Astronomy & spaceflight
Asteroids & comets
Rubble from the dawn of the solar system: why comet tails point away from the Sun and where shooting stars come from.
When the solar system formed about 4.6 billion years ago, dust clumped into ever larger chunks that eventually became the planets. But not all the material found a home. Billions of rocky boulders and lumps of ice were left over: the asteroids and comets. They are a kind of rubble from the dawn of the solar system, and that makes them gold for research, because they have barely changed since then.
The asteroid belt
Most asteroids in the asteroid belt between the paths of Mars and Jupiter. Its largest resident is Ceres, about 940 kilometres across, big enough to count as a dwarf planet. The vast majority of chunks are much smaller though, and all the asteroids in the belt together add up to only about three percent of the Moon's mass. Even with millions of them, the belt is almost empty: on average, many hundreds of thousands of kilometres lie between two asteroids. fly straight through without any slalom.
Comets, dirty snowballs
Comets come from the icy outskirts of the solar system and are made of ice, dust and rock, which is why they are often called dirty snowballs. Their orbits are strongly stretched out: they spend most of their time far out in the cold and then race past the Sun for a short while. When a comet gets close to the Sun, its ice evaporates and a huge shell of gas and dust grows around the core, which is only a few kilometres across. The most famous one is Halley's comet, which returns about every 76 years and in between travels as far out as 35 . One astronomical unit is the average distance from the Earth to the Sun, about 150 million kilometres.
The satellite keeps falling toward the planet, but moves sideways so fast that it keeps missing it. Rule of thumb: an orbit is falling around the planet.
The tail points away from the Sun
A comet's tail is not an exhaust plume trailing behind it. Sunlight and the solar wind, a stream of charged particles, push gas and dust away from the comet's head. That is why the tail always points away from the Sun, no matter which way the comet is flying. When the comet moves away from the Sun again, it therefore flies tail first. Often you can even see two tails: a straight bluish one made of gas and a curved whitish one made of dust.
Shooting stars
A shooting star is not a falling star, it is usually just a grain of dust. It plunges into Earth's atmosphere at several dozen kilometres per second and burns up at an altitude of about 80 to 100 kilometres in a bright streak of light. When Earth crosses the dusty trail of a comet, there are especially many of them: a meteor shower, like the Perseids every August. Only larger chunks survive the flight through the atmosphere. Whatever reaches the ground is called a .
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
Where is the asteroid belt?
In which direction does a comet's tail point?
Halley's comet last came close to the Sun in 1986. Assume an orbit of 76 years: which year does that give for the next return?
Match each object to its fitting description.
Shooting stars burn up at an altitude of about 100 kilometres. A passenger plane flies at about 10 kilometres. How many times as high do the shooting stars burn up?
Even though there are millions of asteroids, the belt is almost …, because on average hundreds of thousands of kilometres lie between two chunks.