ASTR-301 Lessons

Professor Gagarina enters the classroom in a flurry of parchment to take her place at the front of the class.  The large, ticking orrery is still upon her desk but several new posters adorn the classroom walls.  These new images are colorful, featuring various objects flying through space or making fiery impacts with other, much larger objects.

Welcome!  When we last left our newly formed solar system, it was not the harmonious and, comparatively, empty place that we are accustomed to occupying; instead, many more planets were there.  Some were rocky and close to the Sun, others were further out and made up of gas and ice.  The massive cloud that had occupied that space collapsed into many varied objects.  How then, did our Earth emerge from the fray?

There Can Only Be Eight

                                                                       Our solar system and Pluto Source

The current orbits of our planets are not perfect circles.  The true path of each planet around the Sun is an oval sort of shape, with the Sun in a central position, but not the perfect center of the oval.  These orbits were generated by the spinning of the cloud of dust and gas around the new Sun.  New objects came into being in various places in this spinning cloud and the gravitational pull of the Sun kept them from flying off into space.  However, because the objects were all formed at about the same time and at various places in the spinning cloud, conflicts occurred and not all of these objects survived the creation of the solar system.

The Sun’s gravity may have prevented planets from flying off into space; however, it didn’t stop them from moving outward entirely.  The rate at which the cloud was spinning when the planets formed gave them an outward push.  This momentum, though, is one of many forces acting on the planets.

Gravity also played a part in planetary evolution.  Larger planets moving past smaller objects either forced them out of the way or drew them into orbit.  This first effect of gravity, often called “slingshot” pushed planets out of their original orbits.  Some objects, caught at the right angle, were sent far off into space while others were pushed inward in the direction of the Sun.  This effect can be seen most clearly in the planet Neptune.  Neptune, many scientists believe, actually formed closer to the Sun than Uranus, the second to last planet.  It is believed that Neptune interacted gravitationally with a larger body, perhaps a young Saturn or Jupiter, and was pushed out towards the edge of the solar system.  Gravity also pulled smaller planets into orbit around larger ones, meaning that these large objects lost their planetary title and became moons.  Jupiter and the Galilean moons are the best example of this phenomenon.  Ganymede, the largest, is bigger than the planet Mercury and the others are also similarly huge, but Jupiter’s gravity pulled in the otherwise massive planetoids ensuring their destiny as moons.

Not all planetoids were destined to become moons.  Rocky and icy planets formed by accretion, where small amounts of matter came together to form much larger objects.  Some of these objects were captured gravitationally much earlier in their formation than others.  You may remember from last year that the Sun and Moon pull on the Earth, causing tides and the rise and fall of the sea.  When loosely formed objects faced tidal forces from large planetoids some were pulled apart, losing their shape and becoming little more than massive rocks floating in space.  Some of these rocks were pulled into orbit around planets, forming smaller, funny-shaped moons or becoming part of the rings of the outer planets.

Finally, many other newly formed planets in our solar system were destroyed by brute force.  Confused orbits caused large, solid masses to collide with one another where they intersected.  Sometimes these were fatal blows, destroying one or both objects.  Other times these were glancing blows, with material being thrown out from both objects into space.  This whole process took several million years, and the result is our own solar system with eight unique planets.

When Worlds Collide

Collisions between planets cannot be seen as wholly destructive.  Indeed without such collisions our world and our skies would look very different.  Our Moon was the product of such a collision.

There is still quite a bit of debate about how the Moon came to be.  Analysis of moon rocks and dust brought back from the American Apollo missions has determined that the Moon is 4.5 billion years old, about the same age as the rest of the solar system.  That being said, the material is not so different from Earth’s, so the Moon is not a captured planetoid.  Instead, Moon rocks are very similar to Earth rocks.  The conclusion that many have drawn is that the material that made up the Moon was a part of Earth at one point, until a collision between Earth and a planet the size of Mars knocked off material that came into orbit around Earth, eventually coming together to form the Moon we know today.

Earth and the Moon have a very special magical relationship.  Firstly, the Moon is the closest celestial body to us and reflects the most magic back to Earth of any object, providing us with a powerful source of magic.  Secondly there is a special magical connection between the two bodies, as they are made up of the same material and had once been part of the same celestial object.  This connection generates a kind of magical resonance which means that the magic reflected to us from the Moon is less like the raw power of solar-generated magic and much more fine-tuned for Earth; the magic reflected from the Moon generates constructive interference.  

It’s Raining Fire

All those bits and pieces of failed planets did not simply cease to exist.  Instead they continued to move through the newborn solar system where they were sometimes pulled into orbit as rings or else flung away again by a passing planet’s gravity.  Like the planets, these objects did not coexist peacefully with the rest of the solar system.  Individual chunks of space debris, known as asteroids, impacted with all the planets and each other.  The most spectacular of these collision events is thought to have occurred around five hundred million years after the formation of the solar system.  

Known asteroids in the inner solar system

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Late Heavy Bombardment

Evidence gathered by astronauts on the Moon suggests that around half a billion years after the formation of the inner planets a massive number of asteroids was flung into their path.  The result was a massive wave of asteroid impacts that knocked pieces from the newly formed planets and left deep scars in their faces.  This impact event is also thought to have helped “seed” the Earth with elements and compounds that formed all life on Earth.  Impacts thought to have come from the Late Heavy Bombardment can still be found on the Moon and Mars.

Artist's interpretation of asteroids during the Late Heavy Bombardment

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Chicxulub Crater

Asteroid impacts did not stop after life began on Earth.  This crater, discovered off the coast of the Yucatan Peninsula in the 1970s, is thought to be the result of a massive asteroid whose impact caused the extinction of the dinosaurs.  Asteroids do not make craters themselves, like throwing a stone into sand.  Instead some asteroids are so massive, and moving so quickly, that the shockwave in front of them makes a far bigger impact than the object itself.  An asteroid ten kilometers in diameter left a crater almost twenty times bigger than itself which threw enough debris into Earth’s atmosphere to change the climate drastically and cause the mass extinction that wiped out the dinosaurs.

                                                                                                     Artist's interpretation of the impact that formed the Chicxulub Crater

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Chelyabinsk, Russia

Perhaps the most notable modern example of an asteroid impact is Chelyabinsk in 2013.  Here an object approximately twenty meters in diameter encountered Earth’s atmosphere and produced a massive shockwave as it exploded.  Pieces of the object, meteorites, were found in a nearby lake.  The footage is rather spectacular, but many people were also injured by glass sent flying by the midair explosion.  This impact in particular made it clear that humans on Earth are still very vulnerable to the dangers posed by massive space debris.

This concludes the lesson for the week.  You will have a short quiz to be handed in before leaving and an essay due at the beginning of class next week.  While the subject of interstellar dangers may be unsettling to some, we will next be discussing the special composition of the Earth in depth and also some of the ways in which the Earth protects us from the dangers of space.