Planets - Zoom Astronomy Your age on the Planets The Planets (plus the Dwarf Planet Pluto) Our solar system consists of the sun, eight planets, moons, many dwarf planets (or plutoids), an asteroid belt, comets, meteors, and others. The sun is the center of our solar system ; the planets, their moons, a belt of asteroids , comets , and other rocks and gas orbit the sun. The eight planets that orbit the sun are (in order from the sun): Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , Neptune . Another large body is Pluto , now classified as a dwarf planet or plutoid. A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17° from the ecliptic).

Easy ways to remember the order of the planets (plus Pluto) are the mnemonics: "My Very Excellent Mother Just Sent Us Nine Pizzas" and "My Very Easy Method Just Simplifies Us Naming Planets" The first letter of each of these words represents a planet - in the correct order. The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto (the largest of the dwarf planets). Jupiter is so big that all the other planets could fit inside it. The Inner Planets vs. the Outer Planets The inner planets (those planets that orbit close to the sun) are quite different from the outer planets (those planets that orbit far from the sun). The inner planets are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons. The outer planets include: Jupiter, Saturn, Uranus, Neptune, and Pluto (a dwarf planet). They are mostly huge, mostly gaseous, ringed, and have many moons (again, the exception is Pluto, the dwarf planet, which is small, rocky, and has four moons).

Temperatures on the Planets Generally, the farther from the Sun, the cooler the planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere. Density of the Planets The outer, gaseous planets are much less dense than the inner, rocky planets. The Earth is the densest planet. Saturn is the least dense planet; it would float on water. The Mass of the Planets Jupiter is by far the most massive planet; Saturn trails it. Uranus, Neptune, Earth, Venus, Mars, and Pluto are orders of magnitude less massive. Gravitational Forces on the Planets The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of the planet's radius squared. A Day on Each of the Planets A day is the length of time that it takes a planet to rotate on its axis (360°). A day on Earth takes almost 24 hours.

The planet with the longest day is Venus ; a day on Venus takes 243 Earth days. (A day on Venus is longer than its year; a year on Venus takes only 224.7 Earth days). The planet with the shortest day is Jupiter ; a day on Jupiter only takes 9.8 Earth hours! When you observe Jupiter from Earth, you can see some of its features change. The Average Orbital Speed of the Planets As the planets orbit the Sun , they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun (this is Kepler's Second Law of Planetary Motion ). The Planets in Our Solar System Planet (or Dwarf Planet) In 2005, a large object beyond Pluto was observed in the Kuiper belt. A few astronomers think that there might be another planet or companion star orbiting the Sun far beyond the orbit of Pluto. This distant planet/companion star may or may not exist. The hypothesized origin of this hypothetical object is that a celestial object, perhaps a hard-to-detect cool, brown dwarf star (called Nemesis ), was captured by the Sun's gravitational field.

This planet is hypothesized to exist because of the unexplained clumping of some long-period comet's orbits. The orbits of these far-reaching comets seem to be affected by the gravitational pull of a distant, Sun-orbiting object. Planet Activities and Quizzes An Overview of the Solar System, it's alignment and pictures The inner solar system contains the Sun , Mercury , Venus , Earth and Mars : The main asteroid belt (not shown) lies between the orbits of Mars and Jupiter. The planets of the outer solar system are Jupiter , Saturn , Uranus , and Neptune ( Pluto is now classified as a dwarf planet): The first thing to notice is that the solar system is mostly empty space. The planets are very small compared to the space between them. Even the dots on the diagrams above are too big to be in proper scale with respect to the sizes of the orbits. The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit) . The ecliptic is inclined only 7 degrees from the plane of the Sun's equator.

The above diagrams show the relative sizes of the orbits of the eight planets (plus Pluto) from a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense. (The above diagrams show correct positions for October 1996 as generated by the excellent planetarium program Starry Night ; there are also many other similar programs available, some free. You can also use Emerald Chronometer on your iPhone or Emerald Observatory on your iPad to find the current positions.) Sizes The above composite shows the eight planets and Pluto with approximately correct relative sizes (see another similar composite and a comparison of the terrestrial planets or Appendix 2 for more). One way to help visualize the relative sizes in the solar system is to imagine a model in which everything is reduced in size by a factor of a billion. Then the model Earth would be about 1.3 cm in diameter (the size of a grape). The Moon would be about 30 cm (about a foot) from the Earth.

The Sun would be 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 20 and 30 blocks away. A human on this scale would be the size of an atom but the nearest star would be over 40000 km away. Not shown in the above illustrations are the numerous smaller bodies that inhabit the solar system: the satellites of the planets; the large number of asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also elsewhere; the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic; and the many small icy bodies beyond Neptune in the Kuiper Belt .

With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids. The classification of these objects is a matter of minor controversy. Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites (a.k.a. moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits). Unfortunately, the solar system has been found to be more complicated than this would suggest: there are several moons larger than Pluto and two larger than Mercury; there are many small moons that are probably started out as asteroids and were only later captured by a planet; comets sometimes fizzle out and become indistinguishable from asteroids; the Kuiper Belt objects (including Pluto) and others like Chiron don't fit this scheme well The Earth/Moon and Pluto/Charon systems are sometimes considered "double planets". Other classifications based on chemical composition and/or point of origin can be proposed which attempt to be more physically valid. But they usually end up with either too many classes or too many exceptions.

The bottom line is that many of the bodies are unique; the actual situation is too complicated for simple categorization. In the pages that follow, I will use the conventional categorizations. The eight bodies officially categorized as planets are often further classified in several ways: by composition: terrestrial or rocky planets: Mercury, Venus, Earth, and Mars: The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites. jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune: The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites. by size: small planets: Mercury, Venus, Earth, Mars. The small planets have diameters less than 13000 km. giant planets: Jupiter, Saturn, Uranus and Neptune. The giant planets have diameters greater than 48000 km. The giant planets are sometimes also referred to as gas giants. by position relative to the Sun: inner planets: Mercury, Venus, Earth and Mars. outer planets: Jupiter, Saturn, Uranus, Neptune.

The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system. by position relative to Earth: inferior planets: Mercury and Venus. closer to the Sun than Earth. The inferior planets show phases like the Moon's when viewed from Earth. Earth. superior planets: Mars thru Neptune. farther from the Sun than Earth. The superior planets always appear full or nearly so. classical planets: Mercury, Venus, Mars, Jupiter, and Saturn. known since prehistorical times visible to the unaided eye in ancient times this term also refered to the Sun and the Moon; the order was usually specificied as: Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon, based on the time for them to go "all the way round" the sphere of the "fixed" stars). modern planets: Uranus, Neptune. Your Weight on Other Worlds , another neat calculator from the Exploratorium ConverTable Planets , a nice little Macintosh app to calculate your weight on other planets Galileo SSI Education Module on Planetary Surfaces a good bibliography of print material about the solar system The Big Questions What is the origin of the solar system?

It is generally agreed that it condensed from a nebula of dust and gas. But the details are far from clear. How common are planetary systems around other stars? (Updated June 2014) The number of planets around other stars has increased dramatically since the first discoveries of HD 1144762 b in 1989 and gamma Cephei b in 1988 (confirmed in 2003). exoplanet.eu lists 1,811 planets up to 25 July 2014, including over 400 multiple planet systems. Plus there are over 3,000 additional potential planets indicated by the Kepler spacecraft according to the NASA Exoplanet Archive . The reader will note there may be differences in the reported numbers between the two sites referenced. What conditions allow the formation of terrestrial planets? It seems unlikely that the Earth is totally unique but we still have no direct evidence one way or the other. Is there life elsewhere in the solar system? If not, why is Earth special? (Updated June 2014) We do not yet know of life elsewhere.

One of the things that makes Earth special of particular interest to the exoplanet search is our location with respect to our Sun -- the habitable or so-called “goldilocks zone”. The “goldilocks zone” the area around a star where water would be a liquid on the surface of a planet. The location and extent of which would depend on a number of criteria such as the parent star size and temperature. Once planets in these habitable zones are found the size of the planet is taken into account. The size is what may enable a suitable atmosphere for our familiar life forms. The Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo maintains the Habitable Exoplanet Catalog Is there life beyond the solar system? Intelligent life? Is life a rare and unusual or even unique event in the evolution of the universe or is it adaptable, widespread and common? Answers to these questions, even partial ones, would be of enormous value. Answers to the lesser questions on the pages that follow may help answer some of these big ones. NASA - What is orbit? What is orbit? 09.10.03 Diagram of the planets and other heavenly bodies. Orbit is a word we hear quite often.

Every time the Space Shuttle lifts off the launch pad, we hear it. Every time the Space Shuttle meets up with the International Space Station (ISS), we hear it. And every time a rocket launches a payload, we hear the word "orbit." It's a widely used term, but do you know what an orbit really is? An orbit is a regular, repeating path that one object in space takes around another one. An object in an orbit is called a satellite. A satellite can be natural, like the Earth or the Moon. It can also be man-made, like the Space Shuttle or the ISS. In our solar system, the Earth and the eight other planets orbit the Sun. Most of the objects orbiting the Sun move along or close to an imaginary flat surface. This imaginary surface is called the ecliptic plane. Many planets also have moons. These moons orbit around them. Orbits are elliptical in shape, this means they are similar to an oval. For the planets, the orbits are almost round. The orbits of comets have a different shape. They are highly eccentric or "squashed." Satellites that orbit the Earth are not always the same distance from the Earth. Sometimes they are closer, and at other times they are farther away.

The closest point a satellite comes to the Earth is called its perigee. The farthest point is the apogee. The time it takes a satellite to make one full orbit is called its period. The inclination is the angle the orbital plane makes when compared with the Earth's equator. The space station Skylab orbiting the Earth. An object in motion will stay in motion unless something pushes or pulls on it. This is Isaac Newton's First Law of Motion. Without gravity, an Earth-orbiting satellite would go off into space along a straight line. With gravity, it is pulled back toward the Earth. There is a constant tug-of-war between the satellites tendency to move in a straight line, or momentum, and the tug of gravity pulling it back. An object's momentum and the force of gravity have to be balanced for an orbit to happen. If the forward momentum of one object is too great, it will speed past the other one and not enter into orbit. If momentum is too small, the object will be pulled into the other one and crash. When these forces are balanced, the object is always falling into the planet, but because it's moving sideways fast enough, it never hits the planet.

Escape velocity is the speed an object must go to break free from a planet's gravity and enter into orbit. Escape velocity depends on the mass of the planet. Each planet has a different escape velocity. The object's distance from the planet's center is also important. The escape velocity from the Earth is about 11.3 kilometers (7 miles) per second. Orbital velocity is the speed needed to stay in orbit. At an altitude of 242 kilometers (150 miles), this is about 17,000 miles per hour. This is just a little less than full escape velocity. The Space Shuttle in Low-Earth Orbit. Low-Earth Orbit (LEO) is restricted to the first 100 to 200 miles of space. LEO is the easiest orbit to get to and stay in. This is where the Shuttle and ISS conduct their operations. One complete orbit in LEO takes about 90 minutes. Satellites that seem to be attached to some location on Earth are in Geosynchronous Earth Orbit (GEO). These satellites orbit about 23,000 miles above the equator and complete one revolution around the Earth precisely every 24 hours.

Satellites headed for GEO first go to an elliptical orbit with an apogee about 23,000 miles. Firing the rocket engines at apogee then makes the orbit round. Geosynchronous orbits are also called geostationary. Any satellite with an orbital path going over or near the poles maintains a polar orbit. Polar orbits are usually in low-Earth orbit. They remain in place while the Earth passes under. This means that eventually, the entire Earth's surface passes under a satellite in polar orbit. When a meteorite enters our atmosphere and becomes a "shooting star," it is no longer in an orbit. Some space probes, like Voyager, have reached escape velocity and broken away from the pull of the Sun's gravity. These probes are leaving the solar system. They are not in orbit around a planet or the Sun. Courtesy of NASA's Human Exploration and Development of Space Enterprise Published by NASAexplores: October 25, 2001 Store Shop Windows to the Universe We now offer the Cool It! card game in our Science Store . Cool It! is the new card game from UCS that teaches kids about the choices we have when it comes to climate change.

This drawing shows where trans-neptunian objects are found. Click on image for full size How many planets orbit the sun? I was wondering if there is a new planet? Are there planets (a tenth planet?) after Pluto belonging to our solar system? What are the names of the new planets discovered in the solar system? Are there any objects that orbit the sun that are not in the same plane as the planets, excluding Pluto, comets, asteroids? Do we know any names to any other planets than those in our solar system? If yes, could you give me any examples? As of today, January, 26, 2001, it's known that only 9 planets orbit the Sun. There have been many solar system objects that have been discovered recently, but none of them have been classified as planets. And none of them are the mysterious Planet X! These objects have been called asteroids, minor-planets, trans-neptunian objects, or objects of the Kuiper belt. We'll use the word trans-neptunian object (TNO) because that simply means an object past Neptune. The first TNO to be discovered (beside Pluto!) was discovered in 1992.

Since then, over 300 of these objects have been found! One of the most recent discoveries of a TNO was in October of 2000. Astronomers found a huge TNO and called it 2000 EB 173. It is between 300-700 kilometers in size and as far as non-planets go in the solar system, it is second only in size to asteroid Ceres . So, although no new planets have been discovered in our solar system, there have been many discoveries of planet systems outside our solar system. One such finding was in April 1999 of three planets around the star Upsilon Andromedae . Submitted by Jana (New Jersey, USA), Sean (Ohio, USA), John (Ohio, USA), Saravanan (India), Puneet (India), Cassie (Colorado, USA), Jason (January 26, 2001) Why do the planets orbit the sun? (Beginner) - Curious About Astronomy? Ask an Astronomer Why do the planets orbit the sun?

(Beginner) (Beginner) >" onclick="window.open(this.href,'win2','status=no,toolbar=no,scrollbars=yes,titlebar=no,menubar=no,resizable=yes,width=640,height=480,directories=no,location=no'); return false;" rel="nofollow"> Why do the planets rotate around the Sun? First, please note that "rotate" actually is used to describe an celestial body's spin, and "revolve" is used to describe its orbital motion. For example, the Earth completes one rotation about its axis about every 24 hours, but it completes one revolution around the Sun about every 365 days. Anyway, the basic reason why the planets revolve around, or orbit , the Sun, is that the gravity of the Sun keeps them in their orbits. Just as the Moon orbits the Earth because of the pull of Earth's gravity, the Earth orbits the Sun because of the pull of the Sun's gravity. Why, then, does it travel in an elliptical orbit around the Sun, rather than just getting pulled in all the way? This happens because the Earth has a velocity in the direction perpendicular to the force of the Sun's pull.

If the Sun weren't there, the Earth would travel in a straight line. But the gravity of the Sun alters its course, causing it to travel around the Sun, in a shape very near to a circle. This is a little hard to visualize, so let me give you an example of how to visualize an object in orbit around the Earth, and it's analogous to what happens with the Earth and the Sun. Imagine Superman is standing on Mt. Everest holding a football. He throws it as hard as he can, which is incredibly hard because he's Superman. Just like if you threw a football, eventually it will fall back down and hit the ground. But because he threw it so hard, it goes past the horizon before it can fall. And because the Earth is curved, it just keeps on going, constantly "falling," but not hitting the ground because the ground curves away before it can. Eventually the football will come around and smack Superman in the back of the head, which of course won't hurt him at all because he's Superman. That is how orbits work, but objects like spaceships and moons are much farther from the Earth than the football that Superman threw.

(We're ignoring air resistance with the football example; actual spacecraft must be well above most of a planet's atmosphere, or air resistance will cause them to spiral downward and eventually crash into the planet's surface.) This same situation can be applied to the Earth orbiting the Sun - except now Superman is standing on the Sun (which he can do because he's Superman) and he throws the Earth. The next question, then, is how did Earth get that velocity, since in real life there's no Superman throwing it. For that, you need to go way back to when the Solar System formed . This page was last updated on January 31, 2016. How Far Away is Mars? | Distance to Mars How Far Away is Mars? | Distance to Mars By Tim Sharp, Reference Editor | August 2, 2012 05:48pm ET MORE Mars is the fourth planet from the sun. It has been a favorite object of skywatchers for centuries and of space explorers for decades. The distance to Mars depends on when you measure it, and which vantage point you’re considering. We’ll assume first you are wondering this: Mars orbits the sun in 687 Earth days.

Credit: NASA How far is Mars from Earth? Mars and Earth orbit the sun at different speeds: Earth has an inside track and gets around the sun more quickly. Plus, both have elliptical orbits, rather than perfect circles. So the distance to Mars from Earth is constantly changing. In theory, the closest the planets could come together would be when Mars is at its closest point to the sun (perihelion) and Earth is at its farthest point (aphelion). In that situation, the planets would be 33.9 million miles (54.6 million kilometers) from each other. But that has never happened in recorded history. The closest known approach was 34.8 million miles (56 million km) in 2003. At the other end of the scale, when both planets are at aphelion — their farthest distance from the sun — and the two worlds are on opposite sides of the sun, they can be 250 million miles (401 million km) apart — a whopping 7.37 times the closest possible separation. On average, the distance to Mars from Earth is 140 million miles (225 million km).

In their race around the sun, Earth on its inside track laps Mars every 26 months. This close approach provides an opportunity — a launch window — to send spacecraft to the red planet . Rather than pointing the spacecraft at Mars, engineers aim it in a wide orbit around the sun. The sun's gravity gives the spacecraft a boost — called a gravity assist or slingshot effect —saving time and fuel. The spacecraft's orbit then intersects with Mars. NASA\'s Phoenix Mars Lander captured a sunrise Sept. 5, 2008, the 101st day since landing in the Arctic. How far is Mars from the sun? Mars has a very eccentric orbit; that is, it deviates from a perfect circle more than any other planet's orbit (Pluto, no longer considered a planet, has an even more eccentric orbit). At its farthest distance (aphelion), Mars is 154 million miles (249 million km) from the sun. At its closest (perihelion), Mars is 128 million miles (206 million km) distant. On average, the distance to Mars from the sun is 141 million miles (228 million km).

Mars revolves around the sun in 687 Earth days, which represents a Martian year. Measuring the distance The distance to Mars from Earth was first determined by Giovanni Cassini in 1672 using the parallax method. He sent a colleague, Jean Richer, to French Guiana while he stayed in Paris. They took measurements of the position of Mars, and triangulated those measurements with the known distance between Paris and French Guiana. Their calculations were only 7 percent off from today's more precise instruments. — Tim Sharp, Reference Editor Asteroids – Facts and Information about Asteroids Asteroids: Fun Facts and Information About Asteroids By Charles Q. Choi, Space.com Contributor | November 21, 2014 11:38pm ET MORE OSIRIS clear filter image taken during the flyby of the Rosetta spacecraft at asteroid Lutetia on July 10, 2010. Credit: © ESA 2010 MPS/OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA Asteroids are small, airless rocky worlds revolving around the sun that are too small to be called planets.

They are also known as planetoids or minor planets. In total, the mass of all the asteroids is less than that of Earth's moon. But despite their size, asteroids can be dangerous. Many have hit Earth in the past, and more will crash into our planet in the future. That's one reason scientists study asteroids and are eager to learn more about their numbers, orbits and physical characteristics. If an asteroid is headed our way, we want to know that. Most asteroids lie in a vast ring between the orbits of Mars and Jupiter . This main asteroid belt holds more than 200 asteroids larger than 60 miles (100 kilometers) in diameter. Scientists estimate the asteroid belt also contains more than 750,000 asteroids larger than three-fifths of a mile (1 km) in diameter and millions of smaller ones. Not everything in the main belt is an asteroid — for instance, comets have recently been discovered there, and Ceres , once thought of only as an asteroid, is now also considered a dwarf planet. Many asteroids lie outside the main belt. For instance, a number of asteroids called Trojans lie along Jupiter's orbital path.

Three groups — Atens, Amors, and Apollos — known as near-Earth asteroids orbit in the inner solar system and sometimes cross the path of Mars and Earth. Formation Asteroids are leftovers from the formation of our solar system about 4.6 billion years ago. Early on, the birth of Jupiter prevented any planetary bodies from forming in the gap between Mars and Jupiter, causing the small objects that were there to collide with each other and fragment into the asteroids seen today. Physical characteristics Asteroids can reach as large as Ceres, which is 940 km (about 583 miles) across. On the other hand, one of the smallest, discovered in 1991 and named 1991 BA, is only about 20 feet (6 meters) across. Nearly all asteroids are irregularly shaped, although a few are nearly spherical, such as Ceres. They are often pitted or cratered — for instance, Vesta has a giant crater some 285 miles (460 km) in diameter. The surfaces of most asteroids are thought to be covered in dust . As asteroids revolve around the sun in elliptical orbits, they rotate, sometimes tumbling quite erratically.

More than 150 asteroids are also known to have a small companion moon, with some having two moons. Binary or double asteroids also exist, in which two asteroids of roughly equal size orbit each other, and triple asteroid systems are known as well. Many asteroids seemingly have been captured by a planet's gravity and become moons — likely candidates include among Mars' moons Phobos and Deimos and most of the distant outer moons of Jupiter, Saturn, Uranus and Neptune. The average temperature of the surface of a typical asteroid is minus 100 degrees F (minus 73 degrees C). Asteroids have stayed mostly unchanged for billions of years — as such, research into them could reveal a great deal about the early solar system. Asteroids come in a variety of shapes and sizes. Some are solid bodies, while others are smaller piles of rubble bound together by gravity. One, which orbits the sun between Neptune and Uranus, comes with its own set of rings . Another has not one but six tails . Classification In addition to classifications of asteroids based on their orbits, most asteroids fall into three classes based on composition.

The C-type or carbonaceous are greyish in color and are the most common, including more than 75 percent of known asteroids. They probably consist of clay and stony silicate rocks, and inhabit the main belt's outer regions. The S-type or silicaceous asteroids are greenish to reddish in color, account for about 17 percent of known asteroids, and dominate the inner asteroid belt. They appear to be made of silicate materials and nickel-iron. The M-type or metallic asteroids are reddish in color, make up most of the rest of the asteroids, and dwell in the middle region of the main belt. They seem to be made up of nickle-iron. There are many other rare types based on composition as well — for instance, V-type asteroids typified by Vesta have a basaltic, volcanic crust. Earth impacts Ever since Earth formed about 4.5 billion years ago, asteroids and comets have routinely slammed into the planet. The most dangerous asteroids are extremely rare, according to NASA. An asteroid capable of global disaster would have to be more than a quarter-mile wide.

Researchers have estimated that such an impact would raise enough dust into the atmosphere to effectively create a "nuclear winter," severely disrupting agriculture around the world. Asteroids that large strike Earth only once every 1,000 centuries on average, NASA officials say. Smaller asteroids that are believed to strike Earth every 1,000 to 10,000 years could destroy a city or cause devastating tsunamis. On Feb. 15, 2013, an asteroid slammed into the atmosphere over the Russian city of Chelyabinsk, creating a shock wave that injured 1,200 people. The space rock is thought to have measured about 65 feet (20 meters) wide when it entered Earth's atmosphere. Dozens of asteroids have been classified as " potentially hazardous " by the scientists who track them. Some of these, whose orbits come close enough to Earth, could potentially be perturbed in the distant future and sent on a collision course with our planet. Scientists point out that if an asteroid is found to be on a collision course with Earth 30 or 40 years down the road, there is time to react.

Though the technology would have to be developed, possibilities include exploding the object or diverting it. [ Image Gallery: Potentially Dangerous Asteroids ] For every known asteroid, however, there are many that have not been spotted, and shorter reaction times could prove more threatening. When an asteroid, or a part of it, crashes into Earth, it's called a meteorite. Here are typical compositions: Iron meteorites Sun, Moon and Earth Orbits - The Children's University of Manchester Sun, Moon and Earth Orbits Claim your stamp Learn about our planet, our only natural satellite and our nearest star © 2012 This website is produced by the Student Recruitment, Admissions and International Development Division at The University of Manchester How far is each planet from Earth? (Intermediate) - Curious About Astronomy? Ask an Astronomer How far is each planet from Earth?

(Intermediate) (Intermediate) >" onclick="window.open(this.href,'win2','status=no,toolbar=no,scrollbars=yes,titlebar=no,menubar=no,resizable=yes,width=640,height=480,directories=no,location=no'); return false;" rel="nofollow"> We get lots of emails asking this, but it's actually a difficult question to answer because the planets are moving in their orbits all the time, and so the distance from each planet to Earth is constantly changing. When both planets are on the same side of the Sun, and form a line with the Sun, then they are closest to each other. When two planets are on opposite sides of the Sun, and form a line with the Sun, then they are farthest away. An inferior planet (a planet whose orbit is interior to Earth's orbit) is farthest from Earth near the time of superior conjunction and closest to Earth near the time of inferior conjunction. The diagram below (from this page ), with Earth as the blue-green dot, shows a gray circle representing an inferior planet (Mercury or Venus) at these places in its orbit.

For superior planets (like Mars), shown as the red circle in the diagram above, the planet is closest to Earth at about the time of opposition, and it is farthest from Earth at about the time of conjunction. However, since the planets' orbits are ellipses (not perfect circles), and they are not in exactly the same plane, the planet's minimum or maximum distance from Earth may not occur at exactly the same time as opposition or conjunction. Usually when people ask this question, what they mean is "What is the distance between the orbit of Earth and the orbit of each planet?" or "What is the closest that each planet comes to Earth?" (These are essentially the same question, because the planets can't get any closer than their orbital spacing allows.) You can compute this in a rough way by assuming that the orbits are circular and coplanar, and looking at the planet-to-Sun distance for each planet. Since the distances are so large, we usually express them in Astronomical Units (AU). (An AU is the average distance from Earth to the Sun, about 150 million kilometers or 93 million miles.) The table below lists the distance of each planet from the Sun in AU.

(Numbers were taken from this page .) Planet Pluto (dwarf planet) 39.48 To find the approximate distance between the orbits of two planets in AU, subtract the two planet-Sun distances. For example, Earth orbits at 1 AU from the Sun and Venus orbits at 0.72 AU from the Sun. The difference between these two distances is 1.00 - 0.72 = 0.28 AU. To get the number in kilometers, multiply by the conversion above: 0.28 × (150 million kilometers) = 42 million kilometers. Another example: Jupiter is 5.2 AU from the Sun and Earth is 1.0 AU from the Sun. So the distance between Jupiter's orbit and Earth's orbit is about 5.2 - 1.0 = 4.2 AU or 630 million kilometers. You can use this process for any of the planets using the numbers above. For completeness, I should point out that the circular orbit assumption is better for some planets than others. For example, Mars has a fairly eccentric elliptical orbit, which means that its "closest-approach" distance changes.

That's why in August 2003 , Mars was closer to Earth than it has been in about 60,000 years. But the numbers above are generally what are used in introductory astronomy classes. And that's why the Earth-Venus distance calculated in the previous paragraph (42 million kilometers) is slightly greater than the actual minimum Earth-Venus distance (38 million kilometers) - the planets' eccentric orbits can allow them to get slightly closer than the distance that one finds from the simple calculation. If you'd like to see which planets are closer to Earth right now, you can look at the Solar System Simulator or Solar System Live websites, where you can choose a time and then look down on the Solar System from above to see where the planets are. If you're technically inclined and want exact numbers, you can use the JPL Horizons system to select a planet, select a time range, and select "Observer range and range-rate" as an output quantity. You will get a list of times along with the Earth-object distance in AU. This page was last updated by Sean Marshall on March 31, 2016.

Question: How many planets orbit the sun between it and earth?
Answer: 2