Posted on by Abby Cessna

Tallest Mountains

Olympus Mons. Image credit: NASA/JPL

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There are many tall mountains around the world as well as on other worlds. Mount Everest is the highest mountain in the world at 8,848 meters. Mauna Kea is the tallest mountain in the world. The tallest mountain is measured from base to top while the highest mountain is measured from sea level to the top.  Everest is located in the Himalayan mountain range in Nepal and near Tibet. Mauna Kea is located in Hawaii and is 10,200 meters from base to tip. From sea level though, it is only about 4,205 meters tall.  Mauna Kea is an extinct shield volcano.

These are not the only tall mountains though. K2 is in the Karakoram mountain range on the border of Pakistan and China. It is 8,612 meters tall and is generally considered the second tallest mountain in the world. The Himalayans are home to many tall mountains besides Mount Everest. This includes Mount Kangchenjunga at 8,586 meters and Mount Lhotse I at 8,501 meters.

Most of the world’s tallest mountains are located in Asia; however, there are a number of tall mountains that are located on other continents. The seven tallest mountains in different continents are known as the Seven Summits. Climbing all seven mountains is a mountaineering challenge that was started in the 1980’s.The first of these is Mount Everest. Another summit is Aconcagua, which is a mountain in Argentina in South America. At approximately 6,962 meters, it is the tallest mountain in the Americas. North America’s tallest mountain is Mount McKinley at 6,194 meters. Mount Kilimanjaro can be found in Tanzania in the continent of Africa and is 5,895 meters tall. The large summit of Mount Kilimanjaro is covered with an ice cap that is receding and according to scientists will eventually be gone. Mount Elbrus, the tallest mountain in Europe at 5,642 meters, can be found in Russia. Vinson Massif is Antarctica’s tallest mountain at 4,897 meters. It is also very large being 21 kilometers long and 13 kilometers wide.  Australia-Oceania’s largest mountain can be found in Indonesia. At 4,884 meters, it is Puncak Jaya, which is also known as the Carstensz Pyramid.

The tallest mountain that we know of is not even on Earth. It is located on Mars and is known as Olympus Mon.  A shield volcano, Olympus Mon is 27,000 meters tall. Mars is not the only other planet with tall mountains though. Venus’ Maxwell Montes is 11,000 meters tall. Satellites also have tall mountains including our Moon, which has Mons Huygens at 4,700 meters tall. The moon Io has a mountain, Boösaule Montes, which is approximately 17,000 meters tall.

Universe Today has articles on tallest mountain and tallest mountain in the Solar System.

For more information, you should take a look at what are the world’s tallest mountains and highest mountains.

Astronomy Cast has an episode on Earth you will find interesting.

Sources:
http://en.wikipedia.org/wiki/List_of_highest_mountains

Posted on by Tammy Plotner

Weekend SkyWatcher’s Forecast – December 5-7, 2009

Greetings, fellow SkyWatchers! Are you ready for an mmm mmm good weekend? Well, the “m” stands for Messier and we’re off to study three of the late year’s finest… and a Herschel object as well! Don’t feel like taking out the telescope? Then don’t worry, because all of our weekend studies are easily done with even small binoculars! When ever you’re ready, I’ll see you in the dark…

Friday, December 4, 2009 – On this date in 1978, the Pioneer Venus Orbiter became the first spacecraft to orbit Venus. And, in 1996, the Mars Pathfinder mission was launched.

Tonight we’ll launch toward a bright open cluster known by many names: Herschel VII.32, Melotte 12, Collinder 23, and NGC 752. You’ll find it three finger-widths south (RA 01 57 41 Dec +37 47 06) of Gamma Andromedae.

ngc762

Under dark skies, this 5.7-magnitude open cluster can just be spotted with the unaided eye, is revealed in the smallest of binoculars, and can be completely resolved with a telescope. Chances are NGC 752 was discovered by Hodierna over 350 years ago, but it was not cataloged until Sir William gave it a designation in 1786. But give credit where credit is due, for it was Caroline Herschel who observed it on September 28, 1783! Containing literally scores of stars, galactic cluster NGC752 could be well over a billion years old and is strung out in chains and knots in an X pattern over a rich field. Take a close look at the southern edge for orange star 56; although this is a true binary star, the companion you see is merely optical. Enjoy this unsung symphony of stars tonight!

Saturday, December 5, 2009 – No one is certain, but it is believed that Werner Heisenberg was born on this date in 1901! Heisenberg was a physicist and philosopher who discovered a way to formulate quantum mechanics in terms of matrices. His uncertainty principle won him the Nobel Prize for Physics in 1932.

Is it gone yet? Nope. The Moon will rise a little later this evening, but we’re going to run ahead of it tonight and enjoy some studies in Auriga! Looking roughly like a pentagon in shape, Capella is the brightest of these stars. Due south of Capella is the second brightest star, El Nath. By aiming binoculars at El Nath, go north about one-third the distance between the two and enjoy all the stars! You will note two very conspicuous clusters of stars in this area, and so did Le Gentil in 1749.

m38

Binoculars will reveal the pair in the same field, as will telescopes using lowest power. The dimmest of these is M38 (RA 05 28 43 Dec +35 51 18), and it will appear vaguely cruciform in shape. At roughly 4,200 light-years away, the 100 or so fainter members will require larger aperture to resolve.

m36

About 2.5 degrees to the southeast (RA 05 36 12 Dec +34 08 24) you will see the much brighter M36. More easily resolved in binoculars and small scopes, this ‘‘jewel box’’ galactic cluster is quite young and about 100 light-years closer!

Sunday, December 6, 2009 – Today we note the 1848 birth on this date of Johann Palisa. He discovered 122 asteroids with a 600 refractor, and all were visual observations. Check out some asteroids for yourself over the next few days as they approach easily noted objects. You’ll still find the asteroid Psyche close to Jupiter!

The Moon will be along shortly, but we still have time to set our sights about halfway between Theta Aurigae and El Nath. Our study object will be the open cluster M37 (RA 05 52 19 Dec+32 33 12).

m37

Apparently discovered by Messier himself in 1764, this galactic cluster will appear almost nebula-like to binoculars and very small telescopes, but comes to perfect resolution with larger instruments. About 4,700 light-years away and spanning a massive 25 light-years, M37 is often billed as the finest of the three Aurigan open clusters for bigger scopes. Offering beautiful resolvability, this one contains around 150 members down to magnitude 12 and has a total population in excess of 500.

What makes it unique? As you view, you will note the presence of several red giants. For the most part, open clusters are composed of stars that are all about the same age (usually young), but the brightest star in M37 appears orange in color and not blue! So what exactly is going on here? Apparently, some of these big, bright stars have evolved much faster, consuming their fuel at an incredible rate. Other stars in this cluster are still quite young on a cosmic scale, yet they all left the ‘‘nursery’’ at the same time! In theory, this allows us to judge the relative age of open clusters. For example, M36 is around 30 million years old and M38 about 40, but the presence of the red giants in M37 puts its estimated age at 150 million years!

Enjoy the weekend and keep a look out for stray members of the Geminid meteor shower!

This week’s awesome images are (in order of appearance): NGC 752, M38 and M36 (credit—Palomar Observatory, courtesy of Caltech), Johann Palisa (historical image) and M37 (credit—NOAO/AURA/NSF). We thank you so much!

Posted on by Fraser Cain

Mars Global Surveyor

Mars Global Surveyor

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The Mars Global Surveyor was a spacecraft sent to Mars in 1996. It arrived at Mars and studied the planet for 10 years, until it broke down in 2006, and controllers on Earth lost contact with it. But while it was operating, the spacecraft took thousands of images, and made some major discoveries about Mars.

Mars Global Surveyor was launched on November 7, 1996, and made its orbital insertion on September 11, 1997. It used a technique called aerobraking to reduce its orbit and bring it into an orbit that brought it to an average distance of 378 km from the surface of Mars. It circled the planet in a polar orbit once every 117 minutes, which allowed it to photograph the entire Martian surface.

The spacecraft was equipped with 5 major scientific instruments: Mars Orbiter Camera, Mars Orbiter Laser Altimeter, Thermal Emission Spectrometer, Magnetometer and electron reflectometer and the Ultrastable Oscillator for Doppler measurements. These instruments allowed the spacecraft to study the atmosphere and surface composition of Mars. But it also sent back the highest resolution photographs ever seen of Mars. The newer Mars Reconnaissance Orbiter has returned better images with its larger telescope, but when the first MGS images first came back from Mars, they were stunning.

It made some incredible discoveries about Mars. Thanks to the observations from MGS, astronomers determined that Mars had a layered crust that was more than 10 km thick. It found ancient craters that had been buried and then later exposed by erosion, and it found evidence of ancient lava flows.

But perhaps the biggest discovery was made in 2006, which researchers announced that they had uncovered evidence of recent water activity on Mars. Images from the Mars Global Surveyor showed gullies on Mars which looked like they’d been formed by water. It’s possible that water had erupted out of an underground aquifer and spilled down the slope of a hill before evaporating in the pressure of the Martian atmosphere.

After a decade of service, Mars Global Surveyor went silent on November 2, 2006. It went into safe mode after being issued commands to change the orientation of its solar panels, and it stopped communicating. NASA said that it was, “battery failure caused by a complex sequence of events involving the onboard computer memory and ground commands.” But we’ll never really know what happened to it.

We’ve written many articles about the Mars Global Surveyor for Universe Today. Here’s an article about how we lost contact with the Mars Global Surveyor, and here’s a picture of Earth taken by MGS.

If you’d like more info, check out the Mars Global Surveyor homepage.

Source: NASA

Posted on by Jean Tate

What is Absolute Temperature?

If you measure temperature relative to absolute zero, the temperature is an absolute temperature; absolute zero is 0.

The most widely used absolute temperature scale is the Kelvin, symbolized with a capital K, which uses Celsius-scaled degrees (there’s another one, the Rankine, which is related to the Fahrenheit scale). We write temperatures in kelvins without the degree symbol; absolute zero is 0 K.

Another name for absolute temperature is thermodynamic temperature. Why? Because absolute temperate is directly related to thermodynamics; in fact it is the Zeroth Law of Thermodynamics that leads to a (formal) definition of (thermodynamic) temperature.

Roughly speaking, the temperature of an object (or similar, like the gas in a balloon) measures the kinetic energy of the particles (atoms, molecules, etc) of the matter it’s made up of … in an average sense, and macroscopically. Note that blobs of matter have far more energy than just the kinetic energy of the atoms in the blob – there’s the energy that holds the atoms together in molecules (if there are any), the binding energy of the nuclei (unless the blog is pure hydrogen, with no deuterium), and so on; none of these energies are counted in the blob’s temperature.

You might think that at absolute zero a substance would be in its lowest possible energy state, especially if it is a pure compound (or isotopically pure element). Well, it isn’t quite that simple … leaving aside zero point energy (something quite counter-intuitive, from quantum mechanics), there’s the fact that many solids have several different, stable crystal structures (even at 0 K), but only one with minimal energy. Then there’s helium, which is a liquid at 0 K (the solid phase of a substance has a lower energy than the corresponding liquid phase), unless under pressure.

The Kelvin is one of the International System of Units (SI) base units (there are seven of these), and is defined with reference to the triple point of water (“The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water” is the 1967/8 definition; the current one – adopted in 2005 – expands on this to take account of isotropic variations).

Why is it called the Kelvin? Because William Thompson – Lord Kelvin – was the first to describe an absolute temperature scale, in a paper he wrote in 1848; he also estimated absolute zero was -273o C.

Project Skymath has a nice introduction to absolute temperature.

Some Universe Today material you may find interesting: Absolute Zero, Coldest Temperature Ever Created, and Planck First Light.

Sources: Wikipedia, Hyperphysics

Posted on by Jean Tate

Pillars of Creation

One of the Hubble Space Telescope's most famous images, the "Pillars of Creation" in the Eagle Nebula. Credit: NASA/ESA

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The pillars of creation are a part of the emission nebula, or H II region, M16 (also called the Eagle Nebula).

The iconic Hubble Space Telescope image shown here was taken on April Fool’s Day, 1995, using the WFPC2 camera (you can tell it’s that camera from the W-shaped bite taken out of it). It was snapped as part of a research program by Arizona State University’s Jeff Hester and Paul Scowen, and released to the general public on 2 November (i.e. after the proprietary six-month period was over). Embryonic Stars Emerge from Interstellar “Eggs” – that’s the title of the HubbleSite Press Release; “eggs” is a play on EGGs, Evaporating Gas Globules, “dense, compact pockets of interstellar gas“. Interestingly, the name “pillars of creation” is found only in the image title, and nowhere in the Press Release text!

The pillars of creation – and M16 – are about 7,000 light-years away, and each are several light-years long (of course, there’s no “up” in space, so if you turn the image upside down, you see downward hanging linear features … but ‘stalactites of creation’ just isn’t at all catchy).

This region of M16 has been imaged in the x-ray region of the electromagnetic spectrum, by Chandra, in the infrared by Spitzer, and in infrared hi-def from the ground by the ESO’s VLT ANTU telescope.

Hubble has imaged many similar star-forming regions, complete with their own pillars; for example NGC 602 (in the Small Magellanic Cloud; zooming in on this image is fun – can you spot some of the ‘stalactites of creation’?), NGC 6357 (in our own Milky Way, just a tad further away than M16), and a different pillar (“Stellar Spire”) in the Eagle Nebula. Who knows? Maybe, one day, the Horsehead Nebula may become a pillar of creation too!

Universe Today has many articles on these pillars, Shadows Helped Form the “Pillars of Creation”, The Eagle … Has Arrived, Chandra Gives Another Look at the Pillars of Creation, Spitzer’s Version of the Pillars of Creation, and Eagle Nebula’s Pillars Were Wiped Out Thousands of Years Ago.

The Pillars of Creation also feature in Astronomy Cast episodes Nebulae, Stellar Populations, and Stellar Nurseries.

Posted on by Jean Tate

Megaparsec

velocity vs distance, from Hubble's 1929 paper

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A megaparsec is a million parsecs (mega- is a prefix meaning million; think of megabyte, or megapixel), and as there are about 3.3 light-years to a parsec, a megaparsec is rather a long way. The standard abbreviation is Mpc.

Why do astronomers need to have such a large unit? When discussing distances like the size of a galaxy cluster, or a supercluster, or a void, the megaparsec is handy … just as it’s handy to use the astronomical unit (au) for solar system distances (for single galaxies, 1,000 parsecs – a kiloparsec, kpc – is a more natural scale; for cosmological distances, a gigaparsec (Gpc) is sometimes used).

Reminder: a parsec (a parallax of one arc-second, or arcsec) is a natural distance unit (for astronomers at least) because the astronomical unit (the length of the semi-major axis of the Earth’s orbit around the Sun, sorta) and arcsec are everyday units (again, for astronomers at least). Fun fact: even though the first stellar parallax distance was published in 1838, it wasn’t until 1913 that the word ‘parsec’ appeared in print!

As a parsec is approximately 3.09 x 1016 meters, a megaparsec is about 3.09 x 1022 meters.

You’ll most likely come across megaparsec first, and most often, in regard to the Hubble constant, which is the value of the slope of the straight line in a graph of the Hubble relationship (or Hubble’s Law) – redshift vs distance. As redshift is in units of kilometers per second (km/s), and as distance is in units of megaparsecs (for the sorts of distances used in the Hubble relationship), the Hubble constant is nearly always stated in units of km/s/Mpc (e.g. 72 +/- 8 km/s/Mpc, or 72 +/- 8 km s-1 Mpc-1 – that’s its estimated value from the Hubble Key Project).

John Huchra’s page on the Hubble constant is great for seeing megaparsecs in action.

Given the ubiquity of megaparsecs in extragalactic astronomy, hardly any Universe Today article on this topic is without its mention! Some examples: Chandra Confirms the Hubble Constant, Radio Astronomy Will Get a Boost With the Square Kilometer Array, and Astronomers Find New Way to Measure Cosmic Distances.

Questions Show #7, an Astronomy Cast episode, has megaparsecs in action, as does this other Questions Show.

Posted on by Jon Voisey

How Galaxies Lose Their Gas

Galaxy mergers, such as the Mice Galaxies will be part of Galaxy Zoo's newest project. Credit: Hubble Space Telescope
The Mice galaxies, merging. Credit: Hubble Space Telescope

As galaxies evolve, many lose their gas. But how they do this is a point of contention. One possibility is that it is used to form stars when the galaxies undergo intense periods of star formation known as starburst. Another is that when large galaxies collide, the stars pass through one another but the gas gets left behind. It’s also possible that the gas is pulled out in close passes to other galaxies through tidal forces. Yet another possibility involves a wind blowing the gas out as galaxies plunge through the thin intergalactic medium in clusters through a process known as ram pressure.

A new paper lends fresh evidence to one of these hypotheses. In this paper, astronomers from the University of Arizona were interested in galaxies that displayed long gas tails, much like a comet. Earlier studies had found such galaxies, but it was unclear whether or not this gas tail was pulled out from tidal forces, or pushed out from ram pressure.

To help determine the cause of this the team used new observations from Spitzer to look for subtle differences in the causes of a tail following the galaxy ESO 137-001. In cases where tails are known to be pulled out tidally (such as in the M81/M82 system), there “is no physical reason why the gas would be preferentially stripped over stars.” Stars from the galaxy are pulled out as well and often large amounts of new star formation are induced. Meanwhile, ram pressure tails should be largely free of stars although some new star formation may be expected if there is turbulence in the tail which causes regions of higher density (think like the wake of a boat).

Examining the tail spectroscopically, the team was unable to detect the presence of large numbers of stars suggesting tidal processes were not responsible. Furthermore, the disk of the galaxy seemed relatively undisturbed by gravitational interactions. To support this, the team calculated the relative strengths of the forces acting on the galaxy. They found that, between the tidal forces acting on the galaxy from its parent cluster, and its own centripetal forces, the internal forces where greater, which reaffirmed that tidal forces were an unlikely cause for the tail.

But to confirm that ram pressure was truly responsible, the astronomers looked at other parameters. First they estimated the gravitational force for the galaxy. In order to strip the gas, the force generated by the ram pressure would have to exceed the gravitational one. The energy imparted on the gas would then be measurable as a temperature in the gas tail which could be compared to the expected values. When this was observed, they found that the temperature was consistent with what would be necessary for ram stripping.

From this, they also set limits on how long gas could last in such a galaxy. They determined that in such circumstances, the gas would be entirely stripped from a galaxy in ~500 million to 1 billion years. However, because the density of the gas through which the galaxy would slowly become denser as it passed through the more central regions of the cluster, they suggest the timescale would be much simpler. While this timescale say seem long, it is still shorter than the time it takes such galaxies to make a full orbit in their cluster. As such, it is possible that even in one pass, a galaxy may lose its gas.

If the gas loss occurs on such short timescales, this would further predict that tails like the one observed for ESO 137-001 should be rare. The authors note that an “X-ray survey of 25 nearby hot clusters only discovered 2 galaxies with X-ray tails.”

Although this new study in no way rules out other methods of removing a galaxy’s gas, this is one of the first galaxies for which the ram stripping method is conclusively demonstrated.

Source:

A Warm Molecular Hydrogen Tail Due to Ram Pressure Stripping of a Cluster Galaxy

Posted on by Jean Tate

Exobiology

Exobiology (same thing as astrobiology) is about life in space (on other planets, and moons; in other solar systems): where it is, what it is, how it started, and how it evolved (all studied scientifically, of course). Because the origin of life right here on Earth, and its early evolution, is essentially unknown, and because of the distinct possibility of similiarities with the origin (and early evolution) of life elsewhere in the universe, exobiology includes research into abiogenesis (and early, and extreme, life on Earth).

Exobiology is very much a multi-disciplinary field, drawing on biology, chemistry, geology (and planetary science), physics, and astronomy.

Because we have a sample of just one – life on Earth – it is difficult to make anything but the most general decisions on what lines of exobiology research are likely to be productive (keep in mind that null results can, of course, be quite productive). Conservatively, looking for planets like Earth in orbit around stars like the Sun (in age as well as mass, metallicity, etc), and looking for clues for fossil life in planetary environments like those found today on Earth (e.g. early Mars) seem better options than investigating possible silicon-based life (to take just one example).

As the number of exosolar (or extrasolar) planetary systems known continues to grow, quickly, discovering the prevalence of Earth-mass planets, in goldilocks orbital zones, seems like a good idea … so today we have the Kepler mission and COROT.

As the early Mars becomes better understood – and the widespread distribution of liquid water then – so today we have plans for the Mars Science Laboratory and ExoMars (the discovery of methane in the Martian atmosphere certainly spurs such developments).

Less conservatively, the discovery of life around black smokers and sites like Lost City (not to mention entire ecosystems within crustal rocks … several km beneath the surface) sparked interest in the possibility of life in Europa, on Titan, even Enceladus (life – albeit rather simple life – we now know does not need to depend, ultimately, on the Sun’s (or another star’s) radiant energy … think chemolithoautotrophs).

Did you know that NASA has an exobiology branch? Check it out! Duke University’s Chemistry Department has an interesting Introduction to Exobiology you might find interesting too.

Universe Today stories on exobiology? Yep, lots; here’s a random selection: Martian Explorers Should Be Looking for Fossils, Did Life Arrive Before the Solar System Even Formed?, Extremophile Hunt Begins in Antarctica, Implications for Exobiologists , and New Targets to Search for Life on Europa.

Any Astronomy Cast episodes on exobiology? Yep … but it’s called Astrobiology.

Sources: NASA, ESA

Posted on by Jean Tate

Asterism

Kemble's Cascade (Credit: Walter MacDonald)

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The Big Dipper is an asterism (well-known to those who live in the northern hemisphere), so is the False Cross (well-known to those who live in the southern hemisphere). Asterisms are easily recognized pattern of *s*t*a*r*s* (but not a constellation).

The sky is full of asterisms easily seen without a telescope or binoculars: Summer Triangle, Great Square of Pegasus, the W in Cassiopeia, Frying Pan, Orion’s Belt, … it’s a long list.

The Southern Cross is not an asterism, strictly speaking, because it’s a constellation (Crux).

An asterism can take in parts of more than one constellation; for example, the Square of Pegasus has three stars in Pegasus (the three brightest, alpha, beta, and gamma Peg), and one in Andromeda (alpha And).

Some well-known asterisms are visible only through a telescope or binoculars; for example the Coathanger, and Kemble’s Cascade.

A couple (at least) of open clusters are also asterisms – the Hyades and the Pleiades (also known as the Seven Sisters).

Some clear, fixed features in the night sky, with well-known names, are not asterisms or constellations … the Coalsack for example, is a dark cloud in the plane of the Milky Way which blocks its light, and the Magellanic Clouds are dwarf, satellite galaxies of our own.

As astronomy in many cultures developed independently of the West (ancient Greece, Rome, etc), many of the commonly recognized constellations in those cultures correspond to asterisms … see if you can recognize some of the Chinese ones!

A particularly interesting kind of constellation is the dark constellation; instead of joining up bright stars to make an easily recognized figure, some cultures linked various dark nebulae in the Milky Way; for example the Emu in the Sky of the Australian Aborigines (and no, these are not asterisms).

SEDS (Students for the Exploration and Development of Space) has a concise list of asterisms easily visible without binoculars, or a telescope (though you may have to go to the opposite hemisphere to see them all!).

Asterisms are mentioned in many of Universe Today’s Weekend SkyWatcher’s Forecasts (August 21-23, 2009, for example), in its articles on Constellations (e.g. Orion), and Kids Astronomy ones (e.g. Finding the Summer Triangle).

Posted on by Fraser Cain

Blood Moon



A blood moon is the first full moon after a harvest moon, which is the full moon closest to the fall equinox. Another name for a blood moon is a hunter’s moon.

Before the advent of electricity, farmers used the light of the full moons to get work done. The harvest moon was a time they could dedicate to bringing in their fall harvest. And so a month later is the blood moon, or the hunter’s moon. This was a good time for hunters to shoot migrating birds in Europe, or track prey at night to stockpile food for Winter.

A full moon occurs every 29.5 days, so a blood moon occurs about a month after the harvest moon. A blood moon is just a regular full moon. It doesn’t appear any brighter or redder than any other full moon. The distance between the Earth and the Moon can change over the course of the month. When the moon is at its closest, a full moon can appear 10% larger and 30% brighter than when it’s further away from the Earth.

A blood moon will actually turn red when it matches up with a lunar eclipse. These occur about twice a year, so blood moons match up with lunar eclipses about every 6 years or so. At the time of this writing, the next blood moon lunar eclipse will be in 2015.

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 a lava tube on the Moon.

If you’d like more info on the Moon, check out NASA’s Solar System Exploration Guide on the Moon, and here’s a link to NASA’s Lunar and Planetary Science page.

We’ve also done several episodes of Astronomy Cast about the Moon. Here’s a good one, Episode 17: Where Does the Moon Come From?