Chapter 8: Interplanetary Bodies

A bolide (an especially bright meteor)

When we think of interplanetary bodies, we tend to think of the large and flashy objects: The large asteroids, Kuiper Belt objects, and comets that regularly grow so bright they can be seen by the unaided eye. While this is what we think of most, the vast majority of interplanetary objects are actually nothing more than small fragments of rock or ice that are much smaller than comets. This is a good thing since it means catastrophic impacts in the Solar System are extremely rare. It's far more common for a tiny piece of rock from space to hit our atmosphere and simply cause a bright flash of light. The typical meteor (often misnamed "shooting star") that you might see flashing across the sky is likely to be the size of a grain of sand, and probably is a particle of cometary debris.

A Perseid Meteor over the Very Large Telescope in northern Chile.

On an average evening, you may see about three meteors an hour. But on certain dates each year, you may see more than 60 meteors each hour! These events, called meteor showers, coincide with the Earth passing through the debris trail left behind by most often a comet, but occasionally by asteroids as well. If you pay attention, you'll notice all the meteors (colloquially called shooting stars) radiate from one direction in the sky. This point they radiate from is the place in the sky where the Earth's atmosphere is hitting the debris. The best-known example is the Perseid shower, which occurs every year around August 12. For a few nights, bright meteors streak across the sky every few minutes from the direction of the constellation Perseus. (Showers are named for the constellation from which the meteors appear to radiate.) Occasionally the showers are so intense that meteors fall too fast to count. During the Leonid shower (named for the constellation Leo) of November 17, 1966, meteors fell like snowflakes in a blizzard for some minutes, at a rate estimated to be more than 2,000 meteors per minute.

G. V. Schiaparelli

In 1866, G.V. Schiaparelli (who was also involved in the controversy over alleged Martian canals) discovered that the Perseid meteor shower occurred whenever the Earth crossed the orbit of a particular comet. This allowed him to link the meteor shower with the Earth passing through the swarm of debris left in the wake of the comet's passage. Other relationships were soon found between recurring meteor showers and specific comets. In 1983, an infrared astronomical telescope in orbit imaged the meteor dust spread along the orbit of Comet Tempel 2, which was the first direct observation of the comet dust scattered in space along a comet's orbit. The debris is not uniformly spread along the comet's orbit. When the Earth passes through a sparse patch, we may see more than the average number of shooting stars. When the Earth passes through a dense volume of debris, however, we get a spectacular light show.

Diagram of the direction of meteors from a Perseid meteor shower. The meteors appear to diverge from a single point in the sky called a "radiant point.

In addition to meteor showers, there are also continuous encounters at a much lower rate with objects that are traveling in random directions through space. You are most likely to see these sporadic meteors in the early morning hours. Since the Earth travels in its orbit at 30 kilometers per second, only rare high-velocity objects will be able to overtake the Earth with sufficient residual speed to create a glowing trail. It's like driving down the highway in a rain shower — more drops will hit the front windshield than the rear window. On a moonless night at a dark site, you can see about 5 meteors per hour before midnight and about 15 per hour just before dawn. Meteors approaching the Earth at shallow angles are more likely to burn up without leaving a noticeable trail, so you are most likely to see the meteors in a shower that are coming straight in. This creates the perspective effect, where the meteor shower appears to diverge from a point in the sky. You may also see bright "meteors" that are actually spacecraft debris falling through the atmosphere.

Zodiacal light seen from Paranal.

The smallest interplanetary particles are microscopic dust grains spread out along the plane of the Solar System, concentrated toward the Sun. If you look west in a very clear rural sky as the last glow of evening twilight disappears, or look east just before sunrise, you can detect the diffuse glow of sunlight reflecting off these particles. It appears as the zodiacal light: a faint glowing band of light extending up from the horizon and along the ecliptic plane. It's brightest on the horizon. The zodiacal light is just sunlight reflecting off a swarm of countless dust grains. They were released by comets in trails along their orbits, eventually spreading into a uniform disk-shaped cloud of dust around the Sun.