Space Facts

The universe is a big big in fact that light from its outer reaches now being emitted will take billions of years to get here. Or you could say it another way: that light we now see from distant galaxies and quasars is billions of years old...emitted at a time in the past long before the Sun or the Earth were here.

Our tour through the cosmos will be easier if we take smaller steps, and occasionally take it all in from a distance. On the subject of distance, there are certain measurements that don't make much sense when expressed in miles or kilometers. Astronomers use a few different units such as the astronomical unit (the distance between the Earth and the Sun, about 93 million miles) and the light year (the distance that a pulse of light travels in one year, about 6 trillion miles).

Our Neighborhood: The Solar System

You live on a planet in space, a planet called Earth. The Earth is one of nine planets that orbit around our star which we call the Sun. This family of objects travels together through space and was formed about 5 billion years ago in a great cloud of gas and dust.As this cloud fell together due to its own gravity, the center heated up and became very dense. This hot, dense object in the center of the cloud became the Sun. Other concentrations in the cloud became the planets that we sometimes see in our night sky.

Can you name the planets in order starting with the nearest one to the Sun? If not then remember this:

"My Very Educated Mother Just Served Us Nine Pizzas"

"Why is this important and what does it mean?", you ask. If you can remember this phrase then you can remember the order of the planets in our Solar System, since the first letter of each word in the phrase stands for a planet's name.










The Sun

The Sun is our nearest star. It is a mass of very hot gas, mostly hydrogen, that produces energy deep in its core and sends that energy throughout the Solar System. The Sun has many different layers in its outer atmosphere that can be seen with telescopes and proper filters. Sunspots are slightly cooler than the surrounding layer, called the photosphere, in which they reside. Solar prominences are extensions of a different part of the atmosphere called the chromospheres. Solar flares also occur in this layer and send out great bursts of energy that can affect the Earth. The Sun makes up more than 99% of the total mass in our Solar System. It is also very large - about 110 times the Earth's diameter. The strong magnetic fields that give rise to the sunspots and prominences are generated by the Sun's rotation. If you were on the equator of the Sun (take your sunglasses!) you would spin around once in about 25 days; an observer near one of the solar poles would measure almost 30 days.


Mercury is the nearest planet to the Sun and is very hot. Since it is very small and has no atmosphere, though, the temperature on Mercury depends a lot on where you are. The night side of Mercury would be approximately -300-degrees F while the day side would be near 800-degrees F! Mercury spins on its axis very slowly, about once per 60 Earth days.


 You might expect that since Venus is a little further from the Sun than Mercury is that it should be a little cooler as well. However, one thing Venus has that Mercury doesn't is a thick atmosphere that traps in the heat from the Sun. A typical day on Venus would be about 900-degrees and the sheer amount of atmosphere means that the pressure is about 90 times what we're used to here on Earth. There's also a poisonous rain of sulfuric acid from the clouds that erodes the surface rocks (and any young prospectors as well). The Soviet Venera spacecraft and the American Magellan orbiter have shown Venus to be a planet of mountains, valleys, impact craters, and volcanic some ways like the Earth. Venus is also about the same size as Earth but has no moons. Venus' day is very long -- about 243 Earth days. Venus can sometimes be seen in the evening after sunset as the "evening star", or before sunrise as the "morning star". Since Venus is closer to the Sun than the Earth is, its shape as seen through a telescope goes through phases (much like the moon) as it orbits the Sun.


The Earth is the only planet in our Solar System that can support human life. This is due to a number of things: our distance from the Sun, an atmosphere that makes liquid water possible and screens out most of the harmful radiation from the sun, and the composition (oxygen, nitrogen) of that atmosphere. Something that is from the Earth is said to be terrestrial. Earth-like planets that are primarily composed of rocky material are called terrestrial planets, as opposed to gaseous or jovian planets like Jupiter. The Earth is a changing planet, changed by the forces of flowing water, wind, glaciers, volcanoes, and moving crustal plates that carry the continents and sea floors with them. As the Earth travels around the Sun once per year, we experience seasonal changes that are caused not by our varying distance from the Sun, but rather the tilt on our axis of about 23.5-degrees. As seen in the picture, this means that the light from the Sun is more direct on the northern hemisphere during our summer and less direct in winter. Thus during our summer, our neighbors in the southern hemisphere are experiencing winter.

The Earth's only natural satellite, the Moon, is similar to the composition of the Earth's crust and is about one quarter the size of our planet. This means that if you could set the moon down in the middle of the United States, its edges would stretch from the west coast to the east coast. The moon's surface was first visited by humans in July of 1969 with the Apollo 11 mission. With the eye you can gaze up at the moon and see the lunar "seas", or maria. These are not seas of water, but rather lowlands into which molten rock flowed in the past following large impacts by asteroids. There are high mountain ranges on the moon, deep valleys, and millions of impact craters -- some of which can be seen with binoculars. As the moon orbits around the Earth once per month, we see its changing phases and its eastward motion against the backdrop of stars.


Sometimes called the Red Planet, Mars is the fourth planet out from the Sun. Mars is about half the size of the Earth, but spins around in about the same amount of time, 25 hours. Mars is also tilted on its axis by about the same amount as the Earth, 25-degrees. Thus Mars also experiences seasons, but they are more dramatic due to its more elliptical, oval shaped orbit.Mars' red color comes from iron oxide (rust) deposits on the surface. Strong winds can blow across the landscape at half the speed of sound and sometimes produce global dust storms. It is apparent from studies of the surface of Mars that it was a much different planet in the past than it is now. There are great river valleys where water once flowed and huge extinct volcanoes the size of Arizona! The liquid water that once existed on Mars required a thicker atmosphere than is present today. The atmospheric pressure on Mars today is only about 1 percent of Earth's and is mainly carbon dioxide (CO2)...not breathable by human standards. The warmest temperatures on Mars' equator in mid-summer are about 80-degrees F at noon, compared to -170-degrees F just before sunrise. The north and south poles of Mars contain frozen ice caps of water and carbon dioxide.

Mars has two satellites named Phobos and Deimos. These are very small, irregularly shaped asteroids less than 10 miles across that cannot be seen easily from Earth.

There has been much speculation on the possible existence of life on Mars. It is unlikely that any type of life as we understand it could exist on Mars today...but perhaps in the past. That's not something we will likely know for a long time. The Viking spacecraft landed on Mars in 1976 and conducted experiments on the atmospheric gases, soil composition, and many other aspects of Mars. The preliminary tests for life (microbes) in the soil were negative.


An asteroid is a chunk of rock or metal, too small to be called a planet, that orbits the Sun. Between the orbits of Mars and Jupiter is the "asteroid belt". Although not a belt in the literal sense, there are thousands of bodies ranging in size from much less than a mile to about 600 miles in various orbits within the belt. The largest of these is named Ceres and was discovered in 1801.There are hundreds of asteroids that do not orbit in the asteroid belt, and much attention has been given lately to the asteroids on orbits that come near the Earth. The dinosaurs were thought to have become extinct after a large asteroid collided with the Earth 65 million years ago.


Jupiter is the largest planet in our Solar System. You could stretch eleven Earths across Jupiter and still have room left over! Taking a trip all the way around Jupiter would be a longer trip than from the Earth to the Moon. Living on Jupiter would be interesting - your day would be only 10 hours long, but your year (the time to orbit the Sun once) would be almost twelve Earth years. The atmosphere of Jupiter is arranged in bands that encircle the planet and is multicolored red, orange, brown, and white due to different gases. There's also a huge hurricane, the Great Red Spot, that could hold two Earths inside. Remember to take your coat since the temperature at the cloud tops of Jupiter is about -200-degrees F.Jupiter has a vast system of moons - about 39 at last count. The largest four of these were discovered by Galileo in 1610 and can occasionally be seen with binoculars. These are Callisto, Europa, Io, and Ganymede, the largest moon in the Solar System.Jupiter was first visited by the spacecraft Pioneer 10 in 1973, but most of what we know about the giant planet was learned by the Voyager crafts in 1979. Voyager increased our knowledge of Jupiter's atmospheric composition and magnetic field, and saw great flashes of lightning on the back side of the planet. Also discovered were several more moons and active volcanoes on the moon Io. The spacecraft Galileo is now in orbit around Jupiter.


Saturn is the second largest planet and resembles Jupiter in its overall structure and composition. One major difference is that Saturn is less dense than Jupiter, and in fact less dense than water; if you could find a body of water large enough, Saturn would bob to the surface and float! Saturn's most distinctive feature is its system of rings that encircle the planet. These rings were first glimpsed by Galileo, but his telescope was not large enough to show them clearly; he thought Saturn had ears. The rings can be seen clearly with a small modern telescope however.The ring system is thought to be the result of a moon which strayed too close to Saturn at some point in the past and was broken apart. There are actually thousands of separate rings and divisions within the rings. Many of the smaller satellites orbit inside the main ring system and control the ring structure itself.

Although Saturn's rings are over 170,000 miles across, they are very thin - only about 100 feet thick. This means that while the rings are very easy to see when the planet is tipped toward us, they disappear when viewed edge on. Such is the case in 1995.Saturn also has a large family of moons, at least 30 known currently. The largest of these is Titan, also the only moon known to have a significant atmosphere (a discovery made at McDonald Observatory). Voyager gave planetary astronomers a wealth of information about Saturn, its rings, and its moons in the early 1980's.

Uranus and Neptune

Uranus and Neptune are not visible without some optical aid such as binoculars. Uranus was discovered in 1781 by William Herschel, and Neptune in 1846 by Adams and Leverrier.These gaseous giants are listed together because they are similar in size, structure, and composition. Uranus is slightly larger than is Neptune and is tilted on its axis by more than 90-degrees. Thus Uranus resembles a ball rolling along in its orbit. The appearance of Uranus is blue-green and relatively featureless. A ring system exists around the planet, evidence for which was gathered from Earth by watching how a star's light is dimmed when Uranus passes in front of it; this is called an occultation. Uranus' system of moons includes nine bodies (four of which were discovered by Voyager II in 1986) with dramatically different landscapes, probably the most bizarre of which is Miranda. This moon appears as if it were broken apart at some time in the past and then slowly reassembled itself by its own gravity.

Neptune presents a more active atmosphere than does Uranus. It has a hurricane-like feature, similar to the Red Spot on Jupiter, called the Great Dark Spot. Accompanying this large storm are a few smaller ones named D2 (dark spot 2) and Scooter. Although astronomers knew of two moons and a partial ring system from Earth-based studies, virtually all we know of Neptune, its rings, and its eight moons was learned in 1989 when Voyager II visited the planet.


Pluto could be called the oddball planet for a number of reasons. It's the only non-gaseous planet in the outer Solar System, is the smallest of all the planets, has the proportionally largest moon, and travels around the Sun in a very strange orbit. Pluto was discovered by Clyde Tombaugh in 1930 at Lowell Observatory in Flagstaff, Arizona, after a long search. Nobody expected Pluto to be as small as it is, much too small to have a gravitational effect on Uranus or Neptune - the reason for suspecting that there was a ninth planet in the first place. Pluto is only about 1,500 miles across - smaller than our moon. The strange orbit that Pluto follows has suggested to some astronomers that it is an ejected moon from Neptune or Uranus. The physical conditions on Pluto would not be very comfortable for human exploration. At the distance of Pluto, some 4 billion miles from the Sun, solar radiation has little heating effect and the surface temperature approaches -350-degrees F. The surface is also covered with ice, part of which evaporates when Pluto is at perihelion (nearest to the Sun) once every 250 years.

The single moon of Pluto, named Charon, was discovered in 1978 by James Christy at the U.S. Naval Observatory. It is the largest moon in the Solar System when compared with the planet it orbits - about half the size of Pluto.

Other Solar System Objects

Besides the planets, moons, and asteroids that we've talked about, there are a variety of other inhabitants of our planetary system that were formed along with the Sun.

Comet Halley, 1910-1986.

Comets are "dirty snowballs" that move around the Sun in very stretched out orbits. Most spend the majority of their time in the outer reaches of the Solar System, but when nearest the Sun they form long tails of gas and dust blown back by the Solar Wind. Many are frequent visitors to the near-Earth area, such as comet Halley that can be seen every 76 years. Numerous new comets are discovered every year by astronomers using large telescopes.

Meteoroids are small asteroids, or rocks, some of which fall into the Earth's atmosphere and can be seen as meteors streaking across the sky. Most are very small (sand-grained size) and are completely burned up high above your head. The few that do strike the ground are known as meteorites. Most meteorites are of stony composition, whereas most of the recovered meteorites are metallic, containing high proportions of iron and nickel.

On any given night from a dark location you can see several meteors per hour. More can be seen at certain times during meteor showers. Shower meteors originate in comets; they are material that eroded from the comet's nucleus in past orbits.

When a meteorite strikes a moon or planet it leaves an impact crater on the landscape. Most terrestrial surfaces show a record of past cratering - the Earth and Moon, Mercury, Venus, Mars, and the satellites of the planets. A recent impact took place on the Earth in 1908 in Siberia; it flattened trees for 25 miles from the center of impact and produced strong shock waves that were felt hundreds of miles away.

Dust is found not only on Earth, but in the Solar System as a whole. Most of this interplanetary dust is concentrated along the plane, or disc, of the planetary orbits. Sunlight reflecting off this dust and seen from the Earth after sunset is called the Zodiacal Light.

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