Universe Today Audio

Have you ever noticed that the Moon always looks the same? Sure, the phase changes, but the actual features on the Moon always look the same from month to month. Does the Moon rotate? What's going on? From our perspective here on Earth, the Moon always shows us the same face because it's tidally locked to our planet. At some point in the distant past, the Moon did rotate from our perspective, but the Earth's gravity kept pulling unevenly at the Moon, slowing its rotation. Eventually the Moon locked into place, always displaying the same side to us. But if you looked down on the Earth-Moon system from the north celestial pole, from the perspective of Polaris, the North Star, you'd see that the Moon actually does rotate on its axis. In fact, as the Moon travels around the Earth in a counter-clockwise orbit every 27.5 days, it also completes one full rotation on its axis - also moving in a counter-clockwise direction. If you look at a time lapse animation of the Moon moving entirely through its phases over the course of a month, you'll notice a strange wobble, as if the Moon is rocking back and forth on its axis a bit. This is known as libration. On average, the Moon is tidally locked to the Earth's surface. But its actual orbit is elliptical, it moves closer and then more distant from the Earth. When the Moon is at its closest point, it's rotation is slower than its orbital speed, so we see an additional 8 degrees on its eastern side. And then when the Moon is at the most distant point, the rotation is faster than its orbital speed, so we can see 8 degrees on the Western side. Libration allowed astronomers to map out more of the Moon's surface than we could if the Moon followed a circular orbit. Until the space age, half the Moon was hidden from us, always facing away. This hemisphere of the Moon was finally first observed by the Soviet Luna 3 probe in 1959, followed by the first human eyes with Apollo 8 in 1968. The two hemispheres of the Moon are very different. While the near side is covered with large basaltic plains called maria, the far side is almost completely covered in craters. The reasons for this difference is still a mystery to planetary scientists, but it's possible that a second Moon crashed into it, billions of years ago, creating the strange surface we see today. So yes, the Moon does rotate. But its rotation exactly matches its orbit around the Earth, which is why it looks like it never does. You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Life on Earth got you down? Thinking you'd like to pick up and move to another planet? I've got bad news for you. Without protection, there's no place in the entire Solar System that wouldn't kill you in few seconds. You're looking at scorching temperatures, poisonous atmospheres, crushing gravity, bone chilling cold, a complete lack of oxygen, killer radiation, and more. The entire Solar System is hostile to life as we know it. If we had to choose from a range of terrible options, what would be the most Earthlike place in the Solar System? We would want a world that has a similar gravity, similar atmospheric pressure and composition, protection from radiation, and a comfortable temperature. Just like the Earth. Let's look at a few candidates: The Moon looks good. It's close and... well, it's close. It's an airless world, so you'd need a spacesuit. Low gravity is bad news for your bones, which will lose mass and become brittle. Temperatures range from freezing cold to scorching hot, and there's no atmosphere or significant magnetic field to protect you from the radiation of space. While we're suggesting moons, how about Titan, Saturn's largest Moon? It's only 15% of Earth's gravity, and the temperatures dip down to minus -179 degrees C; cold enough that it rains liquid methane. Even though the atmosphere is unbreathable, the good news is that the pressure is only a little higher than Earth's. Which means you wouldn't need a pressurized spacesuit, just a really, really warm coat. How about Mars, the target of so many colonization plans and sci fi adventures? The gravity of Mars is only 38% the gravity of Earth; and we don't know what effect a long stay in this gravity would have on the human body. The atmosphere is poisonous carbon dioxide, and the pressure is less than 1% of sea level on Earth. So, you'd better pack a spacesuit. The temperatures can rise as high as a comfortable 35 degrees C, but then plunge down to -143 degrees C at the poles. One big problem with Mars is a total lack of magnetosphere. Radiation from space would be a constant hazard for anyone on the surface of the planet. Perhaps another planet? How about Venus? On the surface, it's right out of the running. The temperature is an oven-like 462 degrees C, with a surface pressure 92 times more than Earth. The atmosphere is almost entirely carbon dioxide, with clouds of sulphuric acid. On the plus side, it has gravity roughly similar to Earth, and a thick atmosphere that would protect you from radiation. Unfortunately, you'd die faster on the surface of Venus than almost anywhere else in the Solar System. But... there is a place on Venus that's downright lovely. Up in the clouds. Amazingly, if you rise up through the clouds of Venus to an altitude of 50-60 kilometers, the atmospheric pressure and temperature are the same as on Earth. The atmosphere would still be toxic carbon dioxide, but breathable air would be a "lifting gas" on Venus. You could float around the skies of Venus in a balloon made of breathable air. Stand out on the deck of your Venusian sky city in shorts and a T-shirt, soaking up the sunlight in regular Earth gravity. Sounds idyllic, right? So, opinions will vary. Some think Mars is the most Earthlike place in the Solar System, but in my opinion, the clouds of Venus are the place to go. I'll see you there. Related Sources Colonization of Venus MarsOne Mission Pros and Cons of Colonizing the Moon

There is almost nothing that could completely destroy the earth. Follow your instincts and ignore anyone raising alarms about its imminent demise. Oh sure, there’s a pile of events that could make life more difficult, and a laundry list of things that could wipe out all of humanity. Including: asteroid strikes, rising temperatures, or global plagues In order to actually destroy the Earth, you would need significantly more energy, and there just happens to be enough, a short 150 million kilometers away: the Sun. The Sun has been in the main sequence of its life for the last 4.5 billion years, converting hydrogen into helium. For stars this massive, that phase lasts for about 10 billion years, meaning we’re only halfway through. When the Sun does finally run out of hydrogen to burn, it’ll begin fusing helium into carbon, expanding outward in the process. It will become a cooler, larger, red giant star, consuming the orbits of Mercury and Venus. Scientists are still unsure if the red giant phase of the Sun will consume the Earth. If it does, the Earth’s story ends there. It’ll get caught up inside the Sun, and spiral inward to its demise. Death by red giant in 5.5 billion years. If the Sun doesn’t consume the Earth then we’ll have a long, cold future ahead of us. The Sun will shrink down to a white dwarf and begin cooling down to the background temperature of the Universe. The Earth and the rest of the surviving planets will continue orbiting the dying Sun for potentially trillions of years. If we’re exceedingly lucky, the Sun will get too close to another star, and the gravitational interactions will capture Earth in orbit, giving our planet a second chance for life. If not, the Earth will continue following the dying Sun around and around the Milky Way for an incomprehensible amount of time. At this point, the main risk to the planet is a collision. Or maybe it’ll spiral inward over vast periods of time to be destroyed by the Sun, or collide with another planet. Or perhaps the entire Solar System will slowly make its way into the supermassive black hole at the center of the Milky Way. One last possibility. Physicists think that protons - the building blocks of atoms - might eventually decay, becoming smaller particles and pure energy. After an undecillion years - a 1 followed by 36 zeros - half of the Earth will have just melted away into energy. But if protons don’t decay, the Earth could theoretically last forever. The bottom line, the Earth was built to last.

Look up into the night sky and count the moons. You can see only one moon, "the" Moon. But does the Earth have any other moons? Around the Solar System, multiple moons are the rule. Jupiter has 67 natural satellites, even Mars has two asteroid-like moons. Could Earth have more than one? Officially, the answer is no. The Earth has a single moon. Today. It's possible Earth had more than one moon in the past, millions or even billions of years ago. Strange terrain on the far side of the Moon could be explained by a second moon crashing into it, depositing a layer of material tens of kilometers deep. Moons could come and go over the billions of years of the Earth's history. For example, Mars has two Moons, but not for long. Phobos, the larger moon, is spiraling inward and expected to crash into the planet within the next 10 million years. And so, in the future, Mars will only have a single Moon, Deimos. It's also possible that the Earth might capture a Moon in the future. Neptune's largest moon, Triton, orbits in the opposite direction from the rest of the moons around the planet. This suggests that Triton was actually a captured Kuiper Belt Object which strayed too close to the planet. In fact, we did capture a 5-metre asteroid called 2006 RH120. It orbited the Earth four times during 2006/2007 before getting ejected again. So we can assume events like this have happened in the past. Additionally, we might have more moons, but they haven't been discovered yet because they're just too small. Researchers have calculated that there could be meter-sized asteroids in orbit around the Earth, remaining in orbit for hundreds of years before gravitational interactions push them out again. And there are other objects that interact with Earth's orbit in strange ways. Scientists don't consider them moons, but they do stick around in our neighbourhood: Asteroid 3753 Cruithne is in an orbital resonance with the Earth. It has a highly eccentric orbit, but takes exactly one year to orbit the Sun. From our perspective, it follows a slow, horse-shoe shaped path across the sky. Since the discovery of Cruithne in 1986, several other resonant near-Earth objects have been discovered. There's 2010 TK7, the Earth's only known Trojan asteroid. It leads the Earth in the exact same orbit around the Sun, in a gravitationally stable point in space. So, the answer... Earth only has a single Moon. Today. We might have had more moons in the past, and we might capture more in the future, but for right now... enjoy the one we've got. Want to learn more? Here are some articles on Universe Today we've written about this topic: What are some objects known as Earth's other moons? Did Earth have more than one moon in the past? Does Earth have many tiny moons? You might also enjoy this episode of Astronomy Cast: Where did the Moon come from?

If you could travel from world to world, from star to star, out into the gulfs of intergalactic space, you’d move away from the warmth of the stars into the vast and cold depths of the void. Better pack a sweater, it’s going to get cold. But, how cold? How cold is space? Unlike your house, car, or swimming pool, the vacuum of space has no temperature. So, how cold is space? That’s a nonsense question. It’s only when you put a thing in space, like a rock, or an astronaut, that you can measure temperature. Remember there are three ways that heat can transfer: conduction, convection and radiation. Heat up one side of a metal bar, and the other side will get hot too; that’s conduction. Circulating air can transfer heat from one side of the room to another; that’s convection. But out in the vacuum of space, the only way heat can transfer is radiation. Photons of energy get absorbed by an object, warming it up. At the same time, photons are radiating away. If the object is absorbing more photons than it emits, it heats up. And if it emits more photons than it absorbs, it cools down. There is a theoretical point at which you can’t extract any more energy from an object, this minimum possible temperature is absolute zero. As we’ll see in a second, you can never get there. Let’s look close to home, in orbit around the planet, at the International Space Station. A piece of bare metal in space, under constant sunlight can get as hot as two-hundred-sixty (260) degrees Celsius. This is dangerous to astronauts who have to work outside the station. If they need to handle bare metal, they wrap it in special coatings or blankets to protect themselves. And yet, in the shade, an object will cool down to below -100 degrees Celsius. Astronauts can experience vast differences in temperature between the side facing the Sun, and the side in shadow. Their spacesuits compensate for this using heaters and cooling systems. Let's talk a little further out. As you travel away from the Sun, the temperature of an object in space plummets. The surface temperature of Pluto can get as low as -240 Celsius, just 33 degrees above absolute zero. Clouds of gas and dust between the stars within our galaxy are only 10 to 20 degrees above absolute zero. And if you travel out far away from everything in the Universe, you can never get lower than a minimum of just 2.7 Kelvin or -270.45 Celsius. This is the temperature of the cosmic microwave background radiation, which permeates the entire Universe. In space? It’s as cold as it can get. Want more resources? We've recorded an episode of Astronomy Cast all about temperature. Here are some other articles we've done: Mars is Really Cold Cold Dark Matter in the Early Universe

The average distance to the Moon is 384,403 km (238,857 miles). Before you put this answer into your homework you've got to understand that the Moon takes an elliptical path around the Earth. That number, 384,403 km, is an average distance that astronomers call the semi-major axis. The Moon can get closer to the Earth and it can get further. At its closest point, known as the perigee, the Moon is only 363,104 km (225,622 miles). And at its most distant point, called apogee, the Moon gets to a distance of 406,696 km (252,088 miles). (Here's a trick to remembering which is which - "apogee", starts with "A", just like going "away"). You can see that the distance from Earth to Moon can vary by 43,592 km. That's a pretty big difference, and it can make the Moon appear dramatically different in size depending on where it is in its orbit. For example, take a look at this animation from the Goddard Space Flight Center Scientific Visualization Studio which shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year (this was created for 2011), at hourly intervals. When it’s a Full Moon, and it’s a close Moon, that’s a Supermoon; also known as a perigee-syzygy. The total illumination from the Moon can change more than thirty percent from Full Moon to Full Moon, depending on how far away the Moon is when things line up. So, how do we know how far away the Moon is? Astronomers calculate the distance to the Moon using the Lunar Laser Ranging experiment. When astronauts visited the Moon more than forty years ago, they left retroreflecting mirrors on the lunar surface. When they shoot a laser at the Moon, the light from the laser is reflected right back at you from one of these devices. For every 100 quadrillion photons shot at the Moon, only a handful come back, but that’s enough. The light is moving at almost 300,000 kilometers per second, it takes a little more than a second to make the journey. And then it takes another second or so to return. Astronomers calculate the exact amount of time it takes for light to make the journey. They know exactly how far the Moon is at any time, down to millimeter accuracy. As a result, Astronomers have discovered that the Moon is slowly drifting away from us, at a glacial 3.8 centimeters each year. Billions of years in the future, the Moon will appear smaller in the sky than it does today. Within a billion years or so, the Moon will be visually smaller than the Sun, and we won't see total solar eclipses any more. We have recorded a whole episode of Astronomy Cast focused just on the Moon. Give it a listen.

Consider this. The Universe is enormous. There are as many as four-hundred billion stars in our galaxy: the Milky Way. And there are more than one-hundred-and-seventy billion galaxies in the observable Universe. Most of those stars have planets, and many of those planets have got to contain useful minerals and fall within their star’s habitable zone where liquid water is present. The conditions for life are probably everywhere. But where are all the aliens? And think about this. The Universe has been around for 13.8 billion years. Human beings originated 200,000 years ago, so we’ve only been around for 0.01% of the age of the Universe. An intelligent species could arise on any one of those countless worlds, and broadcast their existence to the entire galaxy. Once a species developed interstellar travel, they could completely colonize our galaxy within a few tens of millions of years; just a heartbeat in the age of the Universe. So where are they? As far as we know, Earth is the only place in the Universe where life has arisen, let alone developed an intelligent civilization. This baffling contradiction is known as the Fermi Paradox, first described in 1950 by the physicist Enrico Fermi. Scientists have been trying to resolve this mystery for decades, listening for radio signals from other worlds. We’ve only sampled a fraction of the radio spectrum, and so far, we haven’t detected anything that could be a signal from an intelligent species. How can we explain this? Maybe we really are the only planet in the entire Universe to develop life. Maybe we’re the first civilization to reach this level of advancement in the entire galaxy. But with so many worlds out there, that really seems unlikely. Maybe civilizations destroy themselves when they reach a certain point. Nuclear weapons, global warming, killer epidemics, and overpopulation could all end humanity. Asteroids could strike the planet and wipe us out. But would this happen to every single civilization? one-hundred-percent of them? Even if ninety-nine-percent of civilizations destroy themselves, we’d still have a couple that made it through and fully colonized the galaxy. Maybe they’re just too far away, and our signals can’t reach each other. But then, self-replicating probes could traverse those distances and leave a local artifact in every single star system. Maybe we can’t understand their signals or recognize their artifacts. Maybe, but if aliens constructed a series of artifacts on Earth, I think we’d notice them. The aliens would have experience creating obvious structures. Maybe they’re just too alien and we just can’t understand them. Maybe we’re too insignificant, and they don’t think we’re even worth talking to. We don’t need to talk to them to know they exist. If they flew through our Solar System, ignoring us, we’d still know they’re around. Maybe they’re not talking to us on purpose, and we’re really in some kind of galactic zoo. Or aliens have a Prime Directive, and they’re not allowed to talk to us. Again, all the aliens? Not a single one has gotten through and snuck us some evidence? There are many other potential solutions to the Fermi Paradox, but I personally find them all insufficient. The Universe is big, and old, and if extraterrestrial life is anything like us, it wants to multiply and spread out. Perhaps the most unsettling thought is that something happens to 100% of intelligent civilizations that prevents them from exploring and settling the galaxy. Maybe something good, like the discovery of a transportation system to another Universe. Or maybe something bad, like a destructive technology that has destroyed every single civilization before us. How do you feel about the Fermi Paradox? How do you resolve the contradictions? Whatever the solution, it’s really fun to think about. We've recorded a couple of episodes of Astronomy Cast about the Drake Equation and the Fermi Paradox,

Earth is the third planet from the Sun, and the climate here is just right for life. Here in our Solar System, there are planets both hotter and colder than Earth. So... which one is the hottest? You might think it’s Mercury, the planet closest to the Sun. Mercury orbits at a distance of only 58 million kilometers, travelling in a blast-furnace of scorching radiation. Its temperature can skyrocket to 700 Kelvin, or 426 degrees Celsius on the sunward side. In the shadows, temperatures plunge down to 80 Kelvin, which is -173 degrees Celsius Mercury sure is hot, but Venus is hotter. Venus is much further from the Sun, orbiting at a distance of more than 108 million kilometers. Average temperature there is a hellish 735 Kelvin, or 462 degrees Celsius - hot enough to melt lead. Venus remains that same temperature no matter where you go on the planet. At the North Pole? 735 Kelvin. At night? 735 Kelvin. Daytime at the equator? You get the point. So, why is Venus so much hotter than Mercury, even though it’s further away from the Sun? It’s all about the atmosphere. Mercury is an airless world, not unlike the Moon. Venus, has a very thick atmosphere of CO2, which adds incredible pressure, and traps in the heat. Consider our own planet. When you stand at sea level on Earth, you’re experiencing one atmosphere of pressure. But if you could stand on the surface of Venus - and trust me, you don’t want to - you’d experience ninety-two times as much atmospheric pressure. This is the same kind of pressure as being a kilometer underneath the surface of the ocean. Venus also shows us what happens when carbon dioxide levels just keep on rising. Radiation from the Sun is absorbed by the planet, and the infrared heat emitted is trapped by the carbon dioxide, which creates a runaway greenhouse effect. You might think a planet this hot with such extreme temperature and pressure, would be impossible to explore. And if you did, you’d be wrong. The Soviets sent a series of spacecraft called Venera, which parachuted down through the thick atmosphere and returned images from the surface of Venus. Although the first few missions were failures, this taught the Soviets just how hellish the Venusian environment really is. Venera 13 made it down to the surface in nineteen-eighty-one and survived for one-hundred-and-twenty-seven minutes, sending back the first color pictures of Venus’ surface. The hottest planet in our solar system is Venus, When it comes to temperature, distance from the Sun matters, but it takes a backseat to wrapping a planet in a atmospheric blanket of carbon dioxide. We release this explainer videos every Monday and Thursday from the Universe Today YouTube Channel. Click here to Subscribe to the channel.

This is a classic trick question. Ask a friend, “what is the closest star?” and then watch as they try to recall some nearby stars. Sirius maybe? Alpha something or other? Betelgeuse? The answer, obviously, is the Sun; that massive ball of plasma located a mere 150 million km from Earth. Let’s be more precise; what’s the closest star to the Sun? You might have heard that it’s Alpha Centauri, the third brightest star in the sky, just 4.37 light-years from Earth. But Alpha Centauri isn’t one star, it’s a system of three stars. First, there's a binary pair, orbiting a common center of gravity every 80 years. Alpha Centauri A is just a little more massive and brighter than the Sun, and Alpha Centauri B is slightly less massive than the Sun. Then there’s a third member of this system, the faint red dwarf star, Proxima Centauri. It’s the closest star to our Sun, located just a short 4.24 light-years away. Alpha Centauri is located in the Centaurus constellation, which is only visible in the Southern Hemisphere. Unfortunately, even if you can see the system, you can’t see Proxima Centauri. It’s so dim, you need a need a reasonably powerful telescope to resolve it. Let’s get sense of scale for just how far away Proxima Centauri really is. Think about the distance from the Earth to Pluto. NASA’s New Horizons spacecraft travels at nearly 60,000 km/h, the fastest a spacecraft has ever traveled in the Solar System. It will have taken more than nine years to make this journey when it arrives in 2015. Travelling at this speed, to get to Proxima Centauri, it would take New Horizons 78,000 years. Proxima Centauri has been the nearest star for about 32,000 years, and it will hold this record for another 33,000 years. It will make its closest approach to the Sun in about 26,700 years, getting to within 3.11 light-years of Earth. After 33,000 years from now, the nearest star will be Ross 248. What About the Northern Hemisphere? For those of us in the Northern Hemisphere, the closest visible star is Barnard’s Star, another red dwarf in the constellation Ophiuchus. Unfortunately, just like Proxima Centauri, it’s too dim to see with the unaided eye. The closest star that you can see with the naked eye in the Northern Hemisphere is Sirius, the Dog Star. Sirius, has twice the mass and is almost twice the size of the Sun, and it’s the brightest star in the sky. Located 8.6 light-years away in the constellation Canis Major - it’s very familiar as the bright star chasing Orion across the night sky in Winter. How do Astronomers Measure the Distance to Stars? They use a technique called parallax. Do a little experiment here. Hold one of your arms out at length and put your thumb up so that it’s beside some distant reference object. Now take turns opening and closing each eye. Notice how your thumb seems to jump back and forth as you switch eyes? That’s the parallax method. To measure the distance to stars, you measure the angle to a star when the Earth is one side of its orbit; say in the summer. Then you wait 6 month, until the Earth has moved to the opposite side of its orbit, and then measure the angle to the star compared to some distant reference object. If the star is close, the angle will be measurable, and the distance can be calculated. You can only really measure the distance to the nearest stars this way, since it only works to about 100 light-years. The 20 Closest Stars Here is a list of the 20 closest star systems and their distance in light-years. Some of these have multiple stars, but they’re part of the same system. Alpha Centauri – 4.2 Barnard’s Star – 5.9 Wolf 359 – 7.8 Lalande 21185 – 8.3 Sirius – 8.6 Luyten 726-8 – 8.7 Ross 154 – 9.7 Ross 248 – 10.3 Epsilon Eridani – 10.5 Lacaille 9352 – 10.7 Ross 128 – 10.9 EZ Aquarii – 11.3 Procyon – 11.4 61 Cygni – 11.4 Struve 2398 – 11.5 Groombridge 34 – 11.6 Epison Indi – 11.8 Dx Carncri – 11.8

This article comes from our archive, but we updated it with this video. Saturn is my absolute favorite object in the night sky. When I was a child, I had a dog-eared book on the Solar System, which I read over and over, stopping and staring with wonder at the section on Saturn. How could a planet have rings of ice? What would it be like to fly out and visit the planet, to see the rings with your own eyes. How did it get all those strange moons? When I was 14, I purchased my first telescope, a 4-inch Newtonian from a local company in Vancouver. It was summer, and one of the first planets, appearing just after sunset was Saturn. And my telescope had just enough power and magnification to resolve the planet and its famous rings. In fact, when I first looked at Saturn through the eyepiece, I couldn't believe that I was now seeing the planet with my own eyes. It didn't look quite like the photographs, but my imagination could fill in the gaps. From those first observations, my fascination with astronomy and Saturn only grew, leading me to a career in science journalism. It's funny to think how far I've come, and how I can trace everything back to those warm summer nights, looking at Saturn. Think you know everything about Saturn? Think again. Here are 10 facts about Saturn, some you may know, and some you probably didn't know. 1. Saturn is the least dense planet in the Solar System Saturn has a density of 0.687 grams/cubic centimeter. Just for comparison, water is 1 g/cm3 and the Earth is 5.52. Since Saturn is less dense than water, it would actually float like an apple if you could find a pool large enough. Of course, why you'd want to ruin a pool with all that hydrogen, helium and ices... 2. Saturn is a flattened ball Saturn spins so quickly on its axis that the planet flattens itself out into an oblate spheroid. Seriously, you see this by eye when you look at a picture of Saturn; it looks like someone squished the planet a little. Of course, it's the rapid spinning that's squishing it, causing the equator to bulge out. While the distance from the center to the poles is 54,000 km, the distance from the center to the equator is 60,300 km. In other words, locations on the equator are approximately 6,300 km more distant from the center than the poles. We have a similar phenomenon here on Earth, where points on the equator are more distant from the center of the Earth, but on Saturn, it's much more extreme. 3. The first astronomers thought the rings were moons. When Galileo first turned his rudimentary telescope on Saturn in 1610, he could see Saturn and its rings, but he didn't know what he was looking at. He though that the rings might actually be two large moons stuck to either side of Saturn - ears maybe? It wasn't until 1655 that the Dutch astronomer Christian Huygens used a better telescope to observe Saturn. He had the resolution to realize that the moons on either side of Saturn were actually rings: "a thin, flat ring, nowhere touching, and inclined to the ecliptic." Huygens was also the first person to discover Saturn's largest moon, Titan. 4. Saturn has only been visited 4 times by spacecraft Only 4 spacecraft sent from Earth have ever visited Saturn, and three of these were just brief flybys. The first was Pioneer 11, in 1979, which flew within 20,000 km of Saturn. Next came Voyager 1 in 1980, and then Voyager 2 in 1981. It wasn't until Cassini's arrival in 2004 that a spacecraft actually went into orbit around Saturn and captured photographs of the planet and its rings and moons. Unfortunately, there are no future plans to send any more spacecraft to Saturn. A few missions have been proposed, including such radical concepts as a sailboat that could traverse the liquid methane lakes on Titan. 5. Saturn has 62 moons Jupiter has 67 discovered moons, but Saturn is a close second with 62. Some of these are large, like Titan, the second largest moon in the Solar System.

This is an article from our archive, but we've updated it with this spiffy video. Every now and then I go looking for a free aerial view of my home. It's amazing what's available through the internet now, totally free. Thanks to commercial Earth observation satellites, and internet tools that make these photos accessible through the internet, it's easy to see your house from space. In our modern space age, there are more than 8,000 satellites currently orbiting the Earth. The vast majority of these are relaying data to and from the Earth, and many are equipped with high power cameras. Just look up into the sky any night, and you're sure to see satellite after satellite passing overhead. But what are some ways you can get access to these satellite and aerial images of your house? Satellite Images of the Whole Earth If you want to go way out and just see a satellite image of the entire planet, there are some solutions for you: weather satellites. For example, NOAA's Geostationary Operational Environmental Satellites (GOES) release images of an entire hemisphere of planet Earth every 3 hours. From these images you can see major weather patterns affecting parts of the Earth. But you really can't see any specific spot on Earth with any detail. What is really cool about these satellite views is that they're live. The weather systems you're seeing in those images are happening on the planet right now. If you don't want a live view, and really just want to see a beautiful view of the Earth's hemisphere, check out these images produced by NASA. Here's a composite photograph that shows the Earth's Western Hemisphere, and here's a view of the Earth's Eastern Hemisphere. There were also some amazing new satellite images of the Earth released from the European Space Agency's 3rd generation Meteosat spacecraft. Zoom in. Let's see Satellite Pictures of Houses If those whole Earth pictures don't give you enough detail, let's zoom in, and see some pictures of houses from space. The best tool on the market, in my opinion, is the service from Google Maps. All you need is a web browser and a connection to the internet. When you first start up, Google Maps displays a satellite view of North America. You can then zoom in, or pan the camera around to see any location on Earth. You can also type in the address of the location that you want to see. Once you do that, you'll get a free satellite view of your house. You can save the image or print it off. View Larger Map Another cool tool from Google is Google Earth. You can access by going to http://earth.google.com. The main difference between Google Maps and Google Earth is that you have to download and install Earth on your local computer (they have a version for PCs, Mac, Linux, and even the iPhone). Once you've downloaded and installed Google Earth, you can see a 3-dimensional view of Earth that you can zoom in and out and spin around. You can type in your address and get a view of your house from above. I actually like the printing function of Google Earth better, since it's using your printer directly, and not going through the web browser. And if you really hate using products from Google, no problem. There are similar services from Yahoo and Microsoft. Microsoft's mapping service used to be called MSN Maps, and now it's been changed to Bing Maps with their new identity. The Yahoo service is called Yahoo Maps, and it's very similar to Google Maps. The two services do have some big differences, though, and there's a cool application that lets you see the two of them side-by-side. I used it for my home and found that Google Maps has better resolution maps for my city. Where Do All these Pictures Come From? Google Maps and the other internet mapping services are really just customers for the satellite services that actually take these photographs from space. There are a few major services on the market, including GeoEye.

When we look at the night sky, filled with stars, it’s hard to resist counting. Just with the unaided eye, in dark skies, you can see a few thousand. How many stars are there in the entire Universe? Before we get to that massive number, let's consider what you can count with the tools available to you. Perfect vision in dark skies allows us to see stars down to about magnitude 6. But to really make an accurate census of the total number of stars, you'd need to travel to both the Northern and Southern Hemispheres, since only part of the sky is visible from each portion of the Earth. Furthermore, you'd need to make your count over several months, since a portion of the sky is obscured by the Sun. If you had perfect eyesight and traveled to completely dark skies in both the Northern and Southern Hemispheres, and there was no Moon, you might be able to get to count up almost 9,000 stars. With a good pair of binoculars, that number jumps to about 200,000, since you can observe stars down to magnitude 9. A small telescope, capable of resolving magnitude 13 stars will let you count up to 15 million stars. Large observatories could resolve billions of stars. But how many stars are out there? How many stars are there in the Milky Way? According to astronomers, our Milky Way is an average-sized barred spiral galaxy measuring up to 120,000 light-years across. Our Sun is located about 27,000 light-years from the galactic core in the Orion arm. Astronomers estimate that the Milky Way contains up to 400 billion stars of various sizes and brightness. A few are supergiants, like Betelgeuse or Rigel. Many more are average-sized stars like our Sun. The vast majority of stars in the Milky Way are red dwarf stars; dim, low mass, with a fraction of the brightness of our Sun. As we peer through our telescopes, we can see fuzzy patches in the sky which astronomers now know are other galaxies like our Milky Way. These massive structures can contain more or less stars than our own Milky Way. There are spiral galaxies out there with more than a trillion stars, and giant elliptical galaxies with 100 trillion stars. And there are tiny dwarf galaxies with a fraction of our number of stars. So how many galaxies are there? According to astronomers, there are probably more than 170 billion galaxies in the observable Universe, stretching out into a region of space 13.8 billion light-years away from us in all directions. And so, if you multiply the number of stars in our galaxy by the number of galaxies in the Universe, you get approximately 1024 stars. That’s a 1 followed by twenty-four zeros. That’s a septillion stars. But there could be more than that. It's been calculated that the observable Universe is a bubble of space 47 billion years in all directions. It defines the amount of the Universe that we can see, because that’s how long light has taken to reach us since the Big Bang. This is a minimum value, the Universe could be much bigger - it's just that we can't ever detect those stars because they're outside the observable Universe. It's even possible that the Universe is infinite, stretching on forever, with an infinite amount of stars. So add a couple more zeros. Maybe an infinite number of zeroes. That’s a lot of stars in the Universe. Additional Resources: How Many Stars Can you See? Astronomy Cast: How Big is the Universe? How Big is Our Observable Universe Astronomy Cast: The Observable Universe How Many Galaxies in the Universe?

This article comes from the Universe Today archive, but was updated with this spiffy video. How old is the Earth? Scientists think that the Earth is 4.54 billion years old. Coincidentally, this is the same age as the rest of the planets in the Solar System, as well as the Sun. Of course, it's not a coincidence; the Sun and the planets all formed together from a diffuse cloud of hydrogen billions of years ago. In the early Solar System, all of the planets formed in the solar nebula; the remnants left over from the formation of the Sun. Small particles of dust collected together into larger and larger objects - pebbles, rocks, boulders, etc - until there were many planetoids in the Solar System. These planetoids collided together and eventually enough came together to become Earth-sized. At some point in the early history of Earth, a planetoid the size of Mars crashed into our planet. The resulting collision sent debris into orbit that eventually became the Moon. How do scientists know Earth is 4.54 billion years old? It's actually difficult to tell from the surface of the planet alone, since plate tectonics constantly reshape its surface. Older parts of the surface slide under newer plates to be recycled in the Earth's core. The oldest rocks ever found on Earth are 4.0 - 4.2 billion years old. Scientists assume that all the material in the Solar System formed at the same time. Various chemicals, and specifically radioactive isotopes were formed together. Since they decay in a very known rate, these isotopes can be measured to determine how long the elements have existed. And by studying different meteorites from different locations in the Solar System, scientists know that the different planets all formed at the same time. Failed Methods for Calculating the Age of the Earth Our current, accurate method of measuring the age of the Earth comes at the end of a long series of estimates made through history. Clever scientists discovered features about the Earth and the Sun that change over time, and then calculated how old the planet Earth is from that. Unfortunately, they were all flawed for various reasons. Declining Sea Levels - Benoit de Maillet, a French anthropologist who lived from 1656-1738 and guessed (incorrectly) that fossils at high elevations meant Earth was once covered by a large ocean. This ocean had taken 2 billion years to evaporate to current sea levels. Scientists abandoned this when they realized that sea levels naturally rise and fall. Cooling of the Earth - William Thompson, later known as Lord Kelvin, assumed that the Earth was once a molten ball of rock with the same temperature of the Sun, and then has been cooling ever since. Based on these assumptions, Thompson calculated that the Earth took somewhere between 20 and 400 million years to cool to its current temperature. Of course, Thompson made several inaccurate assumptions, about the temperature of the Sun (it's really 15 million degrees Kelvin at its core), the temperature of the Earth (with its molten core) and how the Sun is made of hydrogen and the Earth is made of rock and metal. Cooling of the Sun - In 1856, the German physicist Hermann Ludwig Ferdinand von Helmholtz attempted to calculate the age of the Earth by the cooling of the Sun. He calculated that the Sun would have taken 22 million years to condense down to its current diameter and temperature from a diffuse cloud of gas and dust. Although this was inaccurate, Helmholtz correctly identified that the source of the Sun's heat was driven by gravitational contraction. Rock Erosion - In his book, The Origin of Species by Means of Natural Selection, Charles Darwin proposed that the erosion of chalk deposits might allow for a calculation of the minimum age of the planet. Darwin estimated that a chalk formation in the Weald region of England might have taken 300 million years to weather to its current form. Orbit of the Moon - George Darwin,

I’m just going to warn you, this is a controversial topic. Some people get pretty grumpy when you ask: how many planets are in the Solar System? Is it eight, ten, or more? I promise you this, though, we’re never going back to nine planets... ever. When many of us grew up, there were nine planets in the Solar System. It was like a fixed point in our brains. As kids, memorizing this list was an early right of passage of nerd pride: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. But then in 2005, Mike Brown discovered Eris, an icy object thought to be about the same size as Pluto, out beyond its orbit. That would bring the total number of planets to ten. Right? There’s no turning back, textbooks would need to be changed. In order to settle the dispute, the International Astronomical Union met in 2006, and argued for, and against Pluto’s planethood. Some astronomers advocated widening the number of planets to twelve, including Pluto, its moon Charon, the Asteroid Ceres, and the newly discovered Eris. In the end, they changed the definition of what makes a planet, and sadly, Pluto doesn’t make the cut: Here are the new requirements of planethood status: A planet has to orbit the Sun. Okay fine, Pluto does that. A planet needs enough gravity to pull itself into a sphere. Okay, spherical. Pluto’s fine there too. A planet needs to have cleared out its orbit of other objects. Uh oh, Pluto hasn’t done that. For example, planet Earth accounts for a million times the rest of the material in its orbit, while Pluto is just a fraction of the icy objects in its realm. The final decision was to demote Pluto from planet to dwarf planet. But don’t despair, Pluto is in good company. There’s Ceres, the first asteroid ever discovered, and the smallest of the dwarf planets. The surface of Ceres is made of ice and rock, and it might even have a liquid ocean under its surface. NASA’s Dawn mission is flying there right now to give us close up pictures for the first time. Haumea, named after the Hawaiian goddess of fertility, is about a third the mass of Pluto, and has just enough gravity to pull itself into an ellipsoid, or egg shape. Even though it’s smaller, it’s got moons of its own. Makemake, a much larger Kuiper belt object, has a diameter about two-thirds the size of Pluto. It was discovered in 2005 by Mike Brown and his team. So far, Makemake doesn’t seem to have any moons. Eris is the most massive known dwarf planet, and the one that helped turn our definition of a planet upside-down. It’s 27% more massive than Pluto and the ninth most massive body that orbits the Sun. It even has a moon: Dysnomia. And of course, Pluto. The founding member of the dwarf family. Want an easy way to remember the eight planets, in order? Just remember this mnemonic: my very excellent mother just served us noodles. For all you currently writing angry tweets to Mike Brown, hold on a sec. Changing Pluto’s categorization is an important step that really needed to happen. The more we discover about our Universe, the more we realize just how strange and wonderful it is. When Pluto was discovered 80 years ago, we never could have expected the variety of objects in the Solar System. Categorizing Pluto as a dwarf planet helps us better describe our celestial home. So, our Solar System now has eight planets, and five dwarf planets.

The International Space Station is one of the most complicated machines ever built and the largest object ever assembled in space. At any time there are up to six astronauts on board, each originating from one of fifteen different nations on Earth. It orbits at an altitude of approximately four-hundred kilometers, and completes an orbit around the Earth every ninety-two minutes and fifty seconds. The station has a mass of four-hundred-and-twenty metric tonnes, and contains a dozen pressurized modules, and many more unpressurized modules, trusses and solar panels. It truly is a feat of human ingenuity. But did you know that the International Space Station is one of the brightest objects in the night sky? And it’s easy to see if you know when, and where, to look. In fact, with your ability to find the station you can amaze your friends and neighbours. The best place to start is NASA’s Spot the Station website. Enter your Country, Region, City along with an email address or mobile phone number. Then give your preference for notifications in the evening, morning or both and that’s it. About twelve hours before the station is due to fly overhead, you’ll get a notification from NASA. Depending on your location, you might get notified a couple of times a week, or as rarely as once a month. As soon as you get the notification, create an alarm on your phone for about a minute before the flyover. When the alarm goes off, take your friends outside and look to the West. The station orbits the Earth from West to East, so you’ll see it appear on the Western horizon as a very bright star, moving rapidly across the sky. It will take only few minutes to cross the entire sky. The station moves so quickly if you’re using a telescope you will have a tough time tracking its movement. A nice pair of binoculars will make it look a lot brighter, and even let you see the H-shape of its solar panels. But even viewing it with the naked eye is a great experience. NASA’s website is just one of the many ways you can get notifications. If you use Twitter, follow @twisst. They can figure out your location and then send you a notification when the station is about to fly overhead via Twitter. There are also dozens of Android and iPhone apps that will perform this function; many of which are free to use. If you’ve never seen the station, head on over to NASA and set up a notification right away. Then kick back and let orbital mechanics bring the station to your backyard at a time that’s convenient for you. Want more details? We've got a detailed guide on how to View the International Space Station for Beginners, and How to Photograph the International Space Station.