On some nights you can observe a special astronomical event: it looks as if a star is falling from the sky. Superstitious people even think that anyone who sees such a shooting star could wish for something. But what is really behind this phenomenon? Where do shooting stars come from and what are they?
Actually, shooting stars have nothing to do with stars. The sun, planets and moons are not the only astronomical bodies in our solar system. Many small pieces of rock and metal have also been discovered. They are much smaller and not as round as planets, so they are called small planets or asteroids. Like their big siblings, they orbit the sun. Most of the asteroids can be found in the asteroid belt between the orbits of Mars and Jupiter.
Every now and then two of these asteroids collide. Such a crash creates a lot of debris and splinters. These fly away from the previous orbit, across the solar system. Some of them get close to earth and are pulled by earth’s gravity. As a result they enter Earth’s atmosphere. These solid pieces of debris are also known as meteoroids.
Meteoroids, meteorites and meteors
They would literally fall to earth’s surface from the sky like stones – if it weren’t for the atmosphere. The meteorites are so fast that air cannot escape from their path quickly enough. The air in front a descending meteorite is compressed and therefore extremely hot. The air begins to glow and the meteorite starts to evaporate. We can then observe it as a glowing streak that stretches across the sky – a shooting star.
Most meteoroids are so small that they burn up completely on their way through the atmosphere. If such a body enters the Earth’s atmosphere and vaporizes, it becomes a meteor. These are most often called shooting stars, their glowing trail simply ends in the sky. Larger debris also lose mass on the way, but do not evaporate completely. They impact earth’s surface. So, a meteorite is a piece of debris that survives its passage through the earth’s atmosphere.
The effect meteorites have on Earth depends on how big they are. Small meteorites with a diameter of a few centimeters, for example, will only leave a dent on a car roof.
Largest meteorite impact
The largest known meteorite struck about 65 million years ago. It was several kilometers in diameter and tore a crater 180 kilometers in diameter. The impact hurled so much dust into the air that the sun was darkened for hundreds of years. As a result, plants and animals died out all over the world – this was the end of the dinosaurs.
Fortunately, such large meteorites are very rare, so we don’t have to worry. In addition – unlike the dinosaurs – we can observe the sky with telescopes and discover such large asteroids long before the impact.
On April 29, 2020, a very big asteroid passed relatively close to Earth. Asteroid (52768) 1998 OR2 passed at about 16 times the Earth-moon distance. According to NASA it is the biggest asteroid to pass Earth in 2020.
On February 15, 2013, a rather ordinary cosmic body fell in the Chelyabinsk region of Russia. Unique to this impact is its place and time. This was the first record of a large meteorite falling in a densely populated area. So, a similar meteorite fall has never caused such serious damage before.
Events of this magnitude occur once every 100 years, and according to some sources – and more often, up to five times a century. Scientists believe that bodies of about ten meters in size (about half the size of the Chelyabinsk body) fly into the Earth’s atmosphere about once a year. However, this happens most often over oceans or sparsely populated regions. Such bodies explode and burn at high altitude without causing any harm.
While a shooting star burns up in a few seconds, another phenomenon remains visible for longer. Comets along with their gas and dust tails can be observed in the sky for days or even weeks. In the past, people also attributed many qualities to them – as divine signs, foretellers of calamity or harbingers of joyful events. But the truth is a little less spectacular.
Often astronomers compare comets to dirty snowballs. They come from the outer solar system, far from the warming power of the sun. It is so cold there that water immediately freezes and becomes ice. This is how these lumps of ice and dust form.
A comet also initially travels far away from the sun – until it is redirected by a collision and flies in the direction of the inner solar system. It comes closer to the sun and receives more and more light and warmth over time. As a result, the frozen surface begins to thaw and even evaporate. This creates a shell of water vapor and dust around the comet.
At the same time, the comet is affected by solar wind – the constant stream of electrically charged particles that fly from the Sun at high speeds. They meet the comet’s vapor envelope. This blows the comet’s vapor envelope away, forming an elongated cloud that faces away from the sun. When this cloud is hit by sunlight, it appears as a glowing streak – the tail of the comet.
The comet flies around the sun and then moves away. If it is far enough from the sun, thawing and evaporation will stop. The tail disappears and the comet moves through the vastness of the outer solar system as a dirty snowball. Depending on the comet’s orbit, it takes many decades or even centuries until it comes close to the sun again.