Watch Live: Philae Probe Lands on Comet

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WATCH PHILAE LAND ON A COMET LIVE

LONDON (CNN) — The Philae probe is closing in on a historic touchdown on a comet after snapping a farewell picture of the mother ship and a tweet reading: “Hey @ESA_Rosetta, did you get my first postcard?”

“There is no going back now,” the agency said after the spacecraft’s lander was released for its trip to the surface of a comet which is 500 million kilometers (310 million miles) from Earth.

If it succeeds it will be the first time a spacecraft has landed on a comet.

The Philae lander separated from the mother ship Rosetta around 3:30 a.m. ET to begin its 7-hour descent.

Mission controllers say preparations are going well. In a webcast, flight operations director Andrea Accomazzo said: “We can see the lander going down on the right track.”

Philae, which has spent 10 years fixed to the side of Rosetta during the journey across the solar system, cannot be steered. Once it was released, it was on its own.

Before the spacecraft separation, ESA lander system engineer Laurence O’Rourke told CNN that the orbiter Rosetta had to be in the right position to allow the craft to “free fall” on the correct trajectory to the chosen landing site.

Scientists are hoping the probe will help us learn a lot more about the composition of comets and how they react when they get close to the Sun.

Weighing in at 220 pounds, it might be the size of a domestic washing machine but Philae is considerably smarter.

It is equipped with an array of experiments to photograph and test the surface of Comet 67P as well as finding out what happens when the roasting effect of the Sun drives off gas and dust.

But first it has to reach the landing site.

O’Rourke explained that hours before separation, Philae’s on-board batteries were prepared and a fly wheel started to give the probe stability on its journey to the comet surface. Without the gyroscopic effect of the fly wheel there’s a danger that the lander could turn end over end, he said.

Mission controllers now face a nail-biting wait as touchdown approaches. The comet is so far away that a confirmation signal relayed from Rosetta, which remains in orbit around the comet, will take nearly half an hour to reach Earth.

Scientists should have word around 11 a.m. ET.

The comet’s gravity is so weak that engineers have come up with ingenious solutions to keep Philae in place. At touchdown two harpoons fire out from the legs and screws on each of the three feet help attach it to the comet. Originally, a thruster on top of the craft was supposed to help push it into the surface, but that system isn’t working, according to ESA.

Built by a European consortium, led by the German Aerospace Research Institute (DLR), the landing probe has nine experiments.

If all goes well the first picture should be of the landing site taken by Philae during the final moments of descent, followed by a panoramic image from seven cameras on the top edge of the lander, O’Rourke told CNN.

According to details on ESA’s Rosetta website, sensors on the lander will measure the density and thermal properties of the surface, gas analyzers will help to detect and identify any complex organic chemicals that might be present, while other tests will measure the magnetic field and interaction between the comet and solar wind — high-energy particles given off by the Sun.

Philae also carries a drill that can drive 20cms (8 inches) into the comet and deliver material to its on-board ovens for testing.

O’Rourke told CNN that the landing is fraught with danger. Philae could hit a boulder or a crevice and tip over.

However, mission scientists are already pleased with progress.

ESA project scientists Matt Taylor said: “The orbiter will remain alongside the comet for over a year, watching it grow in activity as it approaches the Sun, getting to within 180 million kilometers (112 million miles) in summer next year, when the comet will be expelling hundreds of kilograms of material every second.

“It’s got an awesome profile — the adventure of the decade-long journey necessary to capture its prey, flying past the Earth, Mars and two asteroids on the way,” he said.

Daniel Brown, an astronomy expert at Nottingham Trent University in the UK, said: “Although we have landed on planets, moons and asteroids, it has never been attempted for a nucleus of a comet — and with good reason. These objects have a very low gravity, are loosely composed of ice, dust and rocks, and are very irregular in shape. They are temperamental in their behaviour and notoriously difficult to predict.”

And science fiction writer Alastair Reynolds added: “This is science fiction made real in terms of the achievement of the mission itself, but Rosetta is also taking us a step closer to answering science fiction’s grandest question of all — are we alone?”