Artist illustration of Falcon Heavy. Credit: SpaceX

Last year, Space Exploration Technologies (SpaceX) became the first commercial company to send a spacecraft into low Earth orbit and have it successfully reenter the Earth's atmosphere--a significant step in the commercialization of space transportation.

SpaceX hopes to carry crew and cargo to the International Space Station when the space shuttles retire this year, using it's Falcon 9 rocket and Dragon capsule. Now the company is vying for more: it's building a heavy-lift rocket, a vehicle comparable to the Saturn V moon rocket, that can carry extremely large payloads to space, like a fully loaded Boeing 737 with 136 passengers.

"Falcon Heavy will carry more payload to orbit or escape velocity than any vehicle in history....this opens a new world of capability for both government and commercial space missions," said Elon Musk, the company's founder, CEO and chief rocket designer, at today's press conference in Washington, DC. Musk said the Falcon Heavy rocket first launch is planned for late 2013 or 2014.

A few fun facts about the Falcon Heavy:

• It's first stage will be made up of three nine-engine cores--the same engines, called Merlin, that are used for the Falcon 9 rocket.
  
• It will generate 3.8 million pounds of thrust at liftoff--equal to 15 Boeing 747 airplanes at full power.
  
• It is designed to meet NASA human rating standards, which means designing to more stringent safety requirements.
• It will be the first rocket in history to use propellant cross-feed from the side boosters to the center core, leaving the center core with most of its propellant after the side boosters separate.
• The side booster stages will have a mass ratio above 30, better than any vehicle of any kind in history.
  
• It will carry twice the payload of a Delta IV Heavy but will cost less than a third as much--it's $1,000 per pound to orbit would set a new world record in affordable spaceflight.

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.

An artist's conception of the Liberty rocket. Credit: ATK

A new rocket that would combine parts from NASA's canceled Ares I rocket as well as the Ariane 5, a well-proven European satellite launcher, could provide a low-cost option for taking crew and cargo to the space station.

The rocket proposal was announced this week by ATK, an aerospace and defense company that manufactures the solid rocket motors for NASA's space shuttles, and Astrium, the European company that makes the Ariane 5. They say the rocket, called Liberty, would be ready for flight by 2015.

The Liberty's main advantage is that it is built on existing technology. For its lower stages the rocket will use a version of the solid rocket boosters designed by ATK for the Ares I, the rocket NASA was building to replace the space shuttles. Ares I was part of the Constellation program to return humans to the moon, which was canceled by the Obama administration in favor of a new course relying on the commercial sector for transportation to low-Earth orbit. Liberty would also be much cheaper than the Ares I because it's upper stage it would use the first stage of the Ariane 5, which has been launched successfully 41 consecutive times.

Charlie Precourt, a former shuttle astronaut who is vice president and general manager of ATK Space Launch Systems, in Aviation Week:

"We will provide unmatched payload performance at a fraction of the cost, and we will launch it from the Kennedy Space Center using facilities that have already been built. This approach allows NASA to utilize the investments that have already been made in our nation's ground infrastructure and propulsion systems for the space exploration program."

ATK and Astrium are also building the rocket to carry a larger payload to low-Earth orbit than the Atlas V, which is built by United Launch Alliance. Other commercial companies, including Boeing and Orbital Sciences Corporation, are looking to use low-end versions of the Atlas V to carry the capsules they are building. Liberty could carry any capsule at a cost less than that of the Atlas V, according to ATK.

The companies have entered the rocket into the second round of NASA's Commercial Crew Development program, a $200 million competition. Liberty's first test flight would be in 2013.