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Monday 31 December 2012

The Sun's Bubble

This guest post is by Roger Duthie, Doctoral Candidate in Space Plasma Physics at the Mullard Space Science Laboratory.

In the late 1970s, Nasa’s Voyager probes began an epic journey through the solar system to explore the outer planets (Jupiter, Saturn, Neptune & Uranus). The “in-situ” measurements sent back from Neptune and Uranus by Voyager 2 are still the only local observations of the magnetospheres of these ice giants. But what is so very special and unique about the Voyager missions is that they are still operational over 35 years on and they are heading out of the solar system into new territory within our galaxy (the Milky Way) at large.

Image courtesy NASA/JPL-Caltech
NASA's Voyager probes are the furthest man-made items from the Earth.
Image Courtesy NASA/JPL-Caltech.
Image copyright: ESA
The Earth's magnetosphere is also important for keeping
ahold of the planet's atmosphere. Image credit: ESA
The solar system of eight planets, plus many minor planets, asteroids and comets, is bathed in plasma ("solar wind") streaming outwards from our single, central star (the Sun). The Sun also provides a magnetic bubble in the form of the heliosphere, much like the magnetosphere which Earth possesses. The Earth's magnetosphere is the region around the planet where its' magnetic field dominates, and to some extent it protects the Earth from the impinging solar wind; the heliosphere plays the same role for the entire solar system, diverting the plasma of our galaxy's interstellar medium (the ISM). Where we are able to extensively probe and measure the structure of the Earth's magnetosphere and interaction with the solar wind with satellites and even observations made from the ground, the outer reaches of the heliosphere are the sole domain of the Voyager spacecraft. It happens that they are both heading out towards the 'nose' of the heliosphere, one into the northern part and one to the southern. The nose is the direction facing upstream within the flow of the ISM; in the downstream direction is the heliosphere's tail.

Friday 21 December 2012

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.

An artist's impression of what our Sun may look like from Earth as a Red Giant Star
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?"

Monday 10 December 2012

Biofuels – Why bother?

A while ago someone figured out that the Earth's ever fluctuating climate was now changing too quickly and blamed it on excessive emissions of carbon dioxide from human activities. The scientific community, policy makers and the general public more or less agreed that something ought to be done to prevent a global catastrophe. One of the main sources of such climate-harming gases is burning fossil fuels for our transport and industries. Conveniently, such fuels are also a cause of great economical and geopolitical distress in the whole world, since every country needs them but only a handful are able to supply them.

One possible solution for these issues is the use of "biofuels". Biofuels are burned to release energy, much like conventional fuels, but whose energy was stored by consuming (sequestering) carbon dioxide in the first place.  In other words, using biofuels only releases the carbon dioxide that was removed from the atmosphere by making it, resulting in no overall increase. The most obvious example of biofuel is wood.

Grass grown as a biofuel crop for Drax power station
This grass is grown as an annual crop for biofuel, burned at Drax power station in Northern England.
Photo credit: Allan Harris (Creative Commons)