Solar System

How far away is the Moon from the Earth?

The moon is 384,400 km or about 239,000 miles from the Earth.
Are there any huge cracks on the Moon?
We do not know of any large cracks in the Moon. But it does have a huge crater. The South Pole - Aitken crater is 2250 km across and 12 km deep. It is the largest impact crater in the solar system.
Can you really walk on an asteroid like in some movies?
Since asteroids are generally small, their gravity is very weak. An astronaut standing on an asteroid would weigh very little and would tend to float more than walk.
Are the colors of the planets important?
The colors of the planets are important because they give us information about what a planet is made of. For example, Mars is a rusty orange color because its soil contains iron oxide (rust). Colors can also tell us about the temperatures of planets and, in the case of the gas planets, give us information about different levels in the atmosphere.

Visible light images can be true or false color. False color images are often used to show features in more detail. Images of the gas planets often are in false color to show the belts and zones more clearly. Images in parts of the spectrum other than visible light (e.g. infrared) use color to show different wavelengths. Infrared images often use different colors to indicate different temperatures (different infrared wavelengths represent different temperatures).
What causes a ring around the moon?
Rings around the moon are caused by moonlight being refracted by ice crystals in the upper atmosphere. This can split the moonlight (which is really sunlight shining off the moon) into colors, like light passing through a prism.
How big would Jupiter look like if it was where our Moon is?
Hmm, how big would Jupiter look like if it was where our Moon is? Well, some simple math can give us an answer, but I'm not sure we could really imagine just how dramatic it would look. The radius of Jupiter is about fifty times the radius of the Moon, and the area of a circle is calculated by pi times the radius squared. 50 X 50 ( fifty squared) is 2500. So, Jupiter would be 2500 times the size of the moon in the sky. As far as how much of the sky that would take up, that amount of space would take up about one third of the full horizon-to-horizon sky. But Jupiter would probably reflect so much more light from the Sun that it would seem to light up the whole sky. Very dramatic indeed!
Do auroras also appear at the South Pole?
Yes, the aurora also appears in the southern latitudes - they are called the Southern Lights or Aurora Australis.
Who took Neil Armstrong's picture when he first set foot on the moon?
The pictures were taken by a camera mounted on the Lunar Module.
Do planets rotate counter clockwise?
Most of the planets in our solar system do rotate counter clockwise as seen from north ecliptic pole - except for Venus (clockwise), Uranus (rotates on its side) and Pluto (which points slightly down).
What are asteroids and how big are they?
Asteroids are rocky-metallic objects which range in size from about the size of pebbles to around 600 miles (~1,000 km) across. Although they orbit the Sun, they are too small to be considered planets. Asteroids are thought to be leftover material from the formation of our solar system. Most are found in the Asteroid Belt, a doughnut-shaped ring which lies between the orbits of Mars and Jupiter. Astronomers have also identified a group of asteroids whose orbits cross Earth's orbit. Several hundred thousand asteroids are known to exist in our solar system, and many are yet to be discovered. Most of the undiscovered asteroids are the smaller ones (less than 100 km across) which are more difficult to detect. It is estimated that there are over a million of these smaller asteroids.
What are comets? What are they made of?
Comets are basically dusty snowballs which orbit the Sun. They are a combination of ices, such as water, carbon dioxide, ammonia and methane, mixed with dust. These materials originated from the time that our solar system was formed. Comets have an icy nucleus surrounded by a large cloud of gas and dust (called the coma). The coma is created as the ice in the nucleus is warmed by the Sun and vaporizes. Comets can develop 2 tails as they travel closer to the Sun, a straight gas tail and a curved dust tail. The gas tail is created by the solar wind, whose magnetic fields pull the gas away from the comet's coma. The dust in the coma is not affected by magnetic fields but is vaporized by the Sun's heat, and forms a curved tail due to the comet's orbit.
Why are asteroids called minor planets?
Asteroids are sometimes called minor planets because they orbit the Sun as planets do, but are too small to be considered planets.
What is the solar wind?
The solar wind is a thin flow of energetic and gas charged particles that stream from the Sun. These blow throughout the solar system at speeds of hundreds of kilometers per second.
I heard that the Great Dark Spot on Neptune vanished. Is this true?
When Voyager 2 flew by Neptune in 1989, it discovered a Great Dark Spot - a rotating "storm system" with extremely high wind speeds. Later in 1994, when the Hubble Space Telescope viewed Neptune, the spot had vanished. A new, smaller dark spot was found in Neptune's northern hemisphere instead.
What is zodiacal dust?
Zodiacal dust is the dust found in our solar system, primarily between the Sun and Jupiter. This dust originates from comets and from asteroid collisions, and can sometimes be seen with the naked eye as a triangular glow above the horizon just before sunrise or after sunset. This glow is called the zodiacal light and is caused by sunlight being scattered off of the various dust particles which travel around the Sun. Each time a comet passes near the Sun, it loses some of its material, contributing to the dust between the planets. When asteroids collide, they also send additional dust into the solar system. How much of the zodiacal dust is due to comets and how much is due to asteroid collisions is not known. Due to the effects of the planets, the disk of dust in our solar system is full of structures such as rings and wakes. Each year the Earth collects about 40,000 tons of interplanetary dust!
How did Johannes Kepler do his calculations of orbital motion without the use the instruments we have available today?
Kepler was a student of Tycho Brahe who used new (at the time) instruments to measure angles and positions of objects in space. There were no telescopes at the time so all of this was done by eye. Tycho gathered a vast amount of data for the time. Kepler used Tycho's data, mathematics and the concept of force to figure out that planets have elliptical orbits.
What is the Great Red Spot an how big is it?
Jupiter's Great Red Spot is a giant high pressure storm which is about three times the size of the Earth.
Do all the planets rotate in the same general direction?
The planets in our solar system all orbit the Sun the same way - with Pluto a bit off the plane but they orbit in the same direction. As for their rotation on their own axes, it's like this: Seen from way above our North Pole, all the planets in general rotate around axes that are orientated like ours in space. Some axes like ours are tilted to the plane of the Solar System (SS) (ours is tilted 23 deg.; Mars, 24 deg.) some, like Jupiter, are almost straight up. The exception is Uranus, which rotates on its side as it goes round the Sun, with its axis pointing nearly aligned with the plane, always in the same direction in space (and thus only twice a year towards the sun). Uranus' orbit is from causes unknown. Perhaps it is the product of a collision, or an early close planetesimal interaction. All the other planets rotate counter clockwise, which is also the direction they orbit the Sun, except Venus. Venus orbits with the other planets but spins backwards. This was only discovered in the Sixties by bouncing radar off the surface under the clouds.
Do we rotate around the same general two dimensional plane, or do we spin at all sorts of angles relative to each other like electrons?
All the planets of our solar system are, to within 7 degrees, in the same plane, except Pluto which if off by 17 degrees. (The comets and asteroids swarm in a more spherical configuration which is a relic of their pre-solar-disk existence.)
Why do the planets of our solar system all rotate in the same direction and basically in the same plane?
First all seven of nine planets, their 40 or so moons and the Jupiter and Saturn rings rotate in the same direction , as do the asteroids in the asteroid belt. Even the gas of the Sun rotates this way - not rigidly like a planet, but differentially, like the whole Solar System, with the outer equator going around in about 25 days, and other parts needing a month to make a Sun day. With the rule so strong the principle must be strong to - and it is. This CCW (Counter ClockWise) Spin is angular momentum left over from the initial collapse of the protoplanetary nebula (chunk of collapsing gas) that led to the formation of the Sun and her planets. The Sun, as with every other star, came from a chunk of a cloud of gas - a small chunk, because these clouds, called Giant Molecular Clouds, have the mass of 10,000 Suns, although in near perfect vacuum density - 10,000 molecules per cc. A part collapses under its own gravity and after a while (a million years). a very complex beast is seen: rather than a simple collapsing ball of gas - an infall/outflow arrangement is seen. It looks like this:

o o o o
\ \ | / /
i     \\|//     i
ii   \|/   ii
dddsddd
ii   /|\   ii
i     //|\\     i
/ / | \ \
o o o o

o and slashes are regions of outflow - they are cones - "biconical outflows or biconical nebula" The "i"s are regions of infall. The d is the dusty (protoplanetary) disk that will eventually form planets and the s is the protostar.

What has this to do with CCW spin?
When any great number of objects (be them molecules or stars) self-gravitate to make a new body in the Universe (be it a protoplanetary disk of a protogalaxy, they have leftover angular momentum. An analogy would be to imagine all the skaters in Rockefeller Center suddenly clasping hands of others around them. (Imagine them starting from a random pattern of skating.) The linked crowd would have some excess angular momentum and the whole mass would find itself in a slow group spin. If they all tugged closer, the spin would increase. (You know this from watching figure skaters bring in their arms.) This is what happens to the central protoplanetary dust disk.

An alternative analogy is a flight of birds suddenly being tied together with strings. They will start spinning as a group. The Sun carries the lion's share of the angular momentum in the SS. The proto-planets, moons, etc. may get some spin kick back from the Sun by the magnetic fields it drags through space. When the planets formed, again chunks of gas from a larger chunk, the parcels spun with the original spin join together to replicate this spin. The outer parts clumped with the inner parts. Although orbiting slower, the outer parts carry more angular momentum and spin the cloudlets in the same group convention so the SSW spin is maintained. This is a somewhat random process, not necessarily strictly followed, and an exception may be Venus.

"Why do planets have moons - and why do they have moons which are smaller than the host planet?"
Small objects can orbit larger objects if they start out with enough velocity (speed) relative to the large object. They always fall-in because of gravity to the larger object but their original speed keeps them orbiting-falling-orbiting-falling. Our Moon does this. Our moon might have been kicked-up from Earth via an early Solar System collision and kicked with enough energy to orbit Earth. You could ask "why does the moon (smaller) orbit Earth and not the Earth orbit around our Moon?" Actually the Earth does orbit Moon - but it's a little circle (smaller than the Earth radius) that Earth goes through. The Law of Gravity says that the smaller object zips around the larger object or else falls-in. The larger objects orbit the small object but just a little. Imagine a dance with a giant swinging a little person. The giant would circle a little around his footsteps and the small person a lot. That is the way it is with the Earth and Moon. Also with the Sun and planets. The swing of Jupiter makes the Sun do a little circle within itself.

There is another analogy to this where you have a stretched rubber sheet: Heavy objects (Earth) - are like a lead ball weight placed on this sheet, make big depressions in the sheet, and smaller objects can circle around constant depth contour of the depression (if they have sufficient energy) like the moon does.

Yet another question that follows from "why do planets have moons?" is "why don't moons have moons?"

It all gets back to the fact that when gas clouds collapse in space they form central concentrations, which become stars, these stars are spinning and are surrounded by spinning gas disks; amazingly flat disks.

This is a configuration of stability of all self-gravitating systems. Gas and particle lumps of higher density within disks can become, through accretion, solid bodies (planetesimal) which further accrete via gravity into planets with moons. So the Solar System formed in a two-step dance where first the Sun and surrounding disk formed and then the planets formed with surrounding disks and then these disks became the moons.

In general it was a big house cleaning project with the dust bunnies clumping up to be planets and moons and the debris that was left over became the comets and asteroids. All this has to do with how gravity makes the shapes it does in the Universe.

This pattern in the structure of the Universe of central objects orbited by other smaller particles in flat (disk) configurations is repeated at every size scale. The orbiting bodies can be gas molecules, rocks, planets, moons or stars, all orbiting something bigger... When I say gas "molecules" I am thinking of black holes surrounded by an orbiting disk of atoms and molecules, and when I say "rocks" I am thinking of Saturn, Jupiter, or Uranus, each surrounded by orbiting rings of rocks (from pebble-size to house-size), when I say "planets" I am thinking of the Solar System where the Sun is surrounded by a mostly flat disk of planets and when I say moons I am thinking of a planet like Jupiter surrounded by 60 moons!! And when I say "stars" I am thinking about the center of the Milky Way being surrounded by 100 billion stars, the Sun being one. In each case the disk of material all orbits in the same direction. If you were to zoom above the Solar System, say above Earth's North Pole, you would see all the planets spinning counter-clockwise around the Sun, and all of them (except one, Uranus) spinning counter-clockwise about it's own axis, and all of the moons (every planet except Venus and Mercury has one or more moons) orbiting the planet counter-clockwise.

If you tied strings to the feet of every member of a flock of birds such that every bird was tied to every other bird by a string, and you clapped your hands and the flock took flight you would see their confused and disorganized pattern of flight soon consolidate and they would start to fly in a flat spinning disk of birds. The reason for this is that when the flock takes flight it is a net spin to the systems that wins. I know that doesn't mean anything so let's try it this way. Imagine a circular ice-pond in the winter and three skaters run at each other and join hands. They will tend to spin together in the direction that fastest skater was headed before they joined hands. The skater(s) with the most energy determines the group's spin. Likewise the flock of birds: their motions in conflicting directions will cancel till the original dominate direction surfaces. Likewise stars forming out of a blob of gas making a galaxy - they will eventually spin together. Likewise for the gas that is making a star and planets.

Further, the system in three-dimensional space will flatten as opposed trajectories to this plane cancel. They reach a minimum energy configuration much like a ball rolling down hill... Um - I am sorry it's so hard to explain in simple words. Maybe think of this - if you have 100 scuba divers in random positions under the water. You asked each to head for the nearest person and then you asked the groups of two to head to the nearest group of two. The distribution would flatten because the joining process would get rid of the outliers. Hmmm, this would make a sphere but the coasting motions of the divers would make a sheet. OK - my analogies have their limits!

Anyway - once you have a disk, if there are irregularities in the disk, this can make self-gravitating concentrations. This is how disk galaxies make stars and how stellar planetary systems make planets.

FINALLY - Moons!

A piece of a Giant molecular cloud in space has collapsed and makes a central star - our Sun - Sol! That star has a disk of material around it. That disk (because nothing is perfect) has lumps and the lumps form first little objects then these join by gravity to make planetesimal and then planets. Smaller lumps have enough energy to orbit the planets - moons and rings.

Why don't moons have moons?
The answer is that the central body (planet) which the moon is going around is stronger and in general will separate the moon's moon from the moon and make an independent moon- that is a new moon.
Is our Sun in a hurry to become a Red giant?
No - this process is well understood - it takes billions of years - like 11 billion. (The book I mentioned predicts that the age of mammals will end in about a billion years as the Sun _slowly_ bloats up as it runs out of fuel [sic]

How much do we understand? A lot! The so called main sequence burning of the Sun must keep on like it has for billions of more years because of the locked balance between the gravitational forces holding the Sun together and the nuclear forces pushing it apart. Astronomers call the Sun a "Polytrope" based on the solution of the hydrostatic balance. We understand this very well. So - not to worry!