Ken Tapping, November 10 2010

Gravity seems pretty simple; Newton summed it up very well. Every object in the universe attracts every other object with a force that increases with the masses of the objects and decreases as they get further apart. However, that simple rule generates amazing subtleties in the universe.

Image source: NASA

At the beginning of the 19th Century, the Solar System was fairly well known. Seven planets had been discovered thus far: Mercury, Venus, Earth, Mars, Jupiter, Saturn and Uranus. All seemed tidy except for one thing, there was a gap between Mars and Jupiter that seemed too large. In addition, an obscure and apparently empirical rule known as “Bode’s Law” predicted there should be a planet there. Then, in 1801, Guiseppe Piazzi found something - an object orbiting the Sun in that gap; it was named Ceres. However, if this was the missing planet, it was a rather small one at only 490 kilometres in diameter. Soon after, more of these objects, named planetoids, asteroids, or, nowadays, dwarf planets, started to turn up in the tens, hundreds and then thousands. Astronomers developed their laboriously obtained photographic plates to find the images of distant galaxies marred by tracks across them – more asteroids. The novelty rapidly wore off and asteroids became known as the “vermin of the skies”.

We now know that asteroids are actually a planet in an arrested stage of development. All the planets, including ours, formed from the coalescence of lots of small rocky bodies. However, the gravitational tugging by Jupiter stopped the asteroids from forming a planet; that is not what one would expect gravity to do. Joseph Louis Lagrange discovered more subtleties in 1772. He found that the gravity field in a system of orbiting bodies had null spots, now called Lagrange Points, where bodies can be parked and they will stay there. For example, there is one of these points 1.5 million kilometres in the direction of the Sun. Because this point stays in the same place with respect to the Earth, it is a marvelous place to put satellites intended to observe the Sun; the satellites can view the Sun all the time but don’t go wandering off into space. Similarly, there is another point 1.5 million kilometres outward from the Earth, where we can put astronomical instruments intended to observe deep space; they never see the Sun because the Earth blocks it out. There are two more points, leading and trailing 60 degrees from the Earth around its orbit. Before we ever sent things up into space, there was undeniable proof Lagrange was right. There are concentrations of asteroids 60 degrees on either side of Jupiter leading and following it around its orbit. These asteroids got trapped in two of Jupiter’s Lagrange Points and we named these bodies after the heroes in Homer’s poem about the Trojan Wars. One would naturally expect that the Greeks would be in one group and the Trojans in the other, with Zeus (Jupiter) sitting in the middle keeping order. However, things did not turn out that way; we have heroes from both sides of the infamous war all mixed up. To further insult the Greeks, we call all of those objects “Trojans”. We continue to find more and more of the incredibly subtle things that gravity can do. Newton would be very proud.

A diagram showing the Sun–Earth L2 point, which lies well beyond the Moon's orbit around the Earth. Image source: NASA