Scientists at the European Space Agency were facing an agonising wait yesterday as they settled down to receive a signal from Europe's Rosetta probe, to confirm it has woken up from years of hibernation, before they can celebrate a new milestone in their unprecedented mission to land a spacecraft on a comet.
Dormant systems on the unmanned spacecraft were switched back in preparation for the final stage of its decade-long mission to rendezvous with comet 67P/Churyumov-Gerasimenko. They had been powered down in 2011 to conserve energy, leaving scientists in the dark about the probe's fate until now.
The earliest the agency might receive the probe's all-clear call was early morning Hong Kong time. If no signal is received, they will today try to manually restart the probe from the ground.
"We don't know the status of the spacecraft," said Paolo Ferri, head of mission operations at the agency. "There is a possibility that we're not going to hear anything. Two-and-a-half years are a long-time. We're talking about sophisticated electronics and mechanics. We've taken all possible precautions for this not to happen but ... problems may have happened."
While waiting for the first signal travelling the 800 million kilometres back to earth, scientists were holding a social media competition, asking space enthusiasts to compose and perform songs to "wake up Rosetta". The top entries are to be beamed to the spacecraft and the winner invited to witness the landing from the ESA's mission control room.
"There's apprehension and excitement. Some people have put their lives into this," said Matt Taylor, project scientist on Rosetta at the agency's Netherlands division. "But it's a bit like a teenager waking up. It takes some time to get out of bed."
Rosetta is named after a block of stone that allowed archaeologists to decipher ancient Egyptian hieroglyphs. Scientists hope the probe's findings will help them understand the composition of comets and thereby discover more about the origins and evolution of our solar system.
Comets are regarded as flying time capsules because they are essentially unchanged for 4.6 billion years. Scientists have speculated that comets - essentially giant, dirty snowballs - may be responsible for the water found on some planets. And like asteroids, comets also pose a theoretical threat to life on earth.
"Over the millennia comets have actually affected our evolution," said Ferri. "There are many theories about comets hitting the earth and causing global catastrophes. So understanding comets is also important to see in the future what could be done to defend the earth from comets."
If all goes as planned, Rosetta will reach 67P in the coming months and fly a series of complicated manoeuvres to observe the comet - a lump of rock and ice about four kilometres in diameter - before dropping a lander onto its icy surface in November.
The Philae lander will dig up samples and analyse them with its on-board instruments.
The probe and its lander will keep sending back data until their batteries die or the debris streaming off the comet irreparably damages their sensitive instruments.
The mission is different from Nasa's Deep Impact probe that fired a projectile into a comet in 2005 so scientists could study the resulting plume of matter. Nasa also managed to land a probe on an asteroid in 2001, but comets are much more volatile places because they constantly release dust and gas that can harm a spacecraft.
Additional reporting by The Guardian
The origin of space probes' names
If Europe's comet-chasing quest goes well, Rosetta, Philae and even Churyumov-Gerasimenko may become household names. Here's an explanation:
ROSETTA: The European Space Agency craft is named after the Rosetta Stone, the inscription carved into a rock that helped 19th-century archaeologists unravel one of the greatest enigmas of their time. The stone, bearing a text in hieroglyphs and Greek, was found by French soldiers in 1799 near the village of Rashid (Rosetta) in the Nile delta. An English physicist, Thomas Young, and a French scholar, Jean-Francois Champollion, were able to figure out most of the hieroglyphs thanks to the Greek equivalent. The mysterious culture of the Pharaohs was at last explained.
PHILAE: Italian girl, Serena Olga Vismara, 15, proposed Philae in a Europe-wide competition to name Rosetta's scientific payload, a refrigerator-sized laboratory that will conduct experiments on the comet's surface. The name comes from an obelisk, found on the island of Philae on the River Nile, that itself was the key to Rosetta. The obelisk has a bilingual inscription bearing the names of Cleopatra and Ptolemy. This gave Champollion the final clue to decipher the hieroglyphs on the Rosetta Stone.
67P/CHURYUMOV-GERASIMENKO: The comet targeted by Rosetta is named for Soviet astronomers credited with discovering it in 1969, Klim Churyumov of the University of Kiev and Svetlana Gerasimenko, of the Institute of Astrophysics in Tajikistan. Churyumov spotted the comet on a photographic plate taken by Gerasimenko. "P" refers to a periodic comet, or a comet whose revolution around the Sun is less than 200 years. "67" refers to a list number kept by the US-based Minor Planet Centre.