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collision
Has the Earth grown or shrunk since it formed?
Submitted by Saavik Ford on Mon, 7/13/2009, 9:43 PM
Not much. The Earth is inconceivably massive: it is around 6x1024 kg which would weigh about 12x1024 pounds. That's 12 billion quadrillion pounds (yes that's a real number!). The Earth gains and loses very small amounts of mass each day. The main gains are from dust that falls to Earth as micrometeorites. In an average year we gain maybe a hundred million kilograms, which sounds large, but as a percentage of the Earth's total mass, this is tiny. It is so tiny that if the Earth accumulated this much every year since it formed, we would only gain about one one hundred thousandth of a percent of the Earth's mass over its entire history so far.
We also lose mass every year, mainly from our atmosphere. A complete inventory is a little hard to come by, but again, the losses are tiny (most of our atmosphere is still here and breathable, after all!).
Of course, when the Earth was very young, we gained a lot of mass in a very short time period - the Earth formed by accreting "planetary embryos" - essentially super-asteroids. The last accretion event large enough to affect, for example the bulk composition or orbit of the Earth was the Moon forming impact. The Solar System began forming about 4.57 billion years ago, and the Moon-forming impact happened between 4.52 and 4.50 billion years ago. So for the vast majority of the history of the Earth, it has been the same mass as it is today.
Why are galaxies colliding in an expanding universe?
Submitted by Tim Paglione on Thu, 5/07/2009, 12:08 PM
Although the Universe is expanding so galaxies are in general quickly moving away from each other, on small scales gravity can still dominate. This is why neighboring galaxies can collide, galaxy clusters form, galaxies hold onto their stars, the Solar System stays together, and you stay on the Earth.
Will Pluto and Neptune ever collide?
Submitted by Saavik Ford on Mon, 4/27/2009, 3:33 PM
Since Pluto spends part of its orbit closer to the Sun than Neptune, you might think that their orbits cross, and eventually that the two of them could collide. But there are two reasons why they will forever remain like two ships in the night.
1) The usual diagram of Neptune and Pluto's orbits is a "top down" view of the solar system, also called a polar view. In this view, the two orbits appear to cross. But this is a two-dimensional picture of a three-dimensional event! A "side" view shows that the orbits of Pluto and Neptune are also inclined to one another. In fact, Pluto's and Neptune's orbits never actually cross each other - at their closest, Pluto's orbit is "above" Neptune's by a few hundred million miles. This is like the difference between looking at a road map showing that two roads cross, but then driving to the "intersection" only to find out that one road is an overpass and you cannot actually get from one road to another.
2) The orbits of Pluto and Neptune around the Sun are in resonance. This means that every time Neptune orbits the Sun twice, Pluto goes around exactly three times. When one of them is near the "crossing point" (remember, they don't actually cross) the other one is somewhere else entirely on its orbit. The inclination of Pluto's orbit is not entirely stable - over time it will change and this might eventually cause a true crossing with the orbit of Neptune. But the resonance, which is extremely stable, will prevent the two of them from ever being in the same neighborhood at the same time.
If you'd like to learn about this in more detail, I recommend checking out the wikipedia page on Pluto's orbit.
