For nearly two thousand years western culture viewed the heavens as eternal and perfect. The heavenly bodies moved around the Earth – and in circular orbits, for the circle was a perfect geometric shape. Change only occurred on the imperfect Earth.

Changing outlook
Nicolaus Copernicus (1473-1543) argued that the Sun was at the center of the Solar System, not the Earth. Then Johannes Kepler (1571-1630) showed mathematically that the planets orbited the Sun in ellipses, not circles. Over half a century after Kepler's death Isaac Newton's gravitational theory explained why the objects in the Solar System moved as they did.

After the “Copernican revolution” there was still a sense of an eternal unchanging cosmos. But during the last century another revolution has been going on, one that has brought us to the view that there is change and evolution throughout the cosmos. And unexpectedly, much of this change involves collisions.

Impacts of meteoroids, asteroids and comets
Meteoroids enter the Earth's atmosphere all the time. We rarely notice them, though sometimes we may be lucky enough to see the delightful spectacle of a meteor shower. These occur as Earth moves through the debris stream left by a comet. The small bits of comet leave glowing trails as they are heated in the atmosphere.

Scientists were reluctant to accept that cratering on Earth or the Moon was due to impacts. They thought craters showed volcanic activity. Early in the 20th century Daniel Barringer said that Meteor Crater in Arizona was an impact crater, but it took over half a century to get the evidence that confirmed his hypothesis.

It's common knowledge that a large impact may have hastened the end of the dinosaurs. But a large comet or asteroid doesn't need to hit the ground to have an effect.

In 1908 the shock wave from the explosive break-up of a large body flattened 2150 square kilometers (830 sq mi) of Siberian forest. This was the "Tunguska event.” In February 2013 a similar event shook up the Russian city of Chelyabinsk. Since this was a populated area, it was lucky that the asteroid was smaller than the Tunguska object. Nonetheless the shock wave damaged over 7000 buildings, and about 1500 people were injured.

When a comet comes visiting from the outer Solar System, it can be a spectacular sky object, though most of them need a telescope to see. Comet Hale-Bopp (designated the Great Comet of 1997) was very bright and probably seen by more people than any other comet in history.

On the other hand, in 1994 comet Shoemaker-Levy 9 collided with giant Jupiter and left impact scars that were visible for several months.

Collision and creation
We think that planets form from the debris surrounding a young star. Collisions between ever-larger pieces of debris form asteroids or perhaps planets. Once there is enough mass, gravity pulls the object into a sphere. This is how Earth formed about four and half billion years ago.

In addition, the most widely-accepted hypothesis for the formation of our Moon involves a giant impact. It proposes that in the chaotic early Solar System a Mars-sized body hit the Earth. The impact generated enough heat to melt a portion of the Earth's crust and send it flying out into space to orbit the Earth as a ring. The debris coalesced to form the Moon.

Recycling stars
We know that stars aren't eternal. They form, they shine, and finally, they are extinguished. Some dying stars slough off their outer layers as they cool. Others explode as supernovae. In either case the materials are eventually recycled into new stars and planets. This normal recycling doesn't involve collisions, but there's another way of making new stars from old.

Astronomers wouldn't expect stars ever to collide. Space is really big. For example, the Sun's nearest neighbor is over four light years away. That's about 40 trillion kilometers (24 trillion miles). But in a globular cluster things are different. A globular cluster is a large collection of stars held together by gravity in a spherical shape. In such a cluster ten cubic light years could hold a hundred thousand stars.

Globular clusters are very old and have no gas and dust to form new stars. So it's strange to find the occasional hot blue star there, because such a star is a young star that shines brightly and burns out quickly. Stellar collisions seem to be the only plausible explanation.

When galaxies collide
Although the force of gravity weakens with distance, it still has an effect over unimaginably large distances. This is why galaxies are found in clusters, and in turn, the clusters form superclusters. It's also why galaxies collide.

We know that galaxies collide because many of them have been caught in the act. Such a collision is too slow for us to watch. However through observations with large telescopes we can see effects from the interaction, such as the changes of shape of the galaxies and streams of gas connecting them. Here is a picture of the Antennae galaxies in collision.

Closer to home, our neighboring galaxy M31 (Andromeda galaxy) is coming our way at about 250,000 mph (400,000 km per hour) in what is expected to be a head-on collision in about four billion years. Click to see images from a computer simulation of the collision between Andromeda and the Milky Way. Individual stars won't collide, but our Solar System will be thrown into a different part of the new galaxy. The final merger will produce a giant elliptical galaxy.