Friday, March 16, 2012

Kepler's First Law

Johannes Kepler was a very famous astronomer. He was one of the first astronomers to understand the physics behind our solar system and how objects orbit one another. If you ever take a course in astronomy, one of the first concepts you will learn about is Kepler's three laws of planetary motion. Today I introduce Kepler's first law of planetary motion.


Kepler's first law: Planets orbit the sun in an ellipse with the sun at one foci .

With the technology and telescopes that we have today, it's easy to show that planets follow a squished circular shaped path around the sun called an ellipse . But how did Kepler know? Back in the early 1600's, astronomer and physicist Tycho Brahe took very precise measurements of the position of Mars in the sky. At the time, astronomers believed that all planets orbited in a circular path with the sun at the center. Assuming this is true, Brahe calculated where he expected Mars to be located in the sky throughout the year. To his surprise, the position of Mars never matched his prediction! He then gave Kepler the task of figuring out why the data and predictions did not match. Kepler discovered that if you model the Earth and Martian orbit using ellipses with the sun at one foci, Brahe's predictions would match up perfectly with his observations! We can quantify how "squished" the circle is using a parameter called eccentricity. An eccentricity of 0 means the planets path is a perfect circle. An eccentricity of 1 means that the planets path is a straight line. Planets orbit with eccentricities between 0 and 1, and most of the planets in our solar system have an eccentricity of <0.1 i.e. almost circular. This law is universal, which means it can be applied to extrasolar planetary systems as well as our own.

Monday, March 12, 2012

Van Allen Belt


Earth is surrounded by a large magnetic field caused by a molten iron core deep inside the planet. It's very similar to the bipolar magnetic field produced by a bar magnet, just on a much larger scale. The field lines extend out one pole, wrap around the earth, and re-enter at the other pole, creating a magnetic barrier around Earth. The Van Allen Belt is the part of this barrier, where most of the high energy particles aimed towards Earth are collected and safely grounded at Earth's poles. It sits about 20,000km above Earth's surface, well within the orbit of the moon. There are actually two Van Allen belts, an inner and outer one, which trap different types of particles.

This is a great thing for humans on Earth, but it poses big problems for satellites, telescopes, and space travel. Telescopes and satellites that travel through the Van Allen belt can be easily damaged by this highly concentrated radiation. This is why most satellites orbit within the belt, and most space telescopes have orbits that do not cross the belt, or cross through it once to get to a further destination.  The Van Allen belt is also a big problem for astronauts. Without special equipment, humans can not safely pass through the belt, as they would instantly be poised by the radiation. Special protective equipment from astronauts and the space shuttle was developed so that astronauts on the Apollo missions could travel safely to the moon and back.