Natural Satellites

A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet (or sometimes another small Solar System body).

In the Solar System there are six planetary satellite systems containing 185 known natural satellites.^[1][2] Four IAU-listed dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris.^[3] As of September 2018, there are 334 other minor planets known to have moons.^[4]

The Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System (although there are minor-planet systems with even greater ratios, notably the Pluto–Charon system). At 3,474 km (2,158 miles) across, the Moon is 0.27 times the diameter of Earth

The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of De revolutionibus orbium coelestium in 1543. Until the discovery of the Galilean satellites in 1610, however, there was no opportunity for referring to such objects as a class. Galileochose to refer to his discoveries as Planetæ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.^{[citation needed]}

The first to use of the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") in 1610. He derived the term from the Latin word satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens.^[6]

The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology. Sputnik was created by Soviet Union, and it was the first satellite ever.^[6] The terms man-made satellite and artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has become linked primarily with artificial objects flown in space – including, sometimes, even those not in orbit around a planet.^{[citation needed]}

Because of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with natural satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in a sense opposed to "artificial") is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite, but not when referring to other natural satellites.

Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites.^[7][8][9] A few recent authors define "moon" as "a satellite of a planet or minor planet", and "planet" as "a satellite of a star" – such authors consider Earth as a "natural satellite of the sun".

There is no established lower limit on what is considered a "moon". Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets.

Mars

What is Mars like?

Mars is a cold desert world. It is half the size of Earth. Mars is sometimes called the Red Planet. It's red because of rusty iron in the ground.

Like Earth, Mars has seasons, polar ice caps, volcanoes, canyons, and weather. It has a very thin atmosphere made of carbon dioxide, nitrogen, and argon.

There are signs of ancient floods on Mars, but now water mostly exists in icy dirt and thin clouds. On some Martian hillsides, there is evidence of liquid salty water in the ground.

Scientists want to know if Mars may have had living things in the past. They also want to know if Mars could support life now or in the future.

The days and seasons are likewise comparable to those of Earth, because the rotational period as well as the tilt of the rotational axis relative to the ecliptic plane are very similar. Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basinin the northern hemisphere covers 40% of the planet and may be a giant impact feature.^[18][19] Mars has two moons, Phobosand Deimos, which are small and irregularly shaped. These may be captured asteroids,^[20][21] similar to 5261 Eureka, a Mars trojan.

There are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Future astrobiology missions are planned, including the Mars 2020 and ExoMarsrovers.^{[22][23][24][25]} Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% of the Earth's,^[26] except at the lowest elevations for short periods.^[27][28] The two polar ice caps appear to be made largely of water.^[29][30] The volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 meters (36 ft).^[31] In November 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior.^[32][33][34]

Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −2.94,^[11] which is surpassed only by Jupiter, Venus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 kilometers (190 mi) across when Earth and Mars are closest because of Earth's atmosphere.^[35]

Moons

Mars has two relatively small (compared to Earth's) natural moons, Phobos (about 22 km (14 mi) in diameter) and Deimos (about 12 km (7.5 mi) in diameter), which orbit close to the planet. Asteroid capture is a long-favored theory, but their origin remains uncertain.^[214]Both satellites were discovered in 1877 by Asaph Hall; they are named after the characters Phobos (panic/fear) and Deimos(terror/dread), who, in Greek mythology, accompanied their father Ares, god of war, into battle. Mars was the Roman counterpart of Ares.^[215][216] In modern Greek, the planet retains its ancient name Ares 

Next best planet to live on after Earth

Earth won't always be fit for occupation. We know that in two billion years or so, an expanding sun will boil away our oceans, leaving our home in the universe uninhabitable—unless, that is, we haven't already been wiped out by the Andromeda galaxy, which is on a multibillion-year collision course with our Milky Way. Moreover, at least a third of the thousand mile-wide asteroids that hurtle across our orbital path will eventually crash into us, at a rate of about one every 300,000 years.

Why?

Indeed, in 1989 a far smaller asteroid, the impact of which would still have been equivalent in force to 1,000 nuclear bombs, crossed our orbit just six hours after Earth had passed. A recent report by the Lifeboat Foundation, whose hundreds of researchers track a dozen different existential risks to humanity, likens that one-in-300,000 chance of a catastrophic strike to a game of Russian roulette: "If we keep pulling the trigger long enough we'll blow our head off, and there's no guarantee it won't be the next pull."

Earth won't always be fit for occupation. We know that in two billion years or so, an expanding sun will boil away our oceans, leaving our home in the universe uninhabitable—unless, that is, we haven't already been wiped out by the Andromeda galaxy, which is on a multibillion-year collision course with our Milky Way. Moreover, at least a third of the thousand mile-wide asteroids that hurtle across our orbital path will eventually crash into us, at a rate of about one every 300,000 years.

Where?

We have many options. The National Space Society, whose more than 12,000 members are committed to establishing settlements in space, suggests that we'll probably first go to a planet that has the resources to support life. After completing a $200-million study in 2000, NASA reported that a colony could be dug several feet beneath our own moon's surface or covered within an existing crater to protect residents from the constant bombardment of high-energy cosmic radiation, which can damage our DNA and lead to cancer. The NASA study envisions an onsite nuclear power plant, solar panel arrays, and various methods for extracting carbon, silicon, aluminum and other useful materials from the lunar surface. The National Space Society, in its own 2008 report "Roadmap to Space Settlement," also identifies the moon as the logical initial stop, citing the presence of life-sustaining ice there as a precursor to permanent lunar bases, hotels and even casinos.

How?

The first challenge is simply to escape the pull of Earth's own gravity. "If you can get your ship into orbit, you're halfway to anywhere," the writer Robert Heinlein said. The space shuttle flew at around $450 million a trip, and today sending unmanned payloads into orbit will still set you back about $12,000 a pound, with much of the cost coming from the fuel burned in those first hundred miles.

To clear this imposing initial hurdle, engineers have dreamed up many rocketless launch systems. At the height of the Cold War, the U.S. Navy, as part of its High Altitude Research Program, investigated the feasibility of using a giant cannon to blast payloads into orbit. Physicist Derek Tidman, meanwhile, envisions using a massive centrifuge, which he calls a "slingatron," to spin objects until they reach a velocity at which they can be flung out of our gravitational well. And many engineers have contemplated the enticing possibility of constructing a "space elevator" that rides up a 62,000-mile-long cable held aloft, like a spinning lasso, by centripetal force. 

When?

Right now, most of the progress toward space settlement is being accomplished in the private sector. Last December, Elon Musk's SpaceX completed a successful test flight of a reusable capsule capable of carrying up to seven people, and the company has a contract with NASA to shuttle cargo to the International Space station at per-pound costs far below the current rate. Virgin Galactic, Space Adventures and other companies have begun offering flights into low Earth orbit and brief stays in space stations, and Bigelow Aerospace has plans to launch an inflatable "space hotel" by 2015.

                             

                                 For any questions/queries contact me at: jagtapprasanna00@gmail.com

The Milky Way

The Milky Way Galaxy

The band of the Milky Way galaxy can be seen at night in areas with dark skies. Here it is seen with several Atacama Large Millimeter/submillimeter Arra (ALMA) antenna. (Credit: ESO/B. Tafreshi

Our Sun (a star) and all the planets around it are part of a galaxy known as the Milky Way Galaxy. A galaxy is a large group of stars, gas, and dustbound together by gravity. They come in a variety of shapes and sizes. The Milky Way is a large barred spiral galaxy. All the stars we see in the night sky are in our own Milky Way Galaxy. Our galaxy is called the Milky Way because it appears as a milky band of light in the sky when you see it in a really dark area.

Tell me more about galaxies

It is very difficult to count the number of stars in the Milky Way from our position inside the galaxy. Our best estimates tell us that the Milky Way is made up of approximately 100 billion stars. These stars form a large disk whose diameter is about 100,000 light years. Our Solar System is about 25,000 light years away from the center of our galaxy – we live in the suburbs of our galaxy. Just as the Earth goes around the Sun, the Sun goes around the center of the Milky Way. It takes 250 million years for our Sun and the solar system to go all the way around the center of the Milky Way.

Read a NASA Blueshift blog post about how many stars there are in the Milky Way

We can only take pictures of the Milky Way from inside the galaxy, which means we don't have an image of the Milky Way as a whole. Why do we think it is a barred spiral galaxy, then? There are several clues.

The first clue to the shape of the Milky Way comes from the bright band of stars that stretches across the sky (and, as mentioned above, is how the Milky Way got its name). This band of stars can be seen with the naked eye in places with dark night skies. That band comes from seeing the disk of stars that forms the Milky Way from inside the disk, and tells us that our galaxy is basically flat.

Several different telescopes, both on the ground and in space, have taken images of the disk of the Milky Way by taking a series of pictures in different directions – a bit like taking a panoramic picture with your camera or phone. The concentration of stars in a band adds to the evidence that the Milky Way is a spiral galaxy. If we lived in an elliptical galaxy, we would see the stars of our galaxy spread out all around the sky, not in a single band.

Additional clues to the spiral nature of the Milky Way come from a variety of other properties. Astronomers measure the amount of dust in the Milky Way and the dominant colors of the light we see, and they match those we find in other typical spiral galaxies. All of this adds up to give us a picture of the Milky Way, even though we can't get outside to see the whole thing.

There are billions of other galaxies in the Universe. Only three galaxies outside our own Milky Way Galaxy can be seen without a telescope, and appear as fuzzy patches in the sky with the naked eye. The closest galaxies that we can see without a telescope are the Large and Small Magellanic Clouds. These satellite galaxies of the Milky Way can be seen from the southern hemisphere. Even they are about 160,000 light years from us. The Andromeda Galaxy is a larger galaxy that can be seen from the northern hemisphere (with good eyesight and a very dark sky). It is about 2.5 million light years away from us, but its getting closer, and researchers predict that in about 4 billion years it will collide with the Milky Way. , i.e., it takes light 2.5 million years to reach us from one of our "nearby" galaxies. The other galaxies are even further away from us and can only be seen through telescopes.

What does Space smell like ??

When astronauts return from space walks and remove their helmets, they are welcomed back with a peculiar smell. An odor that is distinct and weird: something, astronauts have described it, like "seared steak." And also: "hot metal." And also: "welding fumes." 

Our extraterrestrial explorers are remarkably consistent in describing Space Scent in meaty-metallic terms. "Space," astronaut Tony Antonelli has said, "definitely has a smell that's different than anything else." Space, three-time spacewalker Thomas Jones has put it, "carries a distinct odor of ozone, a faint acrid smell." 

Space, Jones elaborated, smells a little like gunpowder. It is "sulfurous."

Add to all those anecdotal assessments the recent discovery, in a vast dust cloud at the center of our galaxy, of ethyl formate -- and the fact that the ester is, among other things, the chemical responsible for the flavor of raspberries. Add to that the fact that ethyl formate itself smells like rum. Put all that together, and one thing becomes clear: The final frontier sort of stinks.

But ... how does it stink, exactly? It turns out that we, and more specifically our atmosphere, are the ones who give space its special spice. According to one researcher, the aroma astronauts inhale as they move their mass from space to station is the result of "high-energy vibrations in particles brought back inside which mix with the air." 

Pearce came to NASA's attention after he recreated, for an art installation on "Impossible Smells," the scents of the Mir space station. (This was, he noted, a feat made more complicated by the fact that cosmonauts tend to bring vodka with them into space -- which affects not only the scent of their breath, but also that of their perspiration.) The result of Pearce's efforts? "Just imagine sweaty feet and stale body odor, mix that odor with nail polish remover and gasoline ... then you get close!"

Those efforts, alas, did not move forward. But had Pearce continued in creating a NASA-commissioned eau de vacuum, he would have had the aid of wonderfully poetic descriptions provided by astronautsthemselves. Such as, for example, this sweet-smelling stuff from wonder-astronaut Don Pettit: 

"Each time, when I repressed the airlock, opened the hatch and welcomed two tired workers inside, a peculiar odor tickled my olfactory senses," Pettit recalled. "At first I couldn't quite place it. It must have come from the air ducts that re-pressed the compartment. Then I noticed that this smell was on their suit, helmet, gloves, and tools. It was more pronounced on fabrics than on metal or plastic surfaces."

Dark Matter

Dark matter is a hypothetical form of matterthat is thought to account for approximately 85% of the matter in the universe and about a quarter of its total energy density. The majority of dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet undiscovered subatomic particles.^{[note 1]} Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen. For this reason, most experts think dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum, making it extremely difficult to detect using usual astronomical equipment.^[1]

The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter.^[2]Other lines of evidence include observations in gravitational lensing,^[3] from the cosmic microwave background, from astronomical observations of the observable universe's current structure, from the formation and evolution of galaxies, from mass location during galactic collisions,^[4] and from the motion of galaxies within galaxy clusters. In the standard Lambda-CDM model of cosmology, the total mass–energy of the universe contains 5% ordinary matter and energy, 27% dark matter and 68% of an unknown form of energy known as dark energy.^[5][6][7][8] Thus, dark matter constitutes 85%^{[note 2]} of total mass, while dark energy plus dark matter constitute 95% of total mass–energy content.^{[9][10][11][12]}

Because dark matter has not yet been observed directly, if it exists, it must barely interact with ordinary baryonic matter and radiation, except through gravity. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, in particular, weakly-interacting massive particles (WIMPs), or gravitationally-interacting massive particles (GIMPs).^[13]Many experiments to directly detect and study dark matter particles are being actively undertaken, but none have yet succeeded.^[14]Dark matter is classified as cold, warm, or hot according to its velocity (more precisely, its free streaming length). Current models favor a cold dark matter scenario, in which structures emerge by gradual accumulation of particles.

Although the existence of dark matter is generally accepted by the scientific community, some astrophysicists,^[15] intrigued by certain observations that do not fit the dark matter theory,^[16] argue for various modifications of the standard laws of general relativity, such as modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity. These models attempt to account for all observations without invoking supplemental non-baryonic matter

Dark matter can be divided into cold, warm, and hot categories.^[107] These categories refer to velocity rather than an actual temperature, indicating how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion—this is an important distance called the free streaming length (FSL). Primordial density fluctuations smaller than this length get washed out as particles spread from overdense to underdense regions, while larger fluctuations are unaffected; therefore this length sets a minimum scale for later structure formation. The categories are set with respect to the size of a protogalaxy (an object that later evolves into a dwarf galaxy): dark matter particles are classified as cold, warm, or hot according to their FSL; much smaller (cold), similar to (warm), or much larger (hot) than a protogalaxy.^[108][109]

Mixtures of the above are also possible: a theory of mixed dark matter was popular in the mid-1990s, but was rejected following the discovery of dark energy.^{[citation needed]}

Cold dark matter leads to a bottom-up formation of structure with galaxies forming first and galaxy clusters at a latter stage, while hot dark matter would result in a top-down formation scenario with large matter aggregations forming early, later fragmenting into separate galaxies;^{[clarification needed]} the latter is excluded by high-redshift galaxy observations.