Death from the skies

Starting 2013 with a bang, we continue our armageddon series about the science behind the end of the world.

On 8 November 2007, the Minor Planet Center issued a routine "Minor Planet Electronic Circular", reporting on observations of a newly discovered object, designated 2007 VN84 - some 4 million km from the Earth. It was heading towards us at a speed of 45,000 km/h - over 12 km every second. It was set to pass within 5,300km of Earth just five days later.

The gravitational pull of planets can change the orbits of smaller bodies. This "slingshot effect" is sometimes used by interplanetary spacecraft. Image courtesy DLR

Obviously, 2007 VN84 didn't hit the Earth, and nobody was hurt. But how bad would it have been if it had? Well, it's unlikely anybody would have been hurt, but it certainly would have been embarrassing. This was no space rock, but a spacecraft, carefully being flown past the Earth by the European Space Agency to change its orbit - in exactly the same way they'd done at Mars, several months earlier (and at the Earth, before that). 2007 VN84 was more commonly known as Rosetta.

This mix-up was embarrassing, but also quite alarming for some. After retiring the designation 2007 VN84, the Minor Planet Center stated: “This incident highlights the deplorable state of availability of positional information on distant artificial objects.”

But what if this had been a real asteroid, heading towards us? Five days is not a lot of notice, though Rosetta is only quite small. A larger asteroid would (hopefully) be spotted sooner. Most people have heard the theory that dinosaurs were wiped out by an asteroid, but is the Earth endangered by a similar collision?

Essentially, we need to ask three questions: "How bad is being hit by an asteroid?", "Is it likely to happen?" and "How much warning would we have?". As it turns out, these questions are very closely connected.

Apocalypse When?

Today marks the end of the Mayan long count calendar. So will the world come to an end today? Probably not. But the world will come to an end eventually, and there are a number of scientists looking into various possible ways it might happen.

All life on Earth depends on the Sun. Without its energy plants couldn't grow, and everything would freeze before you could say "it's a bit chilly today". So it is perhaps fitting that the Sun may also be the doom of the planet, in about 5 billion years. Or maybe 6 billion. Our Sun isn't big enough to explode as a supernova, that most spectacular firework of nature, but it will start to get bigger. Much as we fear running out of oil on Earth today, sooner or later the Sun will run low on hydrogen, and will start to change.

Our Sun is performing a balancing act. Its gravity is trying to make it collapse to a tiny point, but the energy from nuclear fusion counteracts it. You can think of it a bit like a balloon, the elastic skin is trying to squeeze it all together, but the pressure of the air inside is keeping it at a steady size. Take away the gravity (elastic) and it would fly apart - but take away the air pressure, and it contracts. When the Sun runs out of hydrogen, this energy will reduce, and gravity starts winning, making the star smaller (our Sun is a star). But it won't make it to a tiny point, because as it gets smaller the pressure gets higher (try compacting a tin can into the size of a pea) and nuclear fusion starts up again - this time, burning helium.

What would our Sun look like as a Red Giant?
Image credit: Hiro Sheridan (Creative Commons)

But as the core of the Sun collapses in this way it also gets hotter, so the outer layers of the Sun will expand and turn the Sun into a red giant - about 250 times bigger than it is now. There has been quite some debate about exactly how large the Sun will get, and what will happen to the Earth. Mercury and Venus will be swallowed, but the Earth may be left intact. As well as being scorched to a crisp, the Earth's gravity may create a 'tidal bulge' on the surface of the Sun, which could eventually drag the Earth down to fiery doom. Either way, you wouldn't want to be around.

Whether or not humanity will survive to see this happen is another question entirely. Or rather, several questions. There are so many possible ways for humanity to meet its grisly end, that it might feel we should be asking "which one will get us first?"