COMETS AND ORBITS

Eric writes...

Jim- Now here's a question I have no knowledge about: If Halley's comet comes so close to Earth why doesn't it get affected by Earth's gravity and put it into a different path? And how close would a comet have to be to Earth for it to get pulled, or even slightly tugged, by our mass?

Well, Eric, we're in synch here, because I actually know this one. The fact is, Halley's path does get affected by the Earth. As a matter of fact, the Earth's path is affected by Halley's comet. It is just that the amounts are small. Of course, the Earth affects Halley's path more that Halley affect's Earth's path. One thing you have to consider is, even at it's closest pass, Halley is not very close to the Earth.

Let's take a look at Halley's closest approach to Earth. The comet's closest approach to Earth occurred in 837, at a distance of 0.033 astronomical units (4.94 million km; 3.07 million miles). So 3 million miles is not far considering the earth is 93 million miles from the Sun. But still, 3 million miles is pretty far. To try to put this in better perspective, let's shrink the Earth down to the size of a basketball. That would make Halley's comet about 1 mm in diameter or about the size of a pin head. At the closest approach in 837, that pin head would be about 300 feet away from that basketball. If you watched any of the baseball playoffs, and went to Houston and put that basketball on home plate, that pin head would be at the warning track down the left field line, in front of those God awful Crawford boxes. Yet even at those distances, there is gravitational pull by both bodies on each other.

Many of the space probes to the outer planets have used gravitational assist from the inner planets. That is, the probe would be sent past another planet, be drawn into it's gravitational field, gain speed but have a trajectory that would not be sucked into the atmosphere, then is "sling shot" forward with an overall gain of speed. We know that there is no free ride, energy is neither created or destroyed. In the case of the gravitational assist, by the laws of physics, the planet will lose energy. Essentially it will slow down, though it may only be by a billionth of a second.

What it would take to pull a comet into our atmosphere to crash into the Earth, would depend on the speed, mass, and trajectory of the comet. Three things can happen, it can be captured by the gravitational field and crash into the Earth, it can pass by and just be deflected, or if everything comes together just right, it could be captured by Earth's gravitational field and fall into orbit around the Earth.