The six-person crew of the Discovery, before boarding the shuttle earlier today

As dusk approached yesterday, the space shuttle Discovery blasted off. It was an appropriate time of day, as the entire shuttle program has entered its twilight hours. When the Discovery returns after its eleven-day mission, it will be mothballed and shipped off to the Smithsonian, leaving only the Endeavour and Atlantis to each fly once more, with the final mission coming in the summer of this year.

After that, NASA will have to find other ways to get astronauts into space. NASA's had gaps in its spaceflight capabilities before, such as during 1975 to 1981, after the Apollo rockets and spacecraft were retired and before the shuttle started flying. But, unlike the late 1970's, NASA can't just choose to skip sending anyone into space -- today, people and supplies must be routinely sent to the International Space Station (ISS), currently operating with a crew of six in low Earth orbit.

NASA always knew that the shuttles couldn't fly forever, but its attempts to develop a successor failed, most notably including the VentureStar, an ambitious single-stage-to-orbit spacecraft that was cancelled in 2001 after over a billion dollars had been spent in development. This left the agency with no clear alternatives, prompting NASA to begin studying how to keeping the shuttle flying until at least 2020, until the disintegration of the Columbia during re-entry in 2003 cast doubts on the wisdom of prolonging the working life of the vehicles. The agency then begun working on the Constellation program, but the future of that program remains clouded, and only one preliminary test flight of the launch rocket has occurred to date. After this summer, NASA will have to rely solely on hitching rides on Russia's cramped three-person Soyuz spacecraft to get its astronauts to and from the ISS.

Fortunately, a number of strong candidates to replace the shuttle, as least with regards to resupplying the ISS, have emerged in the last few years from the private sector, as outlined in the latest Briefing from Technology Review. SpaceX is furthest along, successfully testing an automated version of its Dragon space capsule and Falcon 9 launch rocket in December. Dragon will begin supplying the ISS this year with cargo, and is designed to be easily upgraded to accommodate astronauts--if NASA is willing to buy tickets.

The three main parachutes on SpaceX's Dragon spacecraft carry to a landing in the Pacific Ocean during a drop test. Credit: Chris Thompson/SpaceX

Space Exploration Technologies (or SpaceX) has successfully drop-tested a capsule designed to take cargo and crew to the International Space Station.

The capsule, called Dragon, was dropped from 4,240 meters to validate its parachutes and splashdown systems. It has three main parachutes, each 35 meters wide, which are used to slow the spacecraft's descent, and thrusters that fire to help it navigate to it's landing location. Eventually, SpaceX wants to bring the capsule down on land. The drop test was the final in a series of tests designed to ensure that the spacecraft is ready for its maiden test flight on the Falcon 9 rocket, scheduled for later this year.

"Data gathered during the drop test will be invaluable as we prepare for the upcoming demonstration flight of the first operational Dragon spacecraft," said Chris Thompson, SpaceX vice president of structures in this Space.com article.

SpaceX received a $1.6 billion contract from NASA to provide the agency with a launch vehicle and spacecraft to carry cargo to the space station, with the option of manned missions, once the space shuttles retire next year. Cargo-carrying flights are scheduled to begin in 2011, and SpaceX says crew capabilities can be ready within three years of NASA's orders.

Today at NASA Dryden Research Center in California, three teams are competing to propel robotic climbers up a vertically tethered cable using high-powered lasers. The competition, called the Power Beaming Challenge, is part of the Space Elevator Games run by the Spaceward Foundation, and is sponsored by NASA's Centennial Challenges program, which will award $2 million in prize money.

According to NASA:

This challenge is a practical demonstration of wireless power transmission. Teams build mechanical devices (climbers) that can propel themselves up a vertical cable. The power supply for the device is not self-contained but remains on the ground. The technical challenge is to transmit the power to the climber and transform it into mechanical motion, efficiently and reliably.

For the competition, a cable will be tethered from a helicopter at a height of one kilometer. Each robotic climbers must weight no more than 50 kilograms and must ascend the cable at a minimum speed of two meters per second.

All three competing teams will use lasers as their power source instead solar power or spotlights, as have been used in the past. Power is limited, so teams must build power dense machines. The best performance to date--competitions were held in 2005, 2006, and 2007--was a robot that travelled at 1.8 meters per second for 100 meters.

The purpose of the competition is to spur development of systems that might ultimately lead to a space elevator--a machine intended to scale a stationary cable from Earth to space. But power beaming also has applications for lunar rovers and space propulsion systems.

This year's competition will run until November 6. You can watch it live here or follow Twitter updates.