an overview of the solar system

8/8/2019 An Overview of the Solar System

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An Overview of the Solar System

Orbits

The solar system consists of the Sun; the eight official planets, at least three "dwarf planets",more than 130 satellites of the planets, a large number ofsmall bodies(the comets and asteroids),and the interplanetary medium. (There are probably also many more planetary satellites that havenot yet been discovered.)

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 theouter solar system are Jupiter,Saturn, Uranus, andNeptune(Pluto is now classified as a dwarfplanet):
http://www.nineplanets.org/sol.htmlhttp://www.nineplanets.org/smallbodies.htmlhttp://www.nineplanets.org/smallbodies.htmlhttp://www.nineplanets.org/sol.htmlhttp://www.nineplanets.org/mercury.htmlhttp://www.nineplanets.org/mercury.htmlhttp://www.nineplanets.org/venus.htmlhttp://www.nineplanets.org/earth.htmlhttp://www.nineplanets.org/mars.htmlhttp://www.nineplanets.org/asteroids.htmlhttp://www.nineplanets.org/jupiter.htmlhttp://www.nineplanets.org/jupiter.htmlhttp://www.nineplanets.org/saturn.htmlhttp://www.nineplanets.org/uranus.htmlhttp://www.nineplanets.org/uranus.htmlhttp://www.nineplanets.org/neptune.htmlhttp://www.nineplanets.org/neptune.htmlhttp://www.nineplanets.org/pluto.htmlhttp://www.nineplanets.org/smallbodies.htmlhttp://www.nineplanets.org/sol.htmlhttp://www.nineplanets.org/mercury.htmlhttp://www.nineplanets.org/venus.htmlhttp://www.nineplanets.org/earth.htmlhttp://www.nineplanets.org/mars.htmlhttp://www.nineplanets.org/asteroids.htmlhttp://www.nineplanets.org/jupiter.htmlhttp://www.nineplanets.org/saturn.htmlhttp://www.nineplanets.org/uranus.htmlhttp://www.nineplanets.org/neptune.htmlhttp://www.nineplanets.org/pluto.htmlhttp://www.nineplanets.org/sol.html


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The first thing to notice is that the solar system is mostly empty space. The planets are very smallcompared to the space between them. Even the dots on the diagrams above are too big to be inproper 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 arevery nearly circular. The orbits of the planets are all more or less in the same plane (called theecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degreesfrom 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 fromabove 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 excellentplanetarium program Starry Night; there are also many othersimilar programs available, somefree.)

Sizes
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The above composite shows the eight planets and Pluto with approximately correct relative sizes(see anothersimilar composite and acomparison of the terrestrial planetsorAppendix 2 formore).

One way to help visualize the relative sizes in the solar system is to imagine a model in whicheverything is reduced in size by a factor of a billion. Then the model Earth would be about 1.3cm in diameter (the size of a grape). The Moon would be about 30 cm (about a foot) from theEarth. 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 largegrapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocksaway; Uranus and Neptune (lemons) 20 and 30 blocks away. A human on this scale would be thesize 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 solarsystem: the satellites of the planets; the large number ofasteroids(small rocky bodies) orbitingthe 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 atrandom orientations to the ecliptic; and the many small icy bodies beyond Neptune in theKuiperBelt. With a few exceptions, the planetary satellites orbit in the same sense as the planets andapproximately in the plane of the ecliptic but this is not generally true for comets and asteroids.

ClassificationHardcopy

The New Solar SystemSummarizes what we've learned from interplanetary explorations in the last 25 years. Myprimary reference forThe Nine Planets.

The Compact NASA Atlas of the Solar SystemThis 'road map' of the solar system is the definitive guide for beginning planetary science.
http://www.jpl.nasa.gov/galileo/sepo/education/nav/ss2.gifhttp://www.jpl.nasa.gov/galileo/sepo/education/nav/terr2.gifhttp://www.jpl.nasa.gov/galileo/sepo/education/nav/terr2.gifhttp://www.jpl.nasa.gov/galileo/sepo/education/nav/terr2.gifhttp://www.nineplanets.org/asteroids.htmlhttp://www.nineplanets.org/asteroids.htmlhttp://www.nineplanets.org/asteroids.htmlhttp://www.nineplanets.org/comets.htmlhttp://www.nineplanets.org/comets.htmlhttp://www.nineplanets.org/kboc.htmlhttp://www.nineplanets.org/kboc.htmlhttp://www.nineplanets.org/kboc.htmlhttp://bill.nineplanets.org/bookstore.htmlhttp://www.amazon.com/exec/obidos/ASIN/0521645875/billarnetthttp://www.amazon.com/exec/obidos/ASIN/052180633X/billarnetthttp://www.jpl.nasa.gov/galileo/sepo/education/nav/ss2.gifhttp://www.jpl.nasa.gov/galileo/sepo/education/nav/terr2.gifhttp://www.nineplanets.org/asteroids.htmlhttp://www.nineplanets.org/comets.htmlhttp://www.nineplanets.org/kboc.htmlhttp://www.nineplanets.org/kboc.htmlhttp://bill.nineplanets.org/bookstore.htmlhttp://www.amazon.com/exec/obidos/ASIN/0521645875/billarnetthttp://www.amazon.com/exec/obidos/ASIN/052180633X/billarnett


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The classification of these objects is a matter of minor controversy. Traditionally, the solarsystem has been divided into planets (the big bodies orbiting the Sun), theirsatellites (a.k.a.moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting theSun) and comets (small icy objects with highly eccentric orbits). Unfortunately, the solar systemhas 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 latercaptured 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 proposedwhich attempt to be more physically valid. But they usually end up with either too many classesor too many exceptions. The bottom line is that many of the bodies are unique; the actualsituation 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 orrocky planets: Mercury, Venus, Earth, and Mars:

The terrestrial planets are composed primarily of rock and metal and haverelatively high densities, slow rotation, solid surfaces, no rings and fewsatellites.

jovian orgas 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 andlots 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.
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closer to the Sun than Earth.

The inferior planets show phases like the Moon's when viewed fromEarth.

Earth.

superior planets: Mars thru Neptune. farther from the Sun than Earth.

The superior planets always appear full or nearly so.

by history:

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 orderwas usually specificied as: Saturn, Jupiter, Mars, Sun, Venus, Mercury

and Moon, based on the time for them to go "all the way round" the sphereof the "fixed" stars).

modern planets: Uranus, Neptune.

discovered in modern times

visible only with optical aid

Earth.

The IAU has recently decided that "classical" should refer to all eight planets

(Mercury thru Neptune, including Earth but not Pluto). This is contrary tohistorical usage but makes some sense from a 21st century perspective.

PicturesNote: most of the images in The Nine Planets are not true color. Most of them were created bycombining several black and white images taken thru various color filters. Though the colorsmay look "right" chances are they aren't exactly what your eye would see.

The Nine Planets montage (larger version of the above)36k jpg

Another relative size comparison (from LANL) 93k gif

Sun and large planet comparison (fromExtrema) 15k jpg

Earth and small body comparison (from Extrema) 13k jpg

Voyager 1 mosaic of the solar system from 4 billion miles out36k jpg; html(caption)

Voyager 1 images of 6 planets from 4 billion miles out 123k jpg; html

Pale Blue Dot, reflections on the above image by Carl Sagan.

More General Overview The largest, smallest, brightest, etc bodies

The history of solar system discovery
http://www.nineplanets.org/history.htmlhttp://venus.nineplanets.org/notes.htmlhttp://www.nineplanets.org/gif/NinePlanets.jpghttp://www.nineplanets.org/gif/NinePlanets.jpghttp://www.solarviews.com/cap/misc/ss.htmhttp://www.nineplanets.org/datamax.htmlhttp://www.nineplanets.org/datamax.htmlhttp://www.nineplanets.org/gif/soletal.jpghttp://www.nineplanets.org/datamax.htmlhttp://www.nineplanets.org/gif/SmallWorlds.jpghttp://nssdc.gsfc.nasa.gov/image/planetary/solar_system/solar_family.jpghttp://nssdc.gsfc.nasa.gov/image/planetary/solar_system/solar_family.jpghttp://www.solarviews.com/cap/misc/vgr_fam3.htmhttp://www.solarviews.com/cap/misc/vgr_fam3.htmhttp://antwrp.gsfc.nasa.gov/apod/ap990505.htmlhttp://nssdc.gsfc.nasa.gov/image/planetary/solar_system/family_portraits.jpghttp://www.solarviews.com/cap/misc/vgr_fam1.htmhttp://nineplanets.org/psc/pbd.htmlhttp://nineplanets.org/psc/pbd.htmlhttp://www.nineplanets.org/datamax.htmlhttp://www.nineplanets.org/history.htmlhttp://www.nineplanets.org/history.htmlhttp://venus.nineplanets.org/notes.htmlhttp://www.nineplanets.org/gif/NinePlanets.jpghttp://www.solarviews.com/cap/misc/ss.htmhttp://www.nineplanets.org/datamax.htmlhttp://www.nineplanets.org/gif/soletal.jpghttp://www.nineplanets.org/datamax.htmlhttp://www.nineplanets.org/gif/SmallWorlds.jpghttp://nssdc.gsfc.nasa.gov/image/planetary/solar_system/solar_family.jpghttp://www.solarviews.com/cap/misc/vgr_fam3.htmhttp://antwrp.gsfc.nasa.gov/apod/ap990505.htmlhttp://nssdc.gsfc.nasa.gov/image/planetary/solar_system/family_portraits.jpghttp://www.solarviews.com/cap/misc/vgr_fam1.htmhttp://nineplanets.org/psc/pbd.htmlhttp://www.nineplanets.org/datamax.htmlhttp://www.nineplanets.org/history.html


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Solar System Introduction from LANL

Solar System Family Portrait fromNSSDC

Solar System Live, the interactive Orrery of the Web.

notes about the most distant object in the solar system and the surface temperatures of the

planets from RGO scale models of the solar system

A Solar System Scale Model Meta Page (links to many others)

Lakeview Museum Community Solar System, the world's largest model of the

solar system

scale model from LPI

Sagan Planet Walkin Ithaca, NY

Build a Solar System, a neat scale model calculator

Silver City, NM Sidewalk Solar System

Solar System Walkin Gainesville, Florida

Eugene Oregon Scale Model Solar System

Bonsall Elementary

PlanetTrek, a solar system scale model for Pasadena

Walk the Solar System, a nice size comparison calculator from the Exploratorium

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 goodbibliography 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? There is now good evidence ofJupiter-sized objects orbiting several nearby stars. What conditions allow the formationof terrestrial planets? It seems unlikely that the Earth is totally unique but we still have nodirect evidence one way or the other.

Is there life elsewhere in the solar system? If not, why is Earth special?

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 itadaptable, widespread and common?

Answers to these questions, even partial ones, would be of enormous value. Answers to thelesser questions on the pages that follow may help answer some of these big ones.
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Our Sun is a normal main-sequenceG2star, one of more than 100billion stars in our galaxy.

diameter: 1,390,000 km.

mass: 1.989e30kg

temperature: 5800 K (surface)15,600,000 K (core)

The Sun is by far the largestobject in the solar system. It contains more than 99.8% of the totalmass of the Solar System (Jupitercontains most of the rest).

It is often said that the Sun is an "ordinary" star. That's true in the sense that there are manyothers similar to it. But there are many more smaller stars than larger ones; the Sun is in the top10% by mass. The median size of stars in our galaxy is probably less than half the mass of theSun.

The Sun is personified in many mythologies: the Greeks called it Heliosand the Romans called itSol.

The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else ("metals")amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to heliumin its core.

The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates onceevery 25.4 days; near the poles it's as much as 36 days. This odd behavior is due to the fact thatthe Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. Thedifferential rotation extends considerably down into the interior of the Sun but the core of theSun rotates as a solid body.

Conditions at the Sun's core (approximately the inner 25% of its radius) are extreme. Thetemperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center ofthe core the Sun's density is more than 150 times that of water.

The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced bynuclear fusionreactions. Each second about 700,000,000 tons of hydrogen are converted to about695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gammarays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted atlower and lower temperatures so that by the time it reaches the surface, it is primarily visiblelight. For the last 20% of the way to the surface the energy is carried more byconvectionthan byradiation.
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The surface of the Sun, called the photosphere, is at a temperature ofabout 5800 K. Sunspots are "cool" regions, only 3800 K (they look darkonly by comparison with the surrounding regions). Sunspots can be verylarge, as much as 50,000 km in diameter. Sunspots are caused bycomplicated and not very well understood interactions with the Sun'smagnetic field.

A small region known as the chromosphere lies above the photosphere.

The highly rarefied region above the chromosphere, called the corona,extends millions of kilometers into space but is visible only during atotal solar eclipse (left). Temperatures in the corona are over 1,000,000K.

It just happens that the Moon and the Sun appear the same size in thesky as viewed from the Earth. And since the Moon orbits the Earth inapproximately the same plane as the Earth's orbit around the Sunsometimes the Moon comes directly between the Earth and the Sun.This is called a solar eclipse; if the alignment is slighly imperfect then

the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it linesup perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partialeclipses are visible over a wide area of the Earth but the region from which a total eclipse isvisible, called the path of totality, is very narrow, just a few kilometers (though it is usuallythousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stayhome, you're likely to see a partial eclipse several times per decade. But since the path of totalityis so small it is very unlikely that it will cross you home. So people often travel half way aroundthe world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesomeexperience. For a few precious minutes it gets dark in the middle of the day. The stars come out.The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth amajor journey.

The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Itsmagnetosphere (also known as the heliosphere) extends well beyond Pluto.

In addition to heat and light, the Sun also emits a low density stream of chargedparticles (mostly electrons and protons) known as the solar wind whichpropagates throughout the solar system at about 450 km/sec. The solar wind andthe much higher energy particles ejected by solar flares can have dramaticeffects on the Earth ranging from power line surges to radio interference to thebeautiful aurora borealis.

Recent data from the spacecraft Ulyssesshow that during the minimum of the solar cycle thesolar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers persecond, than it does at lower latitudes. The composition of the solar wind also appears to differ in

the polar regions. During the solar maximum, however, the solar wind moves at an intermediatespeed.

Further study of the solar wind will be done by the recently launched Wind, ACE and SOHOspacecraft from the dynamically stable vantage point directly between the Earth and the Sunabout 1.6 million km from Earth.

The solar wind has large effects on the tails of comets and even has measurable effects on thetrajectories of spacecraft.
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Spectacular loops and prominences are often visible on the Sun's limb (left).

The Sun's output is not entirely constant. Nor is the amount of sunspot activity.There was a period of very low sunspot activity in the latter half of the 17thcentury called the Maunder Minimum. It coincides with an abnormally cold

period in northern Europe sometimes known as the Little Ice Age. Since the formation of the

solar system the Sun's output has increased by about 40%.The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen inits core. It will continue to radiate "peacefully" for another 5 billion years or so (although itsluminosity will approximately double in that time). But eventually it will run out of hydrogenfuel. It will then be forced into radical changes which, though commonplace by stellar standards,will result in the total destruction of the Earth (and probably the creation of aplanetary nebula).

The Sun's satellitesThere are eight planets and a large number ofsmaller objects orbiting the Sun. (Exactly whichbodies should be classified as planets and which as "smaller objects" has been the source of somecontroversy, but in the end it is really only a matter of definition. Pluto is no longer officially a

planet but we'll keep it here for history's sake.)Distance Radius Mass

Planet (000 km) (km) (kg) Discoverer Date

--------- --------- ------ ------- ---------- -----

Mercury 57,910 2439 3.30e23

Venus 108,200 6052 4.87e24

Earth 149,600 6378 5.98e24

Mars 227,940 3397 6.42e23

Jupiter 778,330 71492 1.90e27

Saturn 1,426,940 60268 5.69e26

Uranus 2,870,990 25559 8.69e25 Herschel 1781

Neptune 4,497,070 24764 1.02e26 Galle 1846

Pluto 5,913,520 1160 1.31e22 Tombaugh 1930

More detailed data and definitions of terms can be found on the data page.

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more Sun images

fromNSSDC

Stanford Solar Center
http://www.nineplanets.org/smallbodies.htmlhttp://www.nineplanets.org/mercury.htmlhttp://www.nineplanets.org/venus.htmlhttp://www.nineplanets.org/earth.htmlhttp://www.nineplanets.org/mars.htmlhttp://www.nineplanets.org/jupiter.htmlhttp://www.nineplanets.org/saturn.htmlhttp://www.nineplanets.org/uranus.htmlhttp://www.nineplanets.org/neptune.htmlhttp://www.nineplanets.org/pluto.htmlhttp://www.nineplanets.org/data.htmlhttp://www.nineplanets.org/pxsol.htmlhttp://www.nineplanets.org/pxsol.htmlhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-solar.htmlhttp://solar-center.stanford.edu/http://www.google.com/http://www.solarviews.com/cap/sun/sun.htmhttp://www.nineplanets.org/smallbodies.htmlhttp://www.nineplanets.org/mercury.htmlhttp://www.nineplanets.org/venus.htmlhttp://www.nineplanets.org/earth.htmlhttp://www.nineplanets.org/mars.htmlhttp://www.nineplanets.org/jupiter.htmlhttp://www.nineplanets.org/saturn.htmlhttp://www.nineplanets.org/uranus.htmlhttp://www.nineplanets.org/neptune.htmlhttp://www.nineplanets.org/pluto.htmlhttp://www.nineplanets.org/data.htmlhttp://www.nineplanets.org/pxsol.htmlhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-solar.htmlhttp://solar-center.stanford.edu/


10/18

Yohkoh Public Outreach Project, lots of good info, images and movies

The University of Michigan Solar and Heliospheric Research Group's Web Space forKids and Non-Scientists

Today's Space Weatherand index of solar images

Solar Data Analysis Center Elemental abundances in the Sun

Solar Eclipse info

from Fred Espenak, Mr. Eclipse

from Solar Data Analysis Center

Sky Online's Eclipse Page

Dale Ireland's Eclipses

images by Bob Yen

Umbraphilia, a personal account of the 1998 eclipse in the Caribbean

National Solar Observatory / Sacramento PeakImage Index

more info and links about sunspots

historical info about sunspots

Virtual Tour of the Sunby Michael Berger

The Sun: a Pictorial Introduction, a slide set by P. Charbonneau and O.R. White

The HK Project

Ulysses Home Page

Spartan 201, NASA's mission to explore the Sun's corona

IACG Campaign IV: including lots of good references

Lives and Deaths of Stars; notes by Nick Strobel of the University of Washington

ESA/NASA'sSOHO - Solar and Heliospheric Observatory home page

articles by John Bahcall, many focusing on the solar neutrino problem

Interview with Solby Robert J. Nemiroff

Solar Folklore, various myths about the Sun
http://www.lmsal.com/YPOPhttp://www.lmsal.com/YPOPhttp://solar-heliospheric.engin.umich.edu/hjenning/Welcome.htmlhttp://www.sec.noaa.gov/today.htmlhttp://umbra.nascom.nasa.gov/sdac.htmlhttp://tlusty.gsfc.nasa.gov/Tlusty2002/solar-abun.htmlhttp://sunearth.gsfc.nasa.gov/eclipse/eclipse.htmlhttp://umbra.nascom.nasa.gov/eclipse/http://umbra.nascom.nasa.gov/eclipse/http://skyandtelescope.com/observing/objects/eclipses/article_80_1.asphttp://www.drdale.com/eclipseshttp://www.comet-track.com/eclipse/secl.htmlhttp://obs.nineplanets.org/tse98/tse98.htmlhttp://nsosp.nso.edu/data/latest_solar_images.htmlhttp://antwrp.gsfc.nasa.gov/apod/ap980322.htmlhttp://es.rice.edu/ES/humsoc/Galileo/Things/sunspots.htmlhttp://www.astro.uva.nl/demo/od95/http://www.hao.ucar.edu/public/slides/slides.htmlhttp://www.mtwilson.edu/Science/HK_Project/hk_main.htmlhttp://helio.estec.esa.nl/ulysses/http://umbra.nascom.nasa.gov/spartan/http://space.umd.edu/IACG_c4/c4_home.htmlhttp://www.astronomynotes.com/evolutn/chindex.htmhttp://sohowww.nascom.nasa.gov/http://sohowww.nascom.nasa.gov/http://www.sns.ias.edu/~jnb/Papers/Popular/popular.htmlhttp://antwrp.gsfc.nasa.gov/htmltest/gifcity/interv.htmlhttp://solar-center.stanford.edu/folklore/folklore.htmlhttp://www.lmsal.com/YPOPhttp://solar-heliospheric.engin.umich.edu/hjenning/Welcome.htmlhttp://www.sec.noaa.gov/today.htmlhttp://umbra.nascom.nasa.gov/sdac.htmlhttp://tlusty.gsfc.nasa.gov/Tlusty2002/solar-abun.htmlhttp://sunearth.gsfc.nasa.gov/eclipse/eclipse.htmlhttp://umbra.nascom.nasa.gov/eclipse/http://skyandtelescope.com/observing/objects/eclipses/article_80_1.asphttp://www.drdale.com/eclipseshttp://www.comet-track.com/eclipse/secl.htmlhttp://obs.nineplanets.org/tse98/tse98.htmlhttp://nsosp.nso.edu/data/latest_solar_images.htmlhttp://antwrp.gsfc.nasa.gov/apod/ap980322.htmlhttp://es.rice.edu/ES/humsoc/Galileo/Things/sunspots.htmlhttp://www.astro.uva.nl/demo/od95/http://www.hao.ucar.edu/public/slides/slides.htmlhttp://www.mtwilson.edu/Science/HK_Project/hk_main.htmlhttp://helio.estec.esa.nl/ulysses/http://umbra.nascom.nasa.gov/spartan/http://space.umd.edu/IACG_c4/c4_home.htmlhttp://www.astronomynotes.com/evolutn/chindex.htmhttp://sohowww.nascom.nasa.gov/http://www.sns.ias.edu/~jnb/Papers/Popular/popular.htmlhttp://antwrp.gsfc.nasa.gov/htmltest/gifcity/interv.htmlhttp://solar-center.stanford.edu/folklore/folklore.html


11/18

Mercury is the closest planet to the Sun and the eighth largest. Mercury is slightly smaller indiameter than the moons Ganymedeand Titan but more than twice as massive.

orbit: 57,910,000 km (0.38 AU) from Sun

diameter: 4,880 km

mass: 3.30e23 kg

The New Solar SystemSummarizes what we've learned from interplanetary explorations in the last 25 years. Myprimary reference forThe Nine Planets.

In Search of Planet VulcanAn account of the non-discovery of a planet interior to Mercury. A much more interesting tale

than you might imagine.In Roman mythology Mercury is the god of commerce, travel and thievery, the Romancounterpart of the Greek godHermes, the messenger of the Gods. The planet probably receivedthis name because it moves so quickly across the sky.

Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It wassometimes given separate names for its apparitions as a morning star and as an evening star.Greek astronomers knew, however, that the two names referred to the same body. Heraclituseven believed that Mercury and Venus orbit the Sun, not the Earth.

Since it is closer to the Sun than the Earth, the illumination of Mercury's disk varies whenviewed with a telescope from our perspective. Galileo's telescope was too small to see Mercury'sphases but he did see the phases ofVenus.

Mercury has been now been visited by two spacecraft, Mariner 10 and MESSENGER. Marriner10 flew by three times in 1974 and 1975. Only 45% of the surface was mapped (and,unfortunately, it is too close to the Sun to be safely imaged byHST). MESSENGERwaslaunched by NASA in 2004 and will orbit Mercury starting in 2011 after several flybys. Its firstflyby in Jan 2008 provided new high quality images of some of the terrain not seen by Marriner10.
http://www.nineplanets.org/sol.htmlhttp://www.nineplanets.org/ganymede.htmlhttp://www.nineplanets.org/ganymede.htmlhttp://www.nineplanets.org/titan.htmlhttp://www.nineplanets.org/data.htmlhttp://www.nineplanets.org/data1.htmlhttp://www.nineplanets.org/data1.htmlhttp://www.amazon.com/exec/obidos/ASIN/0521645875/billarnetthttp://www.amazon.com/exec/obidos/ASIN/0738208892/billarnetthttp://www.pantheon.org/articles/m/mercury.htmlhttp://www.pantheon.org/articles/h/hermes.htmlhttp://www.pantheon.org/articles/h/hermes.htmlhttp://www.nineplanets.org/venus.htmlhttp://messenger.jhuapl.edu/index.htmlhttp://messenger.jhuapl.edu/index.htmlhttp://messenger.jhuapl.edu/index.htmlhttp://messenger.jhuapl.edu/gallery/sciencePhotos/view.php?gallery_id=2http://www.nineplanets.org/sol.htmlhttp://www.nineplanets.org/ganymede.htmlhttp://www.nineplanets.org/titan.htmlhttp://www.nineplanets.org/data.htmlhttp://www.nineplanets.org/data1.htmlhttp://www.nineplanets.org/data1.htmlhttp://www.amazon.com/exec/obidos/ASIN/0521645875/billarnetthttp://www.amazon.com/exec/obidos/ASIN/0738208892/billarnetthttp://www.pantheon.org/articles/m/mercury.htmlhttp://www.pantheon.org/articles/h/hermes.htmlhttp://www.nineplanets.org/venus.htmlhttp://messenger.jhuapl.edu/index.htmlhttp://messenger.jhuapl.edu/index.htmlhttp://messenger.jhuapl.edu/gallery/sciencePhotos/view.php?gallery_id=2


12/18

Mercury's orbit is highly eccentric; atperihelion it is only 46 million km from the Sun but ataphelionit is 70 million. The position of the perihelion precesses around the Sun at a very slowrate. 19th century astronomers made very careful observations of Mercury's orbital parametersbut could not adequately explain them usingNewtonianmechanics. The tiny differences betweenthe observed and predicted values were a minor but nagging problem for many decades. It wasthought that another planet (sometimes called Vulcan) slightly closer to the Sun than Mercurymight account for the discrepancy. But despite much effort, no such planet was found. The realanswer turned out to be much more dramatic: Einstein'sGeneral Theory of Relativity! Its correctprediction of the motions of Mercury was an important factor in the early acceptance of thetheory.

Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keepthat same face to the Sun much as the Moon does to the Earth. But this was shown to be false in1965 by doppler radar observations. It is now known that Mercury rotates three times in two ofits years. Mercury is the only body in the solar system known to have an orbital/rotationalresonance with a ratio other than 1:1 (though many have no resonances at all).

This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an

observer on Mercury's surface. At some longitudes the observer would see the Sun rise and thengradually increase in apparent size as it slowly moved toward the zenith. At that point the Sunwould stop, briefly reverse course, and stop again before resuming its path toward the horizonand decreasing in apparent size. All the while the stars would be moving three times faster acrossthe sky. Observers at other points on Mercury's surface would see different but equally bizarremotions.

Temperature variations on Mercury are the most extreme in the solar system ranging from 90 Kto 700 K. The temperature on Venus is slightly hotter but very stable.

Mercury craters

Mercury is in many ways similar to the Moon: its surface is heavily cratered and very old; it hasnoplate tectonics. On the other hand, Mercury is much denser than the Moon (5.43 gm/cm3 vs3.34). Mercury is the second densest major body in the solar system, afterEarth. Actually Earth'sdensity is due in part to gravitational compression; if not for this, Mercury would be denser thanEarth. This indicates that Mercury's dense iron core is relatively larger than Earth's, probablycomprising the majority of the planet. Mercury therefore has only a relatively thinsilicate mantleand crust.

Mercury's interior is dominated by a large iron core whose radius is 1800 to 1900 km. Thesilicate outer shell (analogous to Earth's mantle and crust) is only 500 to 600 km thick. At leastsome of the core is probably molten.

Mercury actually has a very thin atmosphere consisting of atoms blasted off its surface by thesolar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast tothe Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly beingreplenished.
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13/18

Southwest MercuryThe surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers inlength and as much as three kilometers high. Some cut thru the rings of craters and other featuresin such a way as to indicate that they were formed by compression. It is estimated that thesurface area of Mercury shrank by about 0.1% (or a decrease of about 1 km in the planet'sradius).

Caloris Basin

One of the largest features on Mercury's surface is the Caloris Basin (right); it is about 1300 kmin diameter. It is thought to be similar to the large basins (maria) on the Moon. Like the lunarbasins, it was probably caused by a very large impact early in the history of the solar system.

Weird terrain opposite Caloris Basin

That impact was probably also responsible for the odd terrain on the exact opposite side of the

planet (left).

In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains.Some may be the result of ancient volcanic activity but some may be the result of the depositionof ejecta from cratering impacts.

A reanalysis of the Mariner data provides some preliminary evidence of recent volcanism onMercury. But more data will be needed for confirmation.

Amazingly, radar observations of Mercury's north pole (a region not mapped by Mariner 10)show evidence of water ice in the protected shadows of some craters.

Mercury has a small magnetic field whose strength is about 1% of Earth's.

Mercury has no known satellites.Mercury is often visible with binoculars or even the unaided eye, but it is always very near theSun and difficult to see in the twilight sky. There are several Web sites that show the currentposition of Mercury (and the other planets) in the sky. More detailed and customized charts canbe created with aplanetarium program.

More about MercuryTop of Form
http://www.nineplanets.org/luna.htmlhttp://www.psrd.hawaii.edu/Jan97/MercuryUnveiled.htmlhttp://www.nineplanets.org/see.htmlhttp://astro.nineplanets.org/astrosoftware.htmlhttp://astro.nineplanets.org/astrosoftware.htmlhttp://pds.jpl.nasa.gov/planets/captions/mercury/mercter.htmhttp://pds.jpl.nasa.gov/planets/captions/mercury/merccal.htmhttp://pds.jpl.nasa.gov/planets/captions/mercury/mercury2.htmhttp://www.nineplanets.org/luna.htmlhttp://www.psrd.hawaii.edu/Jan97/MercuryUnveiled.htmlhttp://www.nineplanets.org/see.htmlhttp://astro.nineplanets.org/astrosoftware.html


14/18

Bottom of Form

more Mercury images

fromNSSDC Mariner 10 Image Project

from StarDate

from RGO

fromNSSDC

Mercury Unveiled, a new look at the Mariner 10 data

Mercury Nomenclature Table

(low res) images from the 0.5 m Swedish Vacuum Solar Telescope

ground based images from Boston University

more ground-based images

Mercury Studies, ongoing research

Degas Craterand some links

Open Issues Mercury's density (5.43 gm/cm3) is nearly as high as Earth's. Yet in most other respects it

more closely resembles the Moon. Did it lose its light rocks in some early catastrophicimpact?

No trace of iron has been seen in spectroscopic studies of Mercury's surface. Given its

presumably large iron core this is very odd. Is Mercury much more completelydifferentiated than the other terrestrial planets?

What processes produced Mercury's smooth plains?

Are there any surprises on the other half of the surface we've not seen? Low resolutionradar images obtained from Earth show no surprises, but you never know.

ESA may also build a Mercury orbiter calledBepiColombo but it will launch no soonerthan 2012.

Venus is the second planet from the Sun and the sixth largest. Venus' orbit is the most nearlycircular of that of any planet, with an eccentricity of less than 1%.

orbit: 108,200,000 km (0.72 AU) from Sun

diameter: 12,103.6 km mass: 4.869e24 kg

The New Solar System

Summarizes what we havve learned from interplanetary explorations in the last 25 years. Myprimary reference forThe Nine Planets.

Venus Revealed
http://www.nineplanets.org/pxmerc.htmlhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mercury.htmlhttp://www.earth.nwu.edu/people/robinson/merc.htmlhttp://stardate.utexas.edu/resources/ssguide/mercury.htmlhttp://www.nmm.ac.uk/site/navId/00500300f00khttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mercury.htmlhttp://www.psrd.hawaii.edu/Jan97/MercuryUnveiled.htmlhttp://www.psrd.hawaii.edu/Jan97/MercuryUnveiled.htmlhttp://wwwflag.wr.usgs.gov/USGSFlag/Space/nomen/mercury/mercTOC.htmlhttp://www.astro.uu.se/planet/planet/Mercury/mercury.htmlhttp://sirius.bu.edu/planetary/mercury/http://www.astro.uu.se/planet/planet/Mercury/mercury.htmlhttp://www.lpl.arizona.edu/planatmos/mercstds.htmlhttp://antwrp.gsfc.nasa.gov/apod/ap001216.htmlhttp://sci.esa.int/home/bepicolombo/http://sci.esa.int/home/bepicolombo/http://www.nineplanets.org/sol.htmlhttp://www.nineplanets.org/data.htmlhttp://www.nineplanets.org/help.htmlhttp://www.nineplanets.org/data1.htmlhttp://www.nineplanets.org/data1.htmlhttp://www.amazon.com/exec/obidos/ASIN/0521645875/billarnetthttp://www.amazon.com/exec/obidos/ASIN/0201328399/billarnetthttp://www.nineplanets.org/pxmerc.htmlhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mercury.htmlhttp://www.earth.nwu.edu/people/robinson/merc.htmlhttp://stardate.utexas.edu/resources/ssguide/mercury.htmlhttp://www.nmm.ac.uk/site/navId/00500300f00khttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-mercury.htmlhttp://www.psrd.hawaii.edu/Jan97/MercuryUnveiled.htmlhttp://wwwflag.wr.usgs.gov/USGSFlag/Space/nomen/mercury/mercTOC.htmlhttp://www.astro.uu.se/planet/planet/Mercury/mercury.htmlhttp://sirius.bu.edu/planetary/mercury/http://www.astro.uu.se/planet/planet/Mercury/mercury.htmlhttp://www.lpl.arizona.edu/planatmos/mercstds.htmlhttp://antwrp.gsfc.nasa.gov/apod/ap001216.htmlhttp://sci.esa.int/home/bepicolombo/http://www.nineplanets.org/sol.htmlhttp://www.nineplanets.org/data.htmlhttp://www.nineplanets.org/help.htmlhttp://www.nineplanets.org/data1.htmlhttp://www.nineplanets.org/data1.htmlhttp://www.amazon.com/exec/obidos/ASIN/0521645875/billarnetthttp://www.amazon.com/exec/obidos/ASIN/0201328399/billarnett


15/18

The latest results from Magellan in an accessible and easygoing book. Covers mythology andhistory of our "sister planet" as well as up to date science and a history of the Magellan project.

Venus in Transit

Fascinating account of past transits and the woes that befell those involved.

Venus (Greek: Aphrodite; Babylonian: Ishtar) is the goddess of love and beauty. The planet is sonamed probably because it is thebrightest of the planets known to the ancients. (With a fewexceptions, the surface features on Venus are named for female figures.)

Venus has been known since prehistoric times. It is the brightest object in the sky except for theSun and the Moon. Like Mercury, it was popularly thought to be two separate bodies: Eosphorusas the morning star and Hesperus as the evening star, but the Greek astronomers knew better.(Venus's apparition as the morning star is also sometimes called Lucifer.)

Since Venus is an inferiorplanet, it showsphases when viewed with a telescope from theperspective of Earth. Galileo's observation of this phenomenon was important evidence in favorofCopernicus'sheliocentric theory of the solar system.

Venera 9 surface photo

The first spacecraft to visit Venus was Mariner 2 in 1962. It was subsequently visited by manyothers (more than 20 in all so far), including Pioneer Venus and the Soviet Venera 7 the firstspacecraft to land on another planet, and Venera 9 which returned the first photographs of thesurface. The first orbiter, the US spacecraft Magellan

Magellan radar map (false color)

produced detailed maps of Venus' surface using radar. ESA'sVenus Expressis now in orbit witha large variety of instruments.

Venus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day,slightly longer than Venus' year) and retrograde. In addition, the periods of Venus' rotation andof its orbit are synchronized such that it always presents the same face toward Earth when thetwo planets are at their closest approach. Whether this is a resonance effect or merely acoincidence is not known.

Venus is sometimes regarded as Earth's sister planet. In some ways they are very similar:

Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass).

Both have few craters indicating relatively youngsurfaces.

Their densities and chemical compositions are similar.
http://www.amazon.com/exec/obidos/ASIN/0691115893/billarnetthttp://www.pantheon.org/articles/v/venus.htmlhttp://www.pantheon.org/articles/a/aphrodite.htmlhttp://www.nineplanets.org/mercury.htmlhttp://www.badastronomy.com/bitesize/venus_phase.htmlhttp://www.esa.int/SPECIALS/Venus_Express/SEMBD3808BE_0.htmlhttp://www.esa.int/SPECIALS/Venus_Express/SEMBD3808BE_0.htmlhttp://www.esa.int/SPECIALS/Venus_Express/SEMBD3808BE_0.htmlhttp://www.nineplanets.org/earth.htmlhttp://www.solarviews.com/cap/venus/venus1.htmhttp://nssdc.gsfc.nasa.gov/image/planetary/venus/venera9-10.jpghttp://www.amazon.com/exec/obidos/ASIN/0691115893/billarnetthttp://www.pantheon.org/articles/v/venus.htmlhttp://www.pantheon.org/articles/a/aphrodite.htmlhttp://www.nineplanets.org/mercury.htmlhttp://www.badastronomy.com/bitesize/venus_phase.htmlhttp://www.esa.int/SPECIALS/Venus_Express/SEMBD3808BE_0.htmlhttp://www.nineplanets.org/earth.html


16/18

Because of these similarities, it was thought that below its dense clouds Venus might be veryEarthlike and might even have life. But, unfortunately, more detailed study of Venus reveals thatin many important ways it is radically different from Earth. It may be the least hospitable placefor life in the solar system.

Venus in visible light from Galileo

The pressure of Venus' atmosphere at the surface is 90 atmospheres (about the same as thepressure at a depth of 1 km in Earth's oceans). It is composed mostly of carbon dioxide. Thereare several layers of clouds many kilometers thick composed of sulfuric acid. These cloudscompletely obscure our view of the surface. This dense atmosphere produces a run-awaygreenhouse effect that raises Venus' surface temperature by about 400 degrees to over 740 K (hotenough to melt lead). Venus' surface is actually hotter than Mercury'sdespite being nearly twiceas far from the Sun.

Venus in ultra-violet light

There are strong (350 kph) winds at the cloud tops but winds at the surface are very slow, nomore than a few kilometers per hour.

Venus probably once had large amounts of water like Earth but it all boiled away. Venus is nowquite dry. Earth would have suffered the same fate had it been just a little closer to the Sun. Wemay learn a lot about Earth by learning why the basically similar Venus turned out so differently.

Most of Venus' surface consists of gently rolling plains with little relief. There are also severalbroad depressions: Atalanta Planitia, Guinevere Planitia, Lavinia Planitia. There two largehighland areas: Ishtar Terra in the northern hemisphere (about the size of Australia) andAphrodite Terra along the equator (about the size of South America). The interior of Ishtarconsists mainly of a high plateau, Lakshmi Planum, which is surrounded by the highestmountains on Venus including the enormous Maxwell Montes.

Sif Mons (Magellan radar)

Data from Magellan's imaging radar shows that much of the surface of Venus is covered by lavaflows. There are several large shield volcanoes (similar to Hawaii orOlympus Mons) such as Sif
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17/18

Mons. Recently announced findings indicate that Venus is still volcanically active, but only in afew hot spots; for the most part it has been geologically rather quiet for the past few hundredmillion years.

There are no small craters on Venus. It seems that small meteoroids burn up in Venus' denseatmosphere before reaching the surface. Craters on Venus seem to come in bunches indicating

that large meteoroids that do reach the surface usually break up in the atmosphere.The oldest terrains on Venus seem to be about 800 million years old. Extensive volcanism at thattime wiped out the earlier surface including any large craters from early in Venus' history.

Coronae

Pancake volcanoes

Magellan's images show a wide variety of interesting and unique features including pancakevolcanoes (left) which seem to be eruptions of very thick lava and coronae (right) which seem tobe collapsed domes over large magma chambers.

The interior of Venus is probably very similar to that of Earth: an iron core about 3000 km inradius, a molten rocky mantle comprising the majority of the planet. Recent results from theMagellan gravity data indicate that Venus' crust is stronger and thicker than had previously been

assumed. Like Earth, convection in the mantle produces stress on the surface. However on Venusthe stress is relieved in many relatively small regions instead of being concentrated at theboundaries of largeplates as is the case on Earth.

Venus has no magnetic field, perhaps because of its slow rotation.

Venus has no satellites, and thereby hangs a tale.

Venus is usually visiblewith the unaided eye. Sometimes (inaccurately) referred to as the"morning star" or the "evening star", it is by far the brightest "star" in the sky. There are severalWeb sites that show the current position of Venus (and the other planets) in the sky. Moredetailed and customized charts can be created with aplanetarium program.

On June 8 2004, Venus passed directly between the Earth and the Sun, appearing as a large black

dot travelling across the Sun's disk. This event is known as a "transit of Venus" and is very rare:the last one was in 1882, the next one is in 2012 but after than you'll have to wait until 2117.While no longer of great scientific importance as it was in the past, this event was the impetusfor a majorjourney for many amateur astronomers.

More about VenusTop of Form

Bottom of Form
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18/18

more Venus images

fromNSSDC

Venus Revealed Imagesby David Grinspoon

Magellan mission home page

Venusian Craters and Venusian Volcanic Features from LANL Magellan Venus Explorer's Guide from JPL; about Magellan and Venus prior to the

Magellan data

Guide to Magellan Image Interpretation from JPL

The Magellan Spacecraft at Venusby Andrew Fraknoi of ASP

Venus Nomenclature Table

Magellan Image Browser

maps of Venus, Mars and the Moon

Open Issues There is some evidence of spreading and folding on Venus' surface and of recent volcanic

flows. But there is no evidence ofplate tectonics as seen on Earth. Is this a result of thehigher surface temperature?

The greenhouse effect is much stronger on Venus than Earth because of Venus' densecarbon dioxide atmosphere. But why did Venus evolve so differently from Earth?
http://www.nineplanets.org/pxvenus.htmlhttp://www.nineplanets.org/pxvenus.htmlhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-venus.htmlhttp://www.funkyscience.net/http://www.jpl.nasa.gov/magellan/http://www.solarviews.com/eng/vencrate.htmhttp://www.solarviews.com/eng/venvolc.htmhttp://www2.jpl.nasa.gov/magellan/guide.htmlhttp://history.nasa.gov/JPL-93-24/jpl_93-24.htmhttp://www.astrosociety.org/education/publications/tnl/18/18.htmlhttp://planetarynames.wr.usgs.gov/jsp/SystemSearch2.jsp?System=Venushttp://pds-imaging.jpl.nasa.gov/PDS/public/magellan/midrcd_query.htmlhttp://ralphaeschliman.com/http://www.nineplanets.org/pxvenus.htmlhttp://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-venus.htmlhttp://www.funkyscience.net/http://www.jpl.nasa.gov/magellan/http://www.solarviews.com/eng/vencrate.htmhttp://www.solarviews.com/eng/venvolc.htmhttp://www2.jpl.nasa.gov/magellan/guide.htmlhttp://history.nasa.gov/JPL-93-24/jpl_93-24.htmhttp://www.astrosociety.org/education/publications/tnl/18/18.htmlhttp://planetarynames.wr.usgs.gov/jsp/SystemSearch2.jsp?System=Venushttp://pds-imaging.jpl.nasa.gov/PDS/public/magellan/midrcd_query.htmlhttp://ralphaeschliman.com/

an overview of the solar system

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