Category The Universe, Exploring the Universe, Solar System, The Moon, Space, Space Travel

       Scientists have to know how far away a star is before they can begin to analyze details such as its age, size, temperature and mass. The most effective way of measuring a star’s distance from Earth is called the parallax method. If you are travelling in a car and looking out of the window, nearer objects seem to pass by much more quickly than distant ones. In the same way, as Earth orbits the Sun, nearer stars appear to move more quickly through the sky than those further away. The angle through which a certain star moves over a period of six months is called its parallax. This angle is used by astronomers to work out how far away the star is.

         Parallax is “the best way to get distance in astronomy,” said Mark Reid, an astronomer at the Harvard Smithsonian Center for Astrophysics. He described parallax as the “gold standard” for measuring stellar distances because it does not involve physics; rather, it relies solely on geometry.

          The method is based on measuring two angles and the included side of a triangle formed by the star, Earth on one side of its orbit and Earth six months later on the other side of its orbit, according to Edward L. Wright, a professor at the University of California, Los Angeles.

         It works like this: hold out your hand, close your right eye, and place your extended thumb over a distant object. Now, switch eyes, so that your left is closed and your right is open. Your thumb will appear to shift slightly against the background. By measuring this small change and knowing the distance between your eyes, you can calculate the distance to your thumb.

          To measure the distance of a star, astronomers use a baseline of 1 astronomical unit (AU), which is the average distance between Earth and the sun, about 93 million miles (150 million kilometers). They also measure small angles in arcseconds, which are tiny fractions of a degree on the night sky.

          If we divide the baseline of one AU by the tangent of one arcsecond, it comes out to about 19.2 trillion miles (30.9 trillion kilometers), or about 3.26 light years. This unit of distance is called a parallax second, or parsec (pc). However, even the closest star is more than 1 parsec from our sun. So astronomers have to measure stellar shifts by less than 1 arc second, which was impossible before modern technology, in order to determine the distance to a star.

          We know that earth is bombarded by thousands of meteorites every day, none of which does our planet much damage. Any meteorite up to 10m (33ft) in diameter will normally burn up in the atmosphere before it reaches Earth, separating into tiny fragments. If a meteorite larger than this falls to Earth, it can cause considerable damage — impacting with the energy of five nuclear warheads. Approximately once every 1000 years, a larger meteorite does fall to Earth, and several large craters caused by such impacts can still be seen. One such was the nickel – iron meteorite that created the Barringer Crater in Arizona, USA. The meteorite was an incredible 45m (148ft) wide, creating a crater nearly 1.5km (1 mile) in width. However, it would take an impact by an object roughly 5km (3 miles) wide to cause mass extinctions and threaten life on Earth.

          Most meteorites that are found on the ground weigh less than a pound. While it may seem like these tiny pieces of rock wouldn’t do much damage, a 1-lb. (0.45 kilograms) meteorite traveling upward of 200 mph (322 km/h) can fall through the roof of a house or shatter a car windshield. 

          When the Grimsby meteorite landed in Ontario, Canada in 2009, for example, it broke the windshield of an SUV. In another incident, meteorites crashed into the back end of a Chevy Malibu in Peekskill, New York, in 1992, Cooke and Moorhead said. Thankfully, no one was injured during these events. 

          However, the pieces of rock falling from the sky are not even the greatest concern regarding meteor impacts, Cooke said.

          “What causes the most damage is the shock wave produced by the meteor when it breaks apart in [Earth’s] atmosphere,” Cooke said. “So, you don’t have to watch for the falling rocks — you have to worry about the shockwave.”

          For example, the Chelyabinsk meteor — an asteroid the size of a six-story  building that entered Earth’s atmosphere in February 2013 over Russia — broke apart 15 miles (24 km) above the ground and generated a shock wave equivalent to a 500-kiloton explosion, Cooke said. It injured 1,600 people.

          Another major collision was the Tunguska meteorite, which was larger than Chelyabinsk and 10 times more energetic. The meteorite exploded over the Tunguska River on June 30, 1908, and flattened 5000,000 acres (2,000 square km) of uninhabited forest. Because of its remote location, the event is an example of a meteorite that would have gone undetected had it not been so large, Cooke and Moorhead explained. 

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          Unlike many of the planets, moons and smaller bodies in the Solar System, Earth appears to be covered by very few craters. In the early days of the Solar System, Earth was as much a target for meteorites as any other planet, and suffered intensive cratering in the first one billion years of its existence. However, unlike bodies such as Mercury and the Moon, Earth has many geological processes that “hide” craters. Constant weathering and erosion from winds and water wear away or cover up craters. Some may also be hidden by vegetation or lie under the sea, although in the last hundred years, aerial photography and other forms of imaging have given us a clearer view of many remaining craters.

          Impact craters leave quite an impression on the surface of planets and moons — just think of Earth’s moon, which gets its distinctive appearance from millions of encounters of asteroids over the centuries. But Earth is a different story altogether, with only 128 impact craters recorded in the most recent count. That can’t be right, can it?

          He reports that a new study shows that the low number found by past scientists isn’t “just the result of lazy searching”: it’s the surprising truth about a planet that’s astonishingly crater-free.

          The study looked at the ways Earth erosion affects existing craters and concluded that the current count of 70 craters larger than 6 km (3.7 miles) in diameter should be just about right. That’s a rare instance of a complete geologic record, writes Hand — and one that may discourage people on the hunt for new craters.

          But don’t put away your crater-catching gear just yet. The study’s authors note that just because we’ve already found all of the likely large impact craters on Earth don’t mean there aren’t more to discover. The real opportunity, they write, lies in smaller craters: they estimate that more than 90 craters between .6 miles and 3.7 miles in diameter should still be undiscovered and more than 250 between 0.1 miles and .6 miles.

          NASA notes that Earth is equipped with three processes that eat up craters relatively quickly: erosion, tectonics, and volcanism. These forces leave only the largest scars from meteorites or asteroids — unlike, say, the moon, which can’t gobble up craters. Hand writes that the parameters of the study also play a part in the low number — it looks at just surface craters, not those that lie beneath sediment. And the study also didn’t look at volcanic craters, which formed some of Earth’s most distinctive basins and lakes.

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          Because earth passes through meteor streams at roughly the same time each year, meteor showers can be predicted highly accurately. Astronomers have now even worked out which comets are responsible for each annual shower. Two meteor showers come from the trail left by Halley’s Comet: the Orionids in October and the Eta Aquarids in May. Although meteors in a shower fall to Earth over a large distance, perspective makes them seem to be falling from the same point in the sky, called the radiant.

           Most ‘predictions’ of the rate of meteors per hour during meteor showers are based on both theory and observation. Essentially, a computer model is built containing the trajectories of every known comet – since it is the debris from comets that forms the ‘stream’ of particles we see during a meteor shower.

          This model contains information on the rate that these comets release material, along with the sizes, directions and velocities at which they are released, as well as the gravitational forces that determine their subsequent trajectories through space. The trajectory of the Earth and the conditions of the Earth’s atmosphere are also inputted into the computer model.

          By watching how Earth moves through the meteor stream it is possible to estimate the likely number of meteors that will be visible during a given shower for a given location. But different astronomers use different models. Plus, these models are partly based on difficult measurements of the meteoric particles in the Solar System, so their predictions are often only approximate. But generally, they can be used to reliably predict when a meteor shower is likely to be more or less intense than the average.

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          The average meteorite enters the Earth’s atmosphere at around 50km/s (31mi/s), but particles in the atmosphere cause the speeding rocks to slow down. All but the largest meteorites are decelerated to around 150km/h (93mph) by the time they impact. Larger meteorites will not be slowed by atmospheric friction and hit the ground travelling at deadly speed.

          The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the Solar System falls into Earth’s atmosphere creating temporary incandescence resulting from atmospheric friction. A meteoroid is matter revolving around the sun or any object in interplanetary space that is too small to be called an asteroid or a comet. Even smaller particles are called micrometeoroids or cosmic dust grains, which includes any interstellar material that should happen to enter our solar system. A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized.

          Meteor’s come in a range of sizes, from dust-sized which we see as reflected sunlight in the orbital plane of the Solar System (called zodiacal light) to house-sized.

          When a meteor enters the atmosphere friction causes ablation of its surface (i.e. it burns up). If the meteor is small (fist-sized) it vaporizes before hitting the ground. If larger it survives to impact on the ground, although it will be reduced in size during entry into the atmosphere. About 25 million meteors enter the Earth’s atmosphere every day (duck!). Most burn up and about 1 million kilograms of dust per day settles to the Earth’s surface.

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          A great deal of the material that makes up meteorites comes from short-period comets. As comets travel close to the Sun, they lose material, creating a trail of debris behind them. These trails, called meteoroid streams, can take many hundreds of years to form, but gradually build up to contain a large amount of loose dust and rock fragments. If Earth’s orbit carries it through one of these streams, then hundreds of meteoroids will enter the atmosphere in a very short time, creating a meteor shower.

          Meteor showers occur when the earth’s orbit and that of a comet intersect. What you are seeing is the bits of dust that the comet left behind colliding with the atmosphere at high speed. The friction with the earth’s atmosphere heats up the particles of dust to thousands of degrees until they either vaporize or strike the surface of the earth. In some cases they particles a deflected back into space like a stone skipping on water. When the earth is not in a recent orbital path of a comet there are still loose particles all around the solar system so there is a base rate of about 5 visible meteors an hour in ideal viewing conditions. You can see one strike the atmosphere if its dark enough and you happen to be looking in the right direction. The orbital paths of the earth and many comets intersect once every year and the meteor rate can be much higher when the earth passes through these debris fields. The process of small particles loose in space being swept up by larger bodies like the earth has been going on for billions of years and is what created the sun, planets and comets to begin with. The earth captures 40,000 metric tons of space dust a year currently which is much less than the rate it was 4 billion years ago when the planets were first forming. This makes sense logically as the dust clears the collision rate falls. The geologic record on earth and the moon also support this hypothesis.

          Meteor showers associated with particular comet orbits occur at about the same time each year, because it is at those points in the earth’s orbit that the collisions occur. However, because some parts of the comet’s path are richer in debris than others, the strength of a meteor shower may vary from one year to the next. Typically a meteor shower will be strongest when the earth crosses the comet’s path shortly after the parent comet has passed.

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