Not until this year did another American spacecraft, Clementine, orbit the Moon. But Clementine was not a true descendant of Apollo: most of the funding for the small, unmanned probe came from the US Defense Department, not from Nasa. The spacecraft had been envisaged as a test bed for new missile tracking and detection systems. Any scientific return was almost incidental.
For many geologists and planetary scientists, Clementine represents a future marked by frugality and funding cuts. All recent efforts to push lunar exploration up the Nasa agenda have foundered on the rock of congressional opposition, motivated by the need to reduce the US federal budget deficit. In 1992, Nasa's New Innovations Office produced a detailed plan, known as First Lunar Outpost, for a permanent lunar base. Under the programme, the long-neglected scientific study of the Moon would begin with two small orbiters, modestly budgeted at dollars 150m each.
It was hoped to fly the first of these, the Lunar Resource Mapper, in 1995, followed by a second, similar spacecraft a year later. Neither of these got past the study phase; all funding was withdrawn. The New Innovations Office closed down a few months ago.
All that remains is a small office in Washington DC, grandly entitled Mission From Planet Earth, which sponsors symposia and lunar studies. According to Steve Brody, manager of technology and space systems within this office, there are probably no more than about 30 dedicated lunaphiles within the huge agency who are still studying ways of returning humans to the Moon in the 21st century.
Mr Brody tries to maintain an optimistic front: 'We're trying to build a consensus in the public arena. If we don't bite off too much at one time, maybe we'll build support.'
Now the only light on the Nasa horizon is a possible follow-on Clementine mission jointly funded with the Defense Department. This would involve use of a new propulsion system developed under military contract to power a small landing vehicle. Hover tests on simulated lunar terrain by a miniature version of this vehicle have recently been held by the USAF Phillips Laboratory at Edwards Air Force Base in California. Once on the Moon, a tiny, spider-like rover would be released for exploration of the local environment.
Steve Brody's other main hope lies with a recent resurgence in international interest led by the European Space Agency (ESA) and Japan. Despite similar funding problems, ESA's space science directorate set up a Lunar Study Steering Group. Its 1992 report recommended a four-stage approach to future exploration: a polar orbiter to conduct high-resolution remote sensing and mapping; surface stations and roving vehicles to look in more detail at selected sites; sample returns from the lunar far side and upland regions not visited by Apollo astronauts; and, finally, a permanently occupied lunar base.
The Japanese have made it clear that the Moon will be their major target during the next century. For some years, major Japanese companies such as the Shimuzu Corporation have been studying ways of colonising the Moon. Four years ago, Japan became only the third nation to send a spacecraft to orbit the Moon. The Institute of Space and Aeronautical Science is now developing a new rocket capable of sending larger probes to the Moon and other bodies in the solar system. Among its first flights will be the 1997 Lunar-A mission to fire three instrumented penetrators into the lunar surface.
However, the country's scientists are planning much further ahead. The Space Activities Commission recently unveiled a long- term plan which included lunar orbiters, surface rovers and return of samples to Earth within 15 years. By around 2025 the commission expects Japan to have completed, preferably with international co-operation, a manned base and an astronomical observatory on the Moon.
Meanwhile, several non-governmental organisations have got in on the act. The US aerospace giant General Dynamics has proposed a dollars 10bn programme to return humans to the Moon by the turn of the century. Under the proposal, a four-legged lunar excursion vehicle (LEV) would make two deliveries of scientific and support hardware to the lunar surface before the crew arrived on the third flight. The crew habitation module could be based on a mini-pressurised module developed by Italy for the Alpha space station.
A fledgling space corporation appears set to make one giant leap towards re-establishing a human presence on the Moon. The two- year-old International Space Enterprises, set up by a group of American Moon enthusiasts, has established a joint venture company, ISELA, with the Russian Lavochkin Association. Their aim is to produce a series of robotic spacecraft that can deliver scientific instruments, rovers and other payloads to the lunar surface.
The first contract should be signed next year for a maiden launch in 1997. Early versions of the lander will be able to deliver a payload of 600kg at a cost of dollars 125,000 per kilogram. A larger spacecraft currently being designed will be able to carry 1,500kg to the Moon.
Among their first customers will be LunaCorp of Arlington, Virginia. According to David Gump, a company director, two small rovers will be sent to the Moon in 1997. The project's dollars 120m cost will be recouped from entertainment sales. Negotiations are under way with a theme park and a TV network to provide the ultimate in 'out-of-this-world' experiences - using virtual reality techniques to enable Earthbound fun-seekers to drive vehicles across the arid plains once explored by Armstrong and Aldrin. A far cry from the giant leap the astronauts envisaged 25 years ago.