By the end of the year, space engineers hope to fulfil one of their greatest dreams. They plan to land a privately funded probe on the moon and send a small robot craft trundling over the lunar surface. If they succeed they will open up the exploitation of the moon for mining and ultimately human colonisation – and earn $20m prize money as winners of the Google Lunar XPrize.

Out of the 29 companies that originally entered the competition, only five remain in contention. Each has until the end of 2017, the XPrize deadline, to launch its robot mission.

It is an extraordinarily ambitious task. To date, not a single piece of any competitor’s hardware has flown outside Earth’s atmosphere, while two of the rockets earmarked to send craft to the moon have still to undergo test launches. As a result, many observers now wonder if anyone will win the Google XPrize given its tight timetable.

Naveen Jain, co-founder of Moon Express – one of the five remaining competitors – is bullish, however. “We have tested our lander. We have tried out all our hardware and software and we have raised all the money we need to complete the mission. We are very confident.”

The rules for the Google XPrize are straightforward. The winning robot craft must be the first to land on the moon and must then travel 500 metres over the lunar surface while sending back high-resolution images to Earth. At the same time, at least 90% of the mission must be commercially funded.

The first team to succeed in this undertaking will win $20m, with further bonuses on offer. For example, a spacecraft that not only lands successfully but also survives a lunar night – which lasts for two Earth weeks when temperatures drop to minus 173 degrees – will win an extra prize of several million dollars.

The aim of the exercise is “to incentivise space entrepreneurs to create a new era of affordable access to the moon and beyond,” say the organisers. It was set up 10 years ago, attracting considerable interest from aerospace investors. Most would-be competitors have since dropped out as the technical hurdles of the mission have proved too onerous. The five now left are:

Returning to Earth was the real nail-biting part of the Apollo missions. Competitors won’t have to worry about that. Martin Rees, Astronomer Royal

■ TeamIndus of India. Set up by dotcom entrepreneur Rahul Narayan, this outfit is backed by a several major Indian businesses.

■ Hakuto of Japan, which has developed its own four-wheel rover for trundling over the lunar surface

■ Israel’s SpaceIL. It has received substantial backing from American casino magnate Sheldon Adelson and it wants to create an “Apollo moment” when it lands, thus inspiring the young to pursue careers in science and engineering.

■ Synergy Moon, an international concern with individuals from 15 countries, which will use a Neptune 8 rocket, built and launched by Interorbital Systems, to carry a lander and at least one rover to the moon.

■ Moon Express. Funded by Naveen Jain, the founder of dotcom search giant InfoSpace, it has set its sights on mining the moon for minerals and is the only competitor yet to receive permission from the US Federal Aviation Administration to land a craft on the lunar surface.

Most of the challengers have invested sums up to $60m to develop their technology – though several have to finalise this funding. The benefits of taking part in the competition were nevertheless clear, said the Astronomer Royal, Martin Rees. “Challenge prizes like these are very good ideas. They have played major roles in developing driverless cars and the development of private launch systems like Richard Branson’s Virgin Galactic and they could do the same for roboticised space missions.”

In future, said Rees, humanity would depend more and more on robot craft to assemble telescopes and to build orbiting solar power plants and to operate mines on asteroids. “Such operations are not likely to be in use for many decades, but challenges like these definitely help to get things moving.”

It remains to be seen if any of the missions make it to the moon by the end of the year. All are rudimentary in concept, are largely untested and employ a wide variety of technologies. Moon Express will not use a rover to cross the lunar surface, for example. It will employ the craft’s rocket engines to hop across it. Similarly, its craft will be launched on a Rocket Lab electron launcher, from New Zealand, while Synergy Moon’s rocket will blast off from an ocean launch pad. This wide variety of approaches has left some bemused observers viewing the competition as a lunar Wacky Races.

For his part, Moon Express’s chief executive Bob Richards acknowledged that its lander did look a bit like a flying washing machine. “Actually it looks more like R2-D2. But it will work,” he insisted.

However, one awkward task that faced past lunar efforts – returning to Earth – will not be a problem that will affect these competitors. Yes, they will have to overcome the headaches of launching a spacecraft from Earth and they will have to land it gently on the moon, a world that lacks atmosphere and thus precludes the use of parachutes. However, there is no requirement in the Google XPrize regulations for competitors to bring their craft back to Earth. They will be left on the lunar surface as possible visiting sights for future tourists.

“That does make things a little easier,” said Rees. “Returning to Earth was the real nail-biting part of the Apollo missions. At least competitors won’t have to worry about that.”

As to follow-up missions, competitors are also divided. SpaceIL says that if it wins the XPrize, the money will be invested – not on further space missions but into educational grants. By contrast, Moon Express has pledged that regardless of winning the prize or not it intends to embark on a long-term strategy of mining on the moon. “This effort is just the start,” said Richards. “We see a real future up there.”