Where Are All the Supernovae?

Danish astronomer Tycho Brahe was lucky. In 1572, he saw a bright supernova explosion in the constellation of Cassiopeia. Just 32 years later, his German pupil Johannes Kepler observed a similar event in Ophiuchus. Ever since, the Milky Way has produced not a single visible supernova, although John Flamsteed may have observed a pretty faint one in 1680. Indeed, there have only been eight in the past 2,000 years. Yet astronomers believe there should be at least three per century. So where are they?

In all likelihood, a supernova probably does pop off in the Milky Way every few decades. But since they occur in the thin Galactic disc, their light is heavily obscured by interstellar dust. "The supernovae seen by Tycho and Kepler were relatively close to us in the Galaxy," says David Green of the Cavendish laboratory in Cambridge. "No doubt there have been others much further away, which would have been obscured more."

Supernova explosions leave expanding gaseous remnants, like the famous Crab Nebula. Also, the cores of the exploding stars collapse into dense neutron stars, some of which are detected as pulsars. But counting supernova remnants and pulsars doesn't help much in pinning down the true supernova rote. Ages of older remnants are poorly known; our inventory is far from complete, and most neutron stars go unnoticed.

Still, the predicted supernova rate is consistent with the figure derived from supernovae detected in other galaxies, so it seems likely that we just haven't seen them. But it's not all bad news: future telescopes will operate in a wide range of wavelengths, giving us a better chance of observing them, and therefore proving that they're really happening.

Private spacecraft orbits Earth

The commercial space industry celebrated a landmark achievement on 8 December 2010, when California-based company SpaceX launched a spacecraft into orbit and successfully brought it back to Earth. It's the first time a commercial company has ever recovered a spacecraft from orbit, and it paves the way for private spacecraft missions to ferry supplies to the International Space Station. NASA Administrator Charles Bolden called the feat a 'dramatic step forward'. "We're witnessing the dawn of a new era, whose ultimate result could be routine, safe access to space with industry, academia and governments regularly sending payloads and people to low Earth orbit. That's the goal," he said. SpaceX used its own Falcon 9 vehicle to launch the Dragon spacecraft, which is capable of carrying a payload of six tonnes and could eventually be crewed by up to seven astronauts. Both the Dragon spacecraft and the first stage of the Falcon 9 are designed to be reusable, which is a key part of long-term plans to reduce the costs of spaceflight a factor of 10.

After lifting off from Cape Canaveral Air Force Station in Florida, the Dragon spacecraft orbited the Earth twice before re-entering the atmosphere. An advanced heat shield protected it from temperatures of up to 2,200"C. It then deployed parachutes before gently splashing into the Pacific Ocean off the coast of California.

"This has really been better than I expected there's so much that can go wrong and it all went right," said the company's CEO Elon Musk following the mission, which took less than four hours from lift-off to landing.

Later this year, Space X plans to carry out a five-day mission in which the Dragon will approach within 10 km (6.2 miles) of the International Space Station and communicate with its crew before returning to Earth. After that, a full cargo mission that docks with the station and delivers supplies should take place.

SpaceX has a $1.6 billion contract to make at least 12 flights to the International Space Station, following the retirement of NASA's Shuttle fleet later this year after nearly 30 years of service.