Half a century has passed since Yuri Gagarin ventured into orbit, and over 40 years since we landed on the moon, but since then, we've retreated. Space is too expensive to explore.

The space shuttle fleet is being retired to museums, and our only permanent orbital presence is limited to a handful of scientists. But while manned space exploration appears to have faltered, astronomy has exploded beyond all recognition.

We now know of nearly 600 planets orbiting alien suns, and have a small army of space telescopes at our command.

It's not just professional scientists making breakthroughs, either. Amateur astronomers and armchair enthusiasts also provide vital discoveries. Do we really need expensive manned space flight any more, or can we now discover the secrets of the universe from our PCs and back gardens?

"I actually had to classify the galaxy next to it - next to the strange thing I found," says Dutch schoolteacher Hanny van Arkel of the discovery that made her famous. Hanny's Voorwerp (the Dutch word for 'object') is an unusual patch of superheated gas close to a spiral galaxy in the constellation Leo Minor. It contains no stars, so where is the heat coming from? It seems to be a complete mystery, but it's one that could have been undiscovered for many more years if it weren't for the emerging field of citizen science.

"It's very light, very warm, and it doesn't have any stars, which apparently is exceptional," says van Arkel. "I thought, 'Wait, what was that?' So I clicked the back button and saw this strange, blue sort of cloud. It caught my eye because it was very blue and it had a strange form. It was nothing like the irregular galaxies I'd seen."

Van Arkel found the Voorwerp while classifying galaxies from her home in Holland as part of the Galaxy Zoo project. She contacted one of the astronomers on the project (known as Zookeepers), but neither he nor his colleagues knew what the Voorwerp was either.

In a demonstration of just how much private individuals can help science, time was booked on the Hubble Space Telescope to examine the Voorwerp in more detail. The team controlling Hubble receive around 10,000 requests for time each year, but van Arkel's discovery was so intriguing and important, it became one of only 3,500 requests to be granted.

Van Arkel's involvement in the Galaxy Zoo project was as unexpected as her discovery. "I have a passion for music," she says. "I play guitar, and Queen is one of the bands I like. Brian May is interested in astronomy and put an announcement on his website about a project called Galaxy Zoo. It looked very interesting and I thought: 'Well, I'll have a look.'"

Crowdsourced science

Galaxy zoo

Galaxy Zoo is much more than a simple project to get people interested in astronomy and science in general. It's also part of an attempt to help working scientists overcome a growing problem shared by many different areas of science, as physicist Chris Lintott explains.

"It's something strange that's been caused by an explosion in technology, in computing power, in the availability of cheap cameras and in bandwidth," he says. "I think for the first time in a long while, we almost have too much data. And I think that a lot of the science that happens in astronomy, in ecology [and] in climate science over the next decade will be driven by how creatively we can solve that problem."

Galaxy Zoo helps solve the problem of the mountain of data streaming down to Earth from dozens of space telescopes collecting data at various wavelengths. When it comes to pattern-matching, the average human brain is still far more accurate, faster and cheaper than even the best artificial neural network software. It was therefore logical to ask members of the public to log in and set to work classifying galaxies into groups to help create a full survey of captured galaxies.

Galaxy Zoo launched in 2007, with an initial one million images for people to classify. The team behind the site believed it would take two years for visitors to work through them all, but were very excited by the enthusiasm shown by armchair astronomers the world over. Within 27 hours, pictures of galaxies were being classified and re-classified at a rate of 70,000 an hour.

The original million galaxies were classified 50 million times by 150,000 regular users in the first year alone. Multiple classifications increase the confidence that a galaxy has been correctly categorised.

Galaxy Zoo is part of a larger initiative called the Zooniverse. Its subtitle is 'Real science online', and that's exactly what it is. After all, experimentation is only a small part of science - the bulk of the work revolves around classifying and analysing the collected data to discover what it reveals.

There's no reason that anyone with an interest in science and a little training can't don a virtual lab coat, pitch in and help out. The full Zooniverse contains projects as diverse as watching for massive solar storms to help provide a much needed early warning system for Earth, hunting for planets orbiting other suns, and helping to show how Earth's climate is changing by entering naval observations stretching back to the 18th century.

Getting involved

The training involved in Galaxy Zoo is straightforward, making the project open to just about anyone who's interested in taking part.

When classifying galaxies, you are asked several questions about their appearance. To answer the questions, you simply need to know what they mean, which is explained on the project's website.

The first question is whether the galaxy is smooth (simply a cloud of material that gets brighter towards a central point), has visible features, or is a definite disk shape. There are several examples you can click to see if your classifications are correct.

This idea of teaching by example is remarkably efficient, and by the third classification question, it's difficult to get the answers wrong. It's very easy to see if a galaxy is 'cigar-shaped', for example. Counting the number of arms on a spiral galaxy is just as simple.

This classification system is exactly the same as the one used by working scientists like Meghan Gray, a research fellow in the Faculty of Science at Nottingham University. "Even though we have automated routines to gather some of this information," she says, "those automated routines are still not perfect and there's still a lot to be gained by using [the] eye-brain system, which is really good at picking out particular patterns or particular features."

There's a real need for volunteers in these projects. "Together with a team of maybe about seven or eight other people, we could cover about six or seven thousand galaxies, with each one being done a couple of times to make sure that we agree on the classification," says Gray.

Some databases contain millions of galaxies, though. "That's far too many galaxies for any sane individual to look through, one by one," she says.

Amateur discoveries

There's always the possibility that you might uncover something that nobody else has seen or can explain, and that's exactly what happened in the case of Hanny's Voorwerp. It became an object of interest; something out of the ordinary, and that's always something that excites scientific minds, trained or otherwise.

The word 'amateur' comes from French, meaning 'lover of'. While the term has negative connotations in some fields, astronomy has a long tradition of amateurs making important discoveries, and this looks set to continue into the internet age.

One inspiring example is German-born composer and musician William Herschel. His music led him to study mathematics, and eventually astronomy. After emigrating to England and settling in Bath, where he became organist at the Octagon Chapel in 1766, Herschel designed and built his own reflecting telescope - sometimes spending up to 16 hours a day carefully grinding and polishing the mirrors.

In 1781, during a painstaking search for double stars, Herschel and his sister Caroline discovered Uranus. This led to his election to the Royal Society in 1782, and his appointment as the King's Astronomer.

Since then, many other amateur astronomers have found fame and even fortune. Sir Patrick Moore, despite having written over 70 books and been president of the British Astronomical Association, is a proud amateur. His maps of the moon were even used by NASA to help select possible landing sites for the Apollo missions.

Canadian David H Levy is an amateur astronomer who holds the record for the most comets discovered by a single person (22), including the famous Shoemaker-Levy 9, which broke up in 1994 and smashed into Jupiter's upper atmosphere in 1995.