As we consider the next spacecraft to land on the Moon, let's not overlook an increasing problem in scientific research. The laser reflectors already on the Moon are getting old, and need to be replaced.

Laser reflectors are collections of prisms or mirrors that bounce light back in exactly the same direction of its arrival. This means that it's easy to fire a laser beam at the reflector and see the beam travel straight back to you.

Laser reflectors were placed on the Moon by the Apollo astronauts and were also carried aboard the Lunokhod robot rovers landed by the Soviet Union in the 1970s. They have no moving parts and need no power source. Thus, these laser reflectors are the only experiments left on the Moon by those missions that still work.

Scientists have been using the laser reflectors regularly since they were placed on the Moon. They have shown that the Moon is steadily moving away from the Earth by a few centimetres each year, as it has been doing for millions of years. In turn, this is causing the Earth's day to grow slightly longer. Physics, lunar exploration, tidal studies and Earth sciences all benefit from the use of these reflectors.

The reflectors are robust devices that work well, but we cannot expect them to serve us forever. Abrasions from micrometeorites are slowly ruining their optical surfaces. Dust can also coat them, obscuring them further. The vicious thermal cycles of the Moon also place stresses on their materials. Scientists using the reflectors have noticed that they seem to be deteriorating, and reflections are no longer as strong as they used to be.

No laser reflector has landed on the Moon in well over 40 years. The most recent spacecraft to land on the Moon, China's Chang'e-3 mission, did not carry one. Neither did the Yutu rover that Chang'e-3 deployed. To be fair, Chang'e-3 was packed with more experiments than most lunar missions, and most lunar landers (including some of the Apollo missions) did not carry laser reflectors. We don't really need a lot of them. However, it is vital that there is at least one reflector in reasonably good condition at any given time.

How can we get another reflector to the Moon? An obvious choice would be to place one on the next mission to land there. That could well be Chang'e-4, China's follow-up lander mission to the December 2013 landing of Chang'e-3. We do not know if China is considering a laser reflector for this mission, but perhaps they should.

Laser reflectors are fairly easy and cheap to manufacture, compared to other scientific payloads. The major issue with deploying them on the Moon is their orientation. They must be turned to face the Earth, for obvious reasons. It's important that they are not obscured by nearby mountains or crater rims. Some latitudes on the Moon are difficult to see from the Earth, and it would clearly be wrong to place one on the far side. A lander that positioned its reflector at an awkward angle would be wasting its payload!

On the Apollo missions, astronauts positioned the reflectors manually. The Lunokhod rovers could turn to place their own reflectors at different angles. Placing a reflector on a future rover is one way to achieve this pointing, but the rover must be parked suitably before it shuts down permanently. Often, it's difficult to know when this will happen, but it could be useful to assume a suitable position before lunar night falls. This will ensure the reflector is ready in case the rover is damaged by the cold, as recently happened to the Yutu rover.

Alternatively, a lander with precision controls could orientate itself in the right way during its descent to the surface, ensuring that its solar panels (as well as the laser reflector) were all pointing the right way.

A more extreme method would involve sophisticated robotics. A rover would carry the laser reflector, transport it to a clear location, then place it on the ground. Right now, this is easier said than done.

China is planning two more robot lunar landers after Chang'e-4. These missions will be sample-return spacecraft that will launch rockets into lunar orbit with lunar rocks aboard. Launching spacecraft from the Moon blasts the nearby terrain with rocket exhaust and dust. Thus, these missions are probably not as suitable for carrying laser reflectors.

Other space agencies (especially NASA) are also proposing robot lunar landers of their own for the near future. Hopefully some lunar mission will carry a laser reflector within a decade. If we want this to happen, we need to start planning now. Nobody really knows how long we can depend on the ageing equipment that was launched more than a generation ago.