The Earth is constantly in the line of fire from space rocks. Some pass us by, some crash through the atmosphere and burn up as bright fireballs soaring across the night sky. Sometimes, however, they are just brief visitors, skipping through the atmosphere and bouncing right back out again. These are known as “Earth-grazers”, and it’s rare to catch one in the act.
This little meteoroid was picked up by the Global Meteor Network in the early hours of the morning of September 22, above northern Germany and the Netherlands. According to the European Space Agency (ESA), it got as low as 91 kilometers (56.5 miles) in altitude – far lower than orbiting satellites, which remain in low-Earth orbit between 160 and 2,000 kilometers (100 and 1,240 miles) up – before bouncing back into space.
Earth-grazers only occur a handful of times a year, compared to the thousands of meteors we see, which occasionally land on Earth. So, what is the difference between a meteoroid, meteor, and meteorite?
A meteoroid is a fragment of space rock – a comet or asteroid – that becomes a meteor (shooting star) when it burns up in our atmosphere and disintegrates, the pieces of which only become meteorites if they land on the ground. Although thousands of meteorites have been discovered, only 40 have ever been traced back to their parent body, according to ESA.
This one didn’t get low enough to burn up, managing to somehow escape and whizz back out into space. It entered the atmosphere at 3.53am UTC on September 22 traveling at 34.1 km/s (21 m/s), according to Dennis Vida, who leads the Global Meteor Network. Incredibly, Vida revealed they have traced the meteoroid back to a Jupiter-family orbit, but searches for the parent body have not proven fruitful yet.
So how does a meteoroid "bounce" off Earth's atmosphere rather than being sucked in? First, it has to enter the atmosphere at a shallow angle, like a stone skimming water. It also has to keep its speed to be able to escape Earth's gravity; Earth's escape velocity is 11.2 km/s (7 m/s), which the meteoroid was going comfortably faster.
Just because they don't touch the ground, however, doesn't mean Earth-grazers are completely harmless. The Tunguska event of June 30, 1908 – when a massive explosion flattened 2,150 square kilometers (830 square miles) of Siberian forest, with tremors felt as far away as the UK and US – is believed to have released 30 megatons of energy, enough to level a city.
It was thought to have been caused by the largest asteroid impact in recorded history. However, due to the absence of an impact crater, theories suggested the asteroid disintegrated when it entered the atmosphere and a shockwave caused the event. New research released earlier this year suggests the space body was in fact an Earth-grazer, hitting the atmosphere, causing the shockwave, and skimming right back out again, hence no impact crater. Asteroid Day – a UN initiative to engage and educate people about asteroid science and exploration – is celebrated on its anniversary every year.