Ever have visitors stay for a while and leave a mess? You know the type. They treat your house as theirs, leave stuff lying around, and leave without even an offer to help clean up.
The same thing happens in outer space.
The solar system is normally a fairly clean place. There are a few asteroids floating about but most of the debris has been swept away in collisions with the planets a long time ago. Indeed, one of the criterion the International Astronomical Union uses to determine planetary status is if a celestial body "has cleared the neighbourhood around its orbit."
Planets have been sweeping up space junk for the past 4.5 billion years. One of the theories for the origin of the planets in the first place is based on the aggregation of primordial debris. As the nascent proto-planets grew bigger, they were able to catch more debris and collect more chunks of rocky material, which allowed them to grow bigger and to collect more debris, etcetera.
Every now and then, though, the inner solar system gets a messy visitor. A comet in its close approach to the Sun leaves a trail of debris behind. And, of course, the earth blindly circling the sun, obligingly cleans up after it.
Hollywood movies aside, the vast majority of this space junk is not planet-killing asteroids and massive meteoroids from Planet X. According to the television commercials playing during the Blue Jays games, this summer's smash hit is a show called Salvation in which the world is threatened with life-ending extinction by an approaching asteroid. The science shown on Salvation has been laughable so far but in the real world NASA scientists have been doing their best to determine the orbits of all "near earth objects" for the past two decades in attempt to ensure we are not caught unawares.
More typically, the debris Earth cleans up is very, very small - the size of a grain of sand or little pea-size pieces of stone. This is the dust and debris left behind by comets when they come for a visit to the inner solar system.
In one sense, a comet could be described as the cosmic equivalent of a dirty snowball. The debris it leaves behind is just the dirt boiled off from the outer surface. It is the material we see in tail of a comet as it approaches the sun. This debris dirties up our orbital path around the sun.
In space, where there are very few atoms in any given volume and even less debris or dust, encounters with other material is not very likely despite Hollywood's propensity for drama. As a consequence, even the tiny particles of matter given off by comets can stay in orbit around the Sun, blindly following the path of the comet for millennia until they are swept clean by a planet.
But despite their small size and sparseness, these stellar dust particles have an amazing effect when caught by the Earth.
Suddenly encountering the atmosphere results in jarring collisions. Billions and billions of collisions occur with gas molecules in a matter of milliseconds. Frictional forces start to take their toll. The particle heats to incandescence while ionizing atoms and molecules in its path.
These incoming bits of stone and grains of sand are typically moving at 60 km/s or 220,000 kilometres per hour. Their kinetic energy is colossal - not because of their mass but because of their velocity. (Kinetic energy is one half of the mass times the square of the velocity.) Every bump and nudge by the atoms in the atmosphere takes up a bit of this energy, resulting in an excited state atom that glows.
These tiny particles produce the incandescent streak - the vast arc across the sky - we associate with meteors or "falling stars." Most of the falling stars we see on any given night are the result of debris left over from comets.
From July 17 to Aug. 24, we should witness a few falling stars (weather permitting) as we will be passing through the debris field from the Swift-Tuttle comet. The best viewing is expected to occur on the night of Aug. 12 as the Perseid Meteor shower reaches its peak. In a typical year, this generates one or two meteors every minute.
The Perseids are best seen in the northern hemisphere and come out of the northern portion of the sky. Unfortunately, the presence of moonlight on Aug. 12 will diminish the capacity of sky watchers to see the fainter streaks but the show should still be worthwhile.
So, despite being a messy visitor and leaving a trail of dirt and debris in its wake, a comet can have some wonderful effects even if it is not visiting the inner solar system.
