.

You might be surprised to know that astronomers have actually measured how far the moon is from Earth to within a few centimeters. They're able to do this because the Apollo astronauts left small reflective mirrors on the surface of the Moon during their last mission. Astronomers on Earth fire a laser pulse at the Moon and measure how long it takes to hit the Moon, bounce off the reflectors and return to Earth. By knowing the speed of light, and timing the return journey of the light pulse, they're able to calculate the distance to the Moon with incredible accuracy.

We've written many articles about the Moon for Universe Today. Here's an article about the discovery of water on the Moon, and here's an article about an eclipse seen from the Moon.

If you'd like more information on the Moon, NASA has some great resources. Check out NASA's World Book on the Moon. And here's NASA's Lunar and Planetary Science page on the Moon.

We have also recorded many episode of Astronomy Cast about the Moon. Listen here, Episode 113: The Moon, Part 1.

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You've seen the pictures, now watch the movie! Zoom into the Apollo 11 landing site with the Lunar Reconnaissance Orbiter's latest images of Tranquility Base where humans took their first steps on the Moon. Thrill with the detail! Swoon with the history! Or, just enjoy it.

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The LCROSS team announced today the mission successfully uncovered water during the Oct. 9, 2009 impacts into the permanently shadowed region of Cabeus cater near the moon’s south pole. "Indeed yes, we found water. We didn't find just a little bit we found a significant amount," said Tony Colaprete, principal investigator for LCROSS at a press conference. The team was not able to put a concentration of how much water is held in the lunar regolith, but in a fraction of the 20-30 meter crater the impact made, they were able to observe about 25 gallons (95 liters) of water with spectroscopic data. Colaprete held up a 2-gallon (7 liter) bucket, to demonstrate how much they found.
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The Moon distance From Earth is 384,399 km, or 238,854.5 miles.

Keep in mind that this number is just an average. The Moon actually travels in an elliptical path around the Earth. Sometimes its closer and other times its further. At its closest point, called perihelion, the Moon gets to within 363,000 km of the Earth. And then at its most distant point, the Moon gets out to 406,000 km from the Earth.

This means that the Moon's distance from Earth can vary as much as 43,000 km over the course of its orbit. Depending on when these points occur during its phases, we can have full Moons which look much bigger in the sky and reflect much more light. When the Moon is at its closest point in orbit, and it's in a full Moon, it can look 14% bigger in the sky, and reflect 30% more sunlight.

Do you wonder how we have such an accurate figure for the Moon's distance? During the Apollo mission, astronauts placed reflecting mirrors on the surface of the Moon. Astronomers on Earth can fire lasers at the Moon and then calculate the time it takes the light to reflect back. Since they know the speed of light, and can measure the return time of the journey, astronomers can calculate the distance to the moon within just a few centimeters accuracy.

We have written many articles about the Moon for Universe Today. Here's an article about the discovery of water on the Moon, and here's an article about how people might try to build a greenhouse on the Moon.

If you'd like more resources on the Moon, check out NASA's World Book on the Moon, and the Lunar and Planetary Science page on the Moon.

We have also recorded many episodes of Astronomy Cast about the Moon. Here's a good one, Episode 113: The Moon, Part 1.

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JAXA, the Japan Aerospace Exploration Agency has released all the data from the Kaguya mission to the public. One of the ways to view the data is through a very nifty 3-D virtual brower. It only is available in Japanese for now (English version by the end of November, they say) so it is a little difficult to navigate, but once you figure it out, prepare yourself for loads of fun. First, you need Java. Then…
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Close-up view of Apollo 12 landing site from LRO. Credit: NASA/GSFC/Arizona State University

Wow! Just look at the detail visible in this image of the Apollo 12 landing site taken by the Lunar Reconnaissance Orbiter from its lower mapping orbit of 50 km above the surface. Compared to earlier images taken in September when LRO was in a higher orbit, the Lunar Module descent stage really stands out, as well as the Apollo Lunar Surface Experiment Package (ALSEP). Also visible are the trails left by spacewalking astronauts. From this and other LROC landing site images, it is clear that astronaut activity lowers the albedo, or reflectivity of the surface. Areas of heaviest activity have the lowest albedo, especially around the LM. NASA says this effect is most likely due to compaction of a very loose surface powder by the astronauts just walking around.

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The Lunar Reconnaissance Orbiter maneuvered into its 50-km mapping orbit on September 15, which enables it to take a closer look at the Moon than any previous orbiter. This also allows for comparing previous images taken by LRO when it was at its higher orbit. Here's the Apollo 17 landing site: just look at what is all visible, especially in the image below! These images have more than two times better resolution than the previously acquired images.

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Zoomed in image of the impact plume. The extent of the plume at 15 sec is approximately 6-8 km in diameter. Credit: NASA

Nine science instruments on board the LCROSS spacecraft captured the entire crash sequence of the Centaur impactor before the spacecraft itself impacted the surface of the moon. But from Earth, any evidence of the plume was hidden by the rim of a giant impact basin, a 3 kilometer-high (2-mile) mountain directly in the way for Earth telescopes trained on the impact site, said Dr. Peter Schultz, co-investigator for LCROSS. Additionally, the crater created by the impact was only about 28 meters across (92 feet) but Schultz said the best resolution Earth telescopes can garner is about 180 meters (200 yards) across.
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In late September, a team of scientists announced finding water molecule signatures across much of the Moon's surface. Now, a second instrument on board India's Chandrayaan-1’s lunar orbiter confirms how the water is being produced. The Sub keV Atom reflecting Analyzer (SARA) corroborates that electrically charged particles from the Sun interact with the oxygen present in some dust grains on the lunar surface to produce water. But the results bring out a new mystery of why some protons get reflected and not absorbed.
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It's really interesting to note that there are so many craters on the Moon while Earth has so few despite their clear proximity. Missions on and above the lunar surface revealed how Moon craters were formed by the constant bombardment of meteoroids and asteroids. It's therefore natural to expect the Earth to be exposed to the same kind of barrage.

But why is the Earth spared from such bombardment? Actually, it isn't. Meteoroids, which are space projectiles having sizes between a speck of dust to a boulder, rain on the Earth every minute. Fortunately, our planet has a shield that protects us from them. Most meteoroids disintegrate in the atmosphere as they speed to the Earth's surface. Some of the larger ones even blow up.

Although a few make it through, their numbers are nothing compared to the Moon's. Why? Because the Moon doesn't have an atmosphere. Its surface is extremely vulnerable to meteoroids, that even very small ones are able to take pot shots on it. This has been revealed from the Moon rocks collected by astronauts. The rocks were found to have tiny meteoroid-bombarded craters.

Still, that doesn't account for the larger Moon craters. These great holes were formed due to asteroid (bodies larger than meteoroids but smaller than planets) impacts. The largest known crater on the Moon, the South Pole-Aitken Basin, has a 2,240-km diameter and a 13-km depth. For comparison, the deepest point on Earth, Pacific Ocean's Challenger Deep, is only 11 km deep.

Now, the Earth is not invulnerable to asteroid impacts. In fact, there's a theory that suggests the dinosaurs were wiped out when a big one hit the Earth around 65 million years ago. So if the lunar surface has so many asteroid-caused Moon craters, where are the Earth's? The atmosphere surely couldn't burn out and consume those big rocks.

Apparently, the cavities caused by asteroid impacts on our planet may have been covered by erosion and active plate tectonics. The absence of these geological activities have allowed Moon craters to be well preserved.

One crater has caught the attention of scientists eager to colonize the Moon. ESA's Smart-1, which crashed into the lunar surface in 2006, was able to capture an image of a crater that looked like the perfect spot for future settlement. Being near the Moon's North Pole, Crater Plaskett can receive a substantial amount of solar energy. It also has a relatively predictable temperature.

An asteroid may not have wiped out the dinosaurs. We've got that story here in Universe Today. Want to know where the biggest crater is?

Come October 9, 2009, LCROSS will perform a lunar impact. Find out which crater NASA has chosen for the impact. If you want to know more about the largest crater on the Moon, NASA's got the right stuff.

There are some interesting episodes from Astronomy Cast that we'd like to recommend:

The Source of Atmospheres, the Vanishing Moon, and a Glow After Sunset

The Moon, Part 1

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