Why don’t we experience lunar eclipse every month?



On Earth, we experience two types of eclipses: A solar eclipse happens when the moon moves between the sun and the earth, blocking out the sun’s rays from reaching the earth and casting its shadow on parts of the Earth.



A lunar eclipse occurs when the moon passes directly behind the earth and the earth’s shadow blocks sunlight from reflecting of the moon. During a total lunar eclipse, the earth’s shadow (umbra) completely covers the moon.



The moon takes approximately 28 days to make one orbit around the Earth. At this point, you may wonder why we don’t have a lunar eclipse every month. That’s because, the orbit of the moon (around the earth) lies in a different plane to that of the earth (around the sun). To be precise, the lunar orbit is inclined by 5.14 degrees to the latter.



For a lunar eclipse to happen, the sun, moon and earth must be aligned. This happens rarely given the differences in the orbit plane. On an average, a lunar eclipse happens three to four times in a year. In 2020, we will experience four lunar eclipses.



 



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What was Mercury's last mission?



NASA’s spacecraft MESSENGER blasted off from Cape Canaveral in the U.S. in August 2004. After a convoluted, 7.9-billion kilometre journey designed to bring it into alignment with Mercury at the right angle and speed, it was inserted into that planet’s orbit in March 2011.



It was the first time a spacecraft had been put into Mercury’s orbit.



Mercury, only slightly larger than our moon, is the smallest planet in our solar system and it is the one closet to the Sun.



Though originally supposed to function for just one year, Messenger orbited Mercury for four years. While mapping the sun-scorched surface, it sent back more than 2,70,000 images and masses of data about Mercury’s magnetic fields and atmospheric composition. Among other things, it detected hollows on the planet’s surface, ancient lava flows and ice caps at the poles.



When its propellant fuel began running out, MESSENGER was crashed onto the planet’s surface on April 2015.



Curtain Call



NASA’s 30 – year – old space shuttle programme came to a close with the landing of the last mission – space shuttle Atlantis – on July 21, 2011. The space shuttle fleet of Columbia, Challenger, Discovery, Atlantis and Endeavour together flew 135 missions and helped construct and maintain the International Space Station. Two missions, involving space shuttles Challenger (in 1986) and Columbia (in 2003), ended catastrophically and claimed the lives of 14 astronauts.



 



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How is it possible for the Sun to keep ‘burning’ for billions of years?



In the older days people thought that the Sun was a giant ball of burning coal. However, now we know that this is incorrect. The energy of the sun is produced at its core. At the core the temperature and pressure are so high that hydrogen is converted to helium. This nuclear fusion reaction results in the release of energy in the form of heat and light. So the Sun does not ‘burn’ as paper and wood burns. According to scientists, it will keep on shining for around 5 billion years more!



Most models suggest that as the oceans evaporate, more and more water will be present in the atmosphere instead of on the surface. This will act as a greenhouse gas, trapping even more heat and causing more and more of the oceans to evaporate, until the ground is mostly dry and the atmosphere holds the water, but at an extremely high temperature.



 



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Which is the second largest moon of Saturn?



One of the many things that makes our Earth really special is the presence of an oxygen atmosphere. This oxygen richly contributes to various processes happening here on Earth, making the planet a cradle of life.



Even though we have taken an interest in celestial bodies and explored space, near and far, Earth remained the only one with an oxygen atmosphere for long. That changed in 2010 when a spacecraft for the first time directly captured molecules of an oxygen atmosphere at a world was one of the Saturn’s many moons, Rhea.



The stars of Louis



The discovery of Rhea took place on December 23, 1672 by Italian astronomer, mathematician and engineer Giovanni Cassini. Rhea was the second of four Saturn satellites that Cassini discovered and the third to be found orbiting the ringed planet. Cassini named the four moons that he discovered – lapetus, Tethys and Dione in addition to Rhea – Sidera Lodoicea (“the stars of Louis”), in honour of King Louis XIV of France.



This name, however, never picked up outside of France. The names that we recognize them today with were given over a century later by English astronomer John Herschel, Son of astronomer William Herschel, the discoverer of Uranus, John suggested the name Rhea in 1847.



Saturn’s second largest



Saturn’s second largest satellite after Titan, Rhea has a mean radius of 764 km – equivalent in size to a little over a 10th of our Earth. Like Tethys and Dione, Rhea is also tidally locked in phase with its parent, meaning that one side of the moon always faces Saturn.



For centuries after its discovery, Rhea remained merely a tiny dot to astronomers. It was the Voyager (1 and 2) encounters in 1980 and 1981 that finally threw some light on it. Images from these encounters suggested that Rhea’s features could be divided into two regions. While the heavily centered terrain had craters larger than 40 km across, parts of the equatorial and polar region have craters less than 40 km across.



Following the Voyager encounters there were no missions to the region until the Cassini spacecraft made its way to orbit around Saturn in 2004. Cassini then made a number of targeted close flybys of Rhea in the decade that followed.



Tenuous atmosphere



In 2010, Cassini was able to detect around Rhea a very thin atmosphere, known as an exosphere, that was infused with oxygen and carbon dioxide. Even though the source of carbon dioxide isn’t certain, the oxygen is believed to arise when Saturn’s magnetic field rotates over Rhea. The moon’s water-ice surface is peppered with energetic particles that were trapped in Saturn’s magnetic field, and the chemical reactions that ensure decompose the surface and release oxygen.



When compared to what we have at Earth, oxygen at Rhea’s surface is estimated to be about 5 trillion times less dense. Despite the current minuscule composition, the formation of oxygen and carbon dioxide could well lead to complex chemistry.



The average age of Rhea’s plains is theorized to be around four billion. We’ve known this moon, which has been around for so long, for a few centuries now. During this time, we have learned a little bit about it, making it more than just a dot for us. There is, however, plenty still left to find out.



 



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How Spirit reached Mars?



When you want to find out something, but can’t get to where the action is taking place, what’s the next best thing? You would probably entrust others with the task, asking them to report the findings to you. What if the place we want to study is way beyond anywhere human eyes have reached? We would probably have to employ rovers to get there and relay what it finds back to us.



The Spirit and Opportunity were twins rovers built as part of NASA’s Mars Exploration Program. Also known as the Mars Exploration Rover 2 (MER 2, became MER-A), the Spirit rover was about the size of a golf cart and weighed nearly 185 kg.



Spirit goes first



Spirit and Opportunity were launched within a month of each other in 2003 and sent to opposite sides of Mars, in order to study the history of climate and water at those sites.



Spirit made its way first, launched on June 10, 2003 from Cape Canaveral, Florida. After initially being sent to an intermediary parking orbit round Earth, the spacecraft was thrust into a heliocentric orbit on a trajectory to intercept Mars. A number of course corrections later, Spirit approached Mars for landing.



Bounce, roll and stop



The lander, which was still inside a protective aeroshell, separated from the cruise stage only about 15 minutes before entering the martian atmosphere. A parachute was deployed at an altitude of 6 to 7.5 km and four massive airbags were inflated, but as these alone would not suffice to reduce velocity (atmosphere on Mars is less than 1% the density of Earth’s), retrorockets were fired.



On January 4 2004, the entire package hit the Martian surface at a velocity of 14m/s. It bounced 28 times before rolling to a stop bout 250 m from the point of first impact. The landing happened inside the Gusev crater, about 13.4 km away from the planned target.



Blessing in disguise



Even though Spirit’s original mission was scheduled to last only 90 Martian days – until about April 4, 2004 – mission planners were able to repeatedly extend the mission. One of the reasons for this was the passing of dust devils (strong, well-formed, short-lived whirlwinds) that swept dust from the top of the petal solar panels of Spirit, thereby increasing the power coming into the rover.



Even when Spirit faced stumbling blocks, it turned out to be to its advantage. For when its front wheel stopped working, it turned out to be a blessing in disguise as the non-functional wheel then scraped soil as the rover moved. This exposed bright silica-rich dust, an indicator of contact between soil and water.



Throughout its operating time on Mars, Spirit was able to discover a surprising variety of bedrock, churn up soil that contained more sulphur and a trace of water, unearth a patch of almost pure silica and find evidence of an ancient explosion. This, apart from the fact that the mast-mounted cameras provided never-before-seen images of our neighbouring planet and captured many movies of dust devils in action, helping us better understand wind effects on the Martian surface.



Loses contact



On May 1, 2009, Spirit got stuck in soft soil and after several attempts to get it moving failed, the mission was changed to a stationary science platform in January 2010. But after March 22, 2010, mission controllers were unable to regain contact with the rover.



Recovery efforts were officially concluded on May 25, 2011. The excessive cold is seen as the likely reason for loss of contact, as it would have made the rovers several heaters incapable of performing their duty. By the time it lost contact, Spirit had operated for six years, two months and 19 days, travelling 7.73 km across the Martian plains and collecting a treasure trove of data.



 



Picture Credit : Google