39 years ago today...

Image Credit: NASA

39 years ago, humankind took its first tentative steps into the cosmos when astronauts Neil Armstrong and Edwin "Buzz" Aldrin walked on the surface of the Moon as part of the Apollo 11 mission. (Their moonwalk, while only 2.5 hours long, started about 11pm EDT on July 20 and finished in the early morning of July 21, so there is some ambiguity in the "date" of the moonwalk. But why not celebrate this amazing accomplishment over two days, instead of just one?)

At the end of the Apollo Moon program in December, 1972 (a full year before I was born), few people suspected that it would be nearly 50 years before we returned to the moon (and it could be longer than that, if the Orion project is significantly delayed). To some people, this is a travesty. Other people wonder why we are even considering going back.

I think it is in our nature to explore. From our early Homo sapien ancestors leaving the African continent to colonize new lands, to seafaring peoples of many nations and races that sailed the vast and unfriendly seas, to the astronauts/cosmonauts/taikonauts who risk their lives to sail the vacuum of space, the unknown seems to draw us onward. So, I suspect that it is just a matter of time before we humans are crawling across the face of Mars, and perhaps even considering one-way flights to explore new worlds around other stars. But this "time" may be hundreds or thousands of years from now, and there are many other challenges facing us right here on our home planet. So I think we can afford to be patient, as long as we don't take our eyes off the ultimate goal.

Sp, if you look out late this evening to see a big yellow moon rising, remember that we were there just 39 short years ago.

One really cool movie

Credit: D. Linder / EPOXI / NASA

The video above shows a very unique view of the Earth/moon system. It was taken by the EPOXI Mission, the name of NASA's Deep Impact spacecraft's new mission (now that it's completed its mission to explore Comet Tempel 1). The spacecraft turned its onboard camera toward the Earth in late May when it was 31 million miles away from Earth, and it captured (intentionally) the Moon passing in front of the Earth.

The video above is roughly true color. A cartoon globe in the lower left shows you what parts of the Earth are visible at any given time. If you look closely toward the center right of the Earth, you can see "sunglint," the reflection of sunlight off of the oceans in the direction of the spacecraft.

Notice the color difference between the Earth and the moon. The moon looks dark and reddish. That's the true color of the moon. It looks bright and silvery in our sky, but if we had another bright Earth near the moon in our sky, we'd see how reddish and dark the moon really is.

Also notice the relative size of the Earth and the moon (also correct); it can give you some idea of how small the moon is. The only thing this picture doesn't do is give you an idea of the relative separation of the Earth and moon; the moon is roughly 30 Earth-diameters closer to the spacecraft than the Earth is.

Another version of the video shows the same scene, but it includes some infrared imaging, which brings out vegetation (plants reflect a lot of infrared light). In this version, you can see parts of the continents even better. NASA's Landsat satellites use the same infrared colors to track changes in plant life on Earth, such as these pictures showing the damage caused by illegal logging in Indonesia over a 10 year period.

I find views of the Earth like this are very helpful in reminding myself of our place in the Universe, and how small and unique that place is. Some other views of our planet from outer space:

• The Earth seen from the Moon in the famous photo taken by Apollo 8 astronauts in December, 1968.
• A movie of the rotating Earth taken by the Galileo mission as it swung past the Earth on its way to Jupiter
• A view from Mars by the Mars Reconnaissance Orbiter
• A family portrait of the entire Solar System from the Voyager 1 spacecraft, taken from a distance of 8 billion miles.

Special thanks to Phil Plait for posting the EPOXI videos on YouTube and for bringing my attention to them.

Telescopes on the moon

Image Credit: NASA

We're going back to the moon. It may be 15 or 20 years until we get there, but NASA is headed that direction. Many people, including a lot of astronomers, are opposed to this new lunar exploration. It will be very expensive, and we aren't sure what the point of the exploration is.

A colleague of mine once argued that we astronomers should be pushing NASA to build a telescope on the moon once we return. His idea, a liquid-mirror telescope, is pictured above. Such a telescope would be quite expensive, and the science it could do may be able to be done more cheaply with orbiting telescopes. But the point was that NASA is much more likely to spend a lot of money on the moon, not on new space telescopes, and if astronomers want a piece of that money, the time to start lobbying is now. Otherwise, we could find ourselves out in the cold once lunar exploration ramps up.

It's sort of like a parent shopping at a high-end luxury store asking their thrifty teenager, "Hey, should we buy you this 2nd-generation iPhone for $500?" The teenager responds, "But we can go to the Apple Store and buy the new iPhone 3G, which is more functional and less then half the price!" To which the parent responds, "You're getting this phone or you're getting nothing." What should the kid do? I think most of us would be tempted to take the phone.

Today, Space.com posted an article on a new design for a lunar telescope that has been proposed. This telescope could be built from materials on the moon. I have no doubt that the cost would be very high and the design technologically challenging to build (as would be the liquid-mirror telescope my colleague proposed). I know very little about the new proposal, such as what wavelengths of light it would be best suited for, or what instruments would go on it, or what scientific question(s) it would address. And, no doubt, NASA would make sure it is a very nice telescope, in terms of these capabilities.

But I wonder if we astronomers need to start thinking much harder about this. Do we want to push for a telescope on the moon? If there is a trade-off between a lunar telescope and a more functional, less-expensive space-based telescope, then let's by all means choose the latter. But I do think it is likely that we might be given the choice of a big lunar telescope, or no big telescope at all. And if we wait 15 years to make up our minds, it will be too late. Who knows what the astronauts will be doing on the moon science-wise, but it certainly won't be astronomy. And we'll have missed out.

There is good astronomy that can be done on the moon. One of the most convincing ideas I've heard is to put a radio telescope on the far side of the moon; the moon will block out radio signals from Earth, allowing us to study signals from space that are currently swamped by our FM radios, our iPhones, our satellite TV, and most every other modern bit of wireless communication. Some types of astronomy, like optical astronomy and, perhaps, infrared astronomy, are better done in orbit so that we don't have to worry about astronaut dust and other activity upsetting the instruments. But both of these fields would not suffer from a lunar telescope.

Astronomers are beginning to assemble our "decadal survey," a 10-year look into the future needs and desires of astronomy. This survey is cited in our funding requests to Congress and NASA. Maybe this time around we should debate the various possibilities of lunar telescopes. If we wait until 2020 to ask, it may be too late.

The varmint was right!

On February 2, Punxsutawney Phil (the groundhog) predicted six more weeks of winter. And, by gum, he was right! It's now six-and-a-half weeks later, and spring has sprung (at least in the northern hemisphere). Today the sun will appear to move north of the equator, where it will stay for the next six months. At the north pole, the sun is rising, and at the south pole, the sun is set (stranding scientists at the South Pole Station until their spring comes).

Tomorrow is the full moon. In the Christian tradition, the first Sunday following the first full moon of spring is the Easter holiday (which explains why Easter is so early this year -- the full moon comes only a day after the start of spring). The Easter holiday remains one of the western calendar's few remaining links to lunar calendars (calendars based on the moon's 29-day cycle through its phases). Lunar calendars are still in popular use throughout the world -- the Islamic calendar, Jewish calendar, and Chinese calendar (among numerous others) are all based primarily on the moon's phase. But since the moon's phases don't match up exactly with the solar year (the length of time it takes the Earth to orbit the sun once, and the basis of the Gregorian calendar we use in everyday life), lunar calendars are a bit odd to the uninitiated, requiring the occasional extra month or something similar to bring the two back in line.

Astronomer's observations are often based on the lunar cycle. The full moon is very bright -- you will notice that you can see fewer stars in the sky when the moon is almost full as opposed to when the moon is absent from the sky. The faintest stars are hidden by the moon's glare. Even with telescopes, the moon's glare hides fainter objects. So, when we ask for telescope time, we have to specify if we can do our science when the moon is bright.

When the moon is bright, astronomers tend to look at bright stars, because their light cuts through the moon's glare. More recently, astronomers also tend to use the full moon time to look at objects in infrared light. Although the moon is still very bright in the infrared, the sky itself is always glowing brightly in the infrared, and the moon's light doesn't add much to the overall glow. (There are also stories of infrared astronomers moving the telescope to their next target and finding the moon is in the way!) But those of us who just want to study faint stars or galaxies in visible light are out of luck near the full moon. So, even as the modern world steers further away from lunar calendars, observational astronomers still have to pay close attention to the phases of the moon.

Venus if you will, please send an occultation us to thrill

Image Credit: Alan C. Tough / Sky and Telescope

Tomorrow, (Wednesday March 5), people across much of the midwestern and southern United States have the chance to see the moon pass in front of the planet Venus in broad daylight. If you have binoculars, seeing this event will be fairly easy, but if you are patient and lucky, you may be should be able to see the moon and Venus in the daytime with just your eyes.

The planet Venus is usually bright enough to see in plain daylight (as are the other bright planets and, some claim, the brightest stars). The reason most people never see the planets during the day is that your eyes are fooled by the sky. If you look at a blank sky, your eyes tend to focus at a spot about ten feet away from your face, which puts any planets and stars in the sky out of focus, making them impossible to see against the bright blue sky. But if you know where to look and can focus your eyes properly, Venus is quite easy to see against the sky!

The easiest way to see planets in the daytime sky is to wait for the moon to wander nearby. Many people have seen the moon in the daytime sky, and looking at the moon focuses your eyes just right. So, if a planet is close to the moon in the daylight sky, your eye may be able to pick it out and stay focused. I've used this trick to see Venus in the daytime sky before, and it is quite amazing when it works. (I've also been told that looking at the sky from the bottom of a deep well or mineshaft helps you to see stars in the daytime, but I would not recommend falling down a well just for the sake of seeing stars in the daytime. A good whack on the head can accomplish the same end.)

The trick with tomorrow's event will be finding the moon in the sky. The moon will be about 25 degrees away from the sun -- that's only a little more than the width of your stretched hand (from pinky tip to thumbtip) at arm's length. The moon will also be very hard to see, because it will be a very skinny crescent. To see the moon without hurting your eyes due to the sun, you'll need to hide stand in a shadow. And, the moon will be between the sun and the horizon, so a building shadow will likely hide the moon, as well. You'll need a tree branch or lamppost or some other well-placed shadow.

An occultation means that the Moon will pass between Venus and the observer, so this is sort of like an eclipse of Venus. Venus will be behind the moon for over an hour in most places, so if you look during the occultion, you'll just see a crescent moon (like in the picture above, only even skinnier). Whoop-di-doo. No, the best views are either just before or just after the occultation, when you can see the moon and the brilliant light of Venus right next to it.

What time should you look? It depends where you live. Here is a website with a map and times of the start and end of the occultation. The times are in 24-hour format (e.g., 20:00 = 8:00pm), and in Universal Time (UT). Eastern Standard Time is the UT time minus five hours; so if the occultation starts at 20:25 UT, it starts at 15:25 EST (which is 3:25pm). Likewise, Central Standard time is the UT time minus six hours, and Mountain time is UT time minus seven hours.

The moon will be really, really hard to see without binoculars, because it will be very skinny. If you want to use binoculars, be very, very careful to make sure you don't point them anywhere close to the sun. Looking at the sun, even accidentally, through binoculars will instantly and permanently damage your eyes!

So, in short, if you have clear skies tomorrow afternoon and want a challenge, try going outside to see Venus and the moon in broad daylight. They will be hard to find, but seeing both in the daytime sky is something you'll remember for a long time to come!

For more information, read href="http://www.skyandtelescope.com/observing/highlights/16079237.html"Sky and Telescope's article on the occultation. Good luck!

Total Eclipse of the Moon tomorrow

Image Credit: Sky & Telescope and Sean Walker

During the night of February 20, the Moon will pass into Earth's shadow, causing a total eclipse of the moon visible from the entire nighttime face of the Earth. Everyone in North and South America (except Hawaii) will be able to see the eclipse on Wednesday night, while people in Europe and Africa can see the eclipse Thursday morning.

This total lunar eclipse is the third in a year, and the last total lunar eclipse until December 2010. So, if you have clear skies, go out and enjoy the view!

Totality, or the period of time when the moon is completely within Earth's shadow, begins at 10:00pm Eastern Standard Time (7:00pm Pacific) and lasts for just over 50 minutes. During this time, the moon won't completely disappear, but will be a reddish color, as the lunar surface gets to view every sunset and sunrise on the Earth all at once! This eclipse could have some fairly bright copper colors as well, because the moon doesn't go that far into Earth's shadow.

For about 2 hours on either side of totality, you can see the moon in partial eclipse, when part of the moon will be in Earth's shadow. Earth's shadow will have a noticeable curve to it. It is this curvature that proved to the ancient Greek astronomers that the Earth was round and not flat, because it didn't matter when they could see the eclipse -- the shadow was always round. You can imagine that, if the Earth were shaped like a Frisbee, sometimes the shadow would be round, and sometimes it would be long and thin, depending on exactly how the Frisbee was oriented with respect to the sun.

The great thing about lunar eclipses is that you don't need any special equipment to see them. You don't need telescopes, or binoculars, or even a very dark sky. You just need a clear view of the sky in the direction of the moon! So, again, go out and look at tomorrow's eclipse. Enjoy the colors and the shapes of Earth's shadow, absolutely free of charge to anyone who wants to see.

For more details of the eclipse, you can read articles at Sky & Telescope or from NASA's Eclipse Page.

Blue moon, you saw me standing alone...

Image credit:Kostian Iftica, ExploreTheCosmos.com Today, at 10:04pm EDT, the second full moon of the month of May (as seen from the U.S.) will occur. In popular astronomy, this event (the second full moon in a single month) is called a "blue moon." The moon isn't really blue tonight, it's just a name given to the moon.

But even this name is not technically correct. First, astronomers use Universal Time (Greenwich Mean Time, or the time in Greenwich, England when it is not Daylight Savings Time). And, by that measure, the full moon occurs early in the morning on June 1 (1:04am UT, to be precise). So the astronomical blue moon will be next month, when the second full moon will be June 30 at 1:34pm UT.

Second, using the term "Blue Moon" to refer to the second full moon in a month started as a mistake. This article from Sky & Telescope magazine explains how that mistake was made. In the past, the term "blue moon" referred to the third full moon in a season that had four full moons instead of the normal three. But the mistake is now ingrained in popular culture, and there is no need to try and correct it (in my humble opinion).

Scientifically, there is nothing interesting about a blue moon. It takes the moon 29.5 days to go through a set of phases, and months (except for February) are longer than this. So, it is possible, with the exception of February, to have two full moons in a month. This happens every two and a half years or so, including this (or next) month.

In this way, a blue moon is like Leap Day -- it is an occasional event because our calendar (based on the Earth going exactly once around the sun) doesn't match up exactly with cycles of the moon's phases (a "moonth," if you like) or the Earth's spinning about its axis (a "day").

So, although there is nothing special about this full moon, I am happy for the press. Anything that can get people to go and look at the sky is a bonus. If you go out looking for the blue moon this evening, look for the planet Venus (by far the brightest "star" in the western sky after sunset right now.) The Big Dipper is high in the sky in the early evening, too. Just above and to the right of the moon is the star Antares, the heart of Scorpio. To the left of the moon, the bright "star" is the planet Jupiter. Compare the colors of Jupiter to Antares, and you might be surprised to notice that Antares is definitely reddish in color. (It may be a little hard because of the glare of the moon). And Venus is definitely whiter than either Jupiter or Antares!

Ice Geysers

Image Credit: NASA/JPL/Space Science Institute

I received an email from Tom B. this week asking about this news story claiming that water geysers observed on Saturn's moon Enceladus may be due to friction of ice plates rubbing together:

You may have seen the new suggestion regarding how gas plumes form on Enceladus. It suggests that thick ice plates are rubbing together and creating a vapor that escapes as plumes of water vapor. Would you please provide some explanation of how a place as cold as Enceladus could produce anything so hot as water vapor simply by friction?

I can certainly try to explain it! In the outer reaches of our Solar System, water (usually in the form of ice) is very common. Most of the moons, comets, and even larger objects (like Pluto) are made mostly of water ice. Partly this is because the outer solar system is so cold -- temperatures out there are a chilly -325 degrees Fahrenheit! So if you want to make geysers out of this water, you would need to warm the water up by 350 degrees, right?

Well, not necessarily. In my last post, I was talking about the strange forms of "hot ice" that could exist on a Neptune-mass planet around another star. As part of that post, I included this graph showing the phases of water as a function of temperature and atmospheric pressure.

The moons in the outer solar system (except for Titan) have extremely tenuous atmospheres; essentially they have no air pressure. So, looking at the diagram, you notice that the freezing point of water in very low atmospheric pressures goes way down -- only 200 Kelvin (-100 Fahrenheit). So, on these moons, you don't need to warm up the ice quite as much before it changes into a gas, at which point it will find its way through any cracks of the moon's frozen surface and vent into space.

But we also need to consider these big plates of ice. Enceladus orbits Saturn in a near-perfect circle. But it is not quite perfect -- some times it is slightly closer to Saturn than other times. And when it gets closer to Saturn, it gets squeezed a little more. This energy from squeezing (or gravitational tides) seems to cause plate tectonics on Enceladus, just like on Earth, except with water ice instead of rock making up the plates.

On Earth, plate tectonics is responsible for many volcanoes and for earthquakes. Until this press release, scientists thought that the geysers of Enceladus were like Earth's volcanoes -- ice is melted deep in the moon, and near the edges of these plates, the magma (in this case, water), finds it easier to escape to the surface and be erupted out.

So, finally, to the new idea about the geysers. This new concept blames the friction of the ice plates grinding together for the heating of the ice. We on Earth know of frictional heating -- it is why your hands get warm when you rub them together, and why your car's brakes overheat if you use them too much. Friction happens during earthquakes, too, and during moonquakes on Enceladus. In both cases, tremendous amounts of heat are generated. In an earthquake with a magnitude of 4.0 (enough to cause noticeable shaking, but in most of the U.S., there would be little or no damage -- Californians experience 4.0 earthquakes all the time), there is as much energy released as a ton of TNT. A major earthquake, say magnitude 7.0, releases 32 megatons of energy -- dozens of the largest nuclear weapons worth!

This energy has to go somewhere, and most of it goes into friction and heat. As you might imagine, this heat can be tremendous. But on the Earth, it isn't enough to melt rock, which has to get up to thousands of degrees to melt. So an extra couple of hundred degrees deep in the Earth makes no noticeable change. But on Enceladus, this energy would be enough to change the ice into gas, which would then vent out through the fault line that had the moonquake. Remember, these ice sheets are miles thick and acting much more like rocks on Earth than like the ice floes in our arctic oceans (which do gently rub together, and don't create geysers).

The calculations reported in the news story on Enceladus show that friction alone makes enough energy -- remember we are talking about thousands of tons of ice rubbing against each other. But this isn't proof that the geysers on Enceladus are powered by friction; it just shows that it is possible. Yet one thing we know from decades of modern physics is that when strange things are possible (think of black holes and neutron stars), these things tend to happen.

So, in short, the reason the frigid ice on Enceladus could melt due to friction is (a) you don't need to warm it up too much, and (b) remember that we are talking about tremendous amounts of ice, sheets of ice a dozen miles thick, rubbing against each other in sudden releases of energy. And more tests are needed to show if this idea is correct or not.