Robert Browning (1812 – 1889)
Comet 67P has an orbit that ranges between 800 and 186 million kilometres from the Sun and takes about six and a half years. The Earth is around 150 million kilometres away from the Sun for comparison. 67P is currently around 673 million kilometres away. No launch vehicle is capable of sending a probe directly into such an orbit and so Rosetta had to use the gravity of the inner planets (Mars once and the Earth three times) to accelerate towards its destination. This circuitous route, taking Rosetta through the asteroid belt twice, explains why it has taken over ten years to reach its destination.
Given that the comet has a speed of around 135,000 km/hr, Rosetta’s approach speed was around 2,797 km/hr and that Rosetta’s cameras weren’t even switched on at the time, manoeuvring the probe into a stable orbit was itself an astonishing achievement and, even if Philae had failed, would have nevertheless made the whole mission a huge success.
Landing a probe on the comet’s surface, however, raised even greater challenges. Comet 67P is the size of a large mountain. If an astronaut were stood on it and jumped up they would never come back down, as the comet’s gravitational field is so weak. Landing too softly would therefore have led the probe to bounce back up into space and be lost. Landing too hard, however, could have damaged it and so an optimum descent speed had to be determined with no prior knowledge or experience of the nature of a comet’s surface. As it turned out the landing did indeed prove problematic and Philae bounced several times before finally coming to rest in an unfavourable position where its solar panels are unfortunately shielded from the Sun.
This is the first time a space probe has either orbited or landed on a comet and the mission clearly captured the imagination of the general public, with the post office even introducing a commemorative postmark. As a scientist concerned with the public understanding of science it is interesting to me to see which science stories make the headlines and which don’t. In 2005 another European built probe, this time British led, landed on Saturn’s moon Titan. This was another first and so far unique achievement but did not get anything like the same attention.
I believe that part of the popular appeal of Rosetta/Philae was the prospect that it could shed light on the origins of life on Earth. Life exists here wherever liquid water is found but we don’t know where this water came from. The early Earth was so hot that any water present would have boiled off and escaped into space.
One popular theory is that the water that covers most of our planet today arrived in comets. The latest results from Rosetta suggest that this theory is wrong; the water on P67 is very different to that on Earth. These results also suggest that we may need to rethink our ideas as to the origins of at least some comets altogether.
Comets are of particular importance because they are believed to be amongst the oldest objects in the solar system and therefore made of the primordial material from which all the planets and moons, and ultimately ourselves, are made.
Panspermia is the theory that life arrived on Earth from outer space. It has been supported by various eminent astronomers including Sir Fred Hoyle and Professor Stephen Hawking. In 2013 a team of physicists claimed to have found mathematical and semiotic patterns in the genetic code of all life on Earth that suggested it may have been deliberately seeded here. This theory, that life was deliberately transported to Earth by an extra-terrestrial intelligence, is known as directed transpermia. Carl Sagan proposed in the 1970s that humans could carry out their own directed transpermia. He suggested that genetically modified plants could be sent to Mars where they would reproduce and cover the planet in a heat absorbing and oxygen emitting layer that could make Mars habitable in a few thousand years.
A more likely alternative to directed transpermia is that life arrived here accidentally. In 1996 scientists from NASA claimed to have found a nanobacteria inside a meteorite that had travelled to Earth from Mars. This claim was later refuted, with planetary scientists from the Open University in Britain claiming it was too small, for example. Subsequent discoveries, including even smaller extremophile bacteria within the Earth’s mantle, led a team from the Johnson Space Center to reassert that “there was strong evidence that primitive life had existed on Mars”. Other claims have been made for extra-terrestrial life, none so far achieving universal acceptance.
Even if life itself is unlikely to have arrived here on a comet it may be that the extreme conditions that occur there as it approaches the Sun give rise to pre-biotic molecules that are the precursors to living things. Organic molecules are extremely common in outer space but it was hoped that Philae’s drill would find evidence of more complex molecules that had actually been synthesised inside the comet on one of its previous visits to the inner Solar System. Comets have extremely eccentric orbits that mean they spend long periods in deep space, at temperatures just above absolute zero (-273C), interspersed with periods where they closely approach the Sun and reach extremely hot temperatures.
Unfortunately Philae’s battery ran out before it could complete this experiment but all is not lost. It is hoped that as 67P gets closer to the Sun and light levels increase Philae’s batteries may get recharged from the solar panels and the samples can be analysed. Comet 67P makes its closest approach to the Sun next August. It had originally been assumed that Philae would have stopped working by then but it may be that its unintended landing site might end up shielding it from the Sun as 67P makes its closest approach. We may then be able to directly measure the chemical changes that occur as the comet approaches perihelion (closest approach).
At a time of food banks, increasing homelessness and fire station closures it would not be unreasonable for some people to question the decision to spend money on a project such as this. The Rosetta mission cost €1.4 billion spread over 20 years and between 20 countries. This is less than one fifth of the cost of the aircraft carrier HMS Queen Elizabeth. According to the BASIC think tank we will be spending around six times this every year for the next 50 years replacing our Trident nuclear submarines. It is similar to the amount we will spend on food and drink this Christmas in Britain.
People still talk about the Apollo programme producing non-stick frying pans. There are similar spin-offs from Rosetta. Rosetta is the first space probe to have travelled so far from Earth powered solely by the Sun. Voyager 1, that has now left the solar system, has three mini-nuclear power stations, for example, as has the Cassini probe in orbit around Saturn. Rosetta’s specially developed solar cells, using completely new technology, can generate electricity from light with light levels only 4% of those found on Earth. These will almost certainly have applications here in the future.
Why fund space exploration when we should be finding a cure for cancer? This is a question I’ve been asked many times. Pure science throws up unexpected practical discoveries all the time: gelignite, polyethylene, super-glue, penicillin, super-conductivity, microwave ovens and cornflakes were all accidents. The drive to only fund applied science is short-sighted, narrow minded and just plain wrong, though it is sometimes supported by those on the left who want to see money redirected to where they see most need. In short, if we ever do find a cure for cancer it might very well come from a research programme in a totally different area, maybe even from space exploration.
Missions like Rosetta can enthuse and engage young people, inspiring them to study science and choose science and technology related careers, but for me the real reason we should support such projects goes far beyond these issues, it is a philosophical question about how we see ourselves.
Socialists believe the world can and will be better. We understand what is wrong with it and how to put it right. Rosetta is but a tiny glimpse of what will be possible in the future when the planet’s resources are used for the benefit of society as a whole, when human ingenuity is freed from the shackles of the profit system and when international cooperation is no longer stymied by competition and nationalism.
The mission gets its names from the Rosetta stone and Philae obelisk that allowed Egyptian hieroglyphics to be translated and understood for the first time. I absolutely believe that there can be no more inspirational vision of our future than one in which we explore the heavens and seek answers to some of the most fundamental questions: How did life arise on Earth? Are we alone? Can we ever travel to other worlds?
Reach for the stars
Let’s reach for the stars
Can’t nobody hold us back
They can’t hold us down
Will I. Am (1975 – )