Next Stop: Planet Mars Fly me to the moon? That's not far enough. On September 14, 2011, NASA released designs for a superrocket, the Space Launch System (SLS). This time the final destination will be Mars. The SLS is a huge, liquid-fuelled rocket. If it is fully developed, it will be the most powerful rocket ever built. Its lift capability will be much bigger than that of the space shuttle of Saturn 5, the rocket that sent the Apollo missions to the moon. NASA is planning to launch its first unmanned test flight in 2017. It is hoped that the first crew will fly in 2021 and astronauts will make it to a nearby asteroid  in 2025. NASA hopes to send the rocket and astronauts to Mars from the asteroid by the 2030s, according to the Associated Press (AP). NASA used liquid rockets to send Apollo, Gemini and Mercury into space, but later changed to solid rockets boosters  because they were cheaper. Tragically, however, a booster flaw  caused the space shuttle Challenger to crash in 1986. The new project plans to return to liquid fuel. According to AP, the rockets will at first be able to carry 77 to 110 tons of payload . Eventually they will be able to carry 143 tons into space, maybe even as many as 165 tons, NASA officials said. By comparison, the Saturn 5 booster could lift 130 tons and the space shuttle just 27 tons. However, unlike reusable shuttles, these powerful rockets are mostly one use only. New ones have to be built for every launch. This will be very costly. NASA estimates that it will cost about $3 billion (19 billion yuan) per year, or $18 billion until the first test flight in 2017. NASA hopes to make money by allowing private companies to send astronauts to the International Space Station like giant taxi services, so that the program can be "sustainable". "This is perhaps the biggest thing for space exploration in decades," said Senator Bill Nelson, a former astronaut. "The goal is to fly humans safely beyond low-Earth orbit  and deep into outer space where we cannot only survive, but one day also live." According to NASA's schedule, it will take   _   years to fully complete the SLS program.
about 6 about 10 less than 14 more than 19