With several active missions currently exploring Mars, we've taken our attention away from some of the unsolved mysteries of the past. Mars remains the most dangerous place in deep space for robot space probes, and a sense of mystique has built up over some of our problems.

Roughly half of all the missions ever targeted at Mars have failed. Probes sent even deeper into space, to Jupiter, Saturn and beyond, all have a much better performance record. More troubling is the fact that so many of these missions have failed for reasons that have not been conclusively explained.

Jokes of the "Great Galactic Ghoul", a mythical monster that eats spacecraft, have circulated for decades. Of course, the Ghoul is just a piece of NASA-grown humour, but the rise of this tale shows how confounded mission controllers have become over the years.

Mars missions that fail close to home are usually explainable. Some have failed because of faulty launch vehicles. The most recent example was the Russian Mars 96 mission, which climbed into an unstable low Earth orbit, and caused panic around the globe. Fears arose that the spacecraft would crash into Earth and shower radioactive material on the ground.

I have fond memories of witnessing a press conference in Sydney by a prominent American scientist who had worked on Martian meteorites. By cruel coincidence, this happened just as Australian authorities went on alert for a possible crash of Mars 96 on the continent.

Smelling the juicier story, question time at the conference was hijacked by journalists wanting information about the falling spacecraft. Our scientist, who had plenty of interesting news of his own work, had no affiliation to the mission, but managed to keep his cool. It was almost as if the Ghoul was haunting the scientists of Earth, and feeding closer to our own shores!

Some missions fail in deep space, either through navigational errors, damage by solar storms, or less easily explained phenomena. The long cruise to Mars offers plenty of time and opportunity for things to go wrong. Some fail in stages, allowing a diagnosis to be made, but for some missions, an otherwise healthy spacecraft can suddenly fall silent.

The most frustrating experiences come from missions that fail just as they arrive at Mars. In 1993, the NASA Mars Observer spacecraft was about to enter orbit after a relatively smooth cruise across space. It went silent just as it was preparing to fire its main engine. It was later concluded that something went wrong as the engine ignited, possibly destroying the whole spacecraft.

After the success of the Mars Pathfinder lander in 1997 and the Mars Global Surveyor orbiter in 1998, it seemed that the Ghoul would stay away. He returned with a hearty appetite in 1999, claiming the Mars Climate Orbiter, Mars Polar Lander and two Deep Space Two microprobes.

The failure of Mars Climate Orbiter was diagnosed easily, and traced to faulty commands from Earth. Mars Polar Lander apparently failed due to the premature shutdown of its engines, caused by a design error. The failure of the microprobes was never properly explained.

In 2003, the British Beagle 2 lander separated from the European Mars Express orbiter, which remains operational at Mars. Beagle 2 was never heard from again.

Diagnosing the failure of a spacecraft at a vast distance from Earth will often be difficult. You cannot retrieve wreckage or flight recorders. But investigators have one last tool for missions that have reached the surface: Photography from orbit!

The Mars Reconnaissance Orbiter, a spacecraft with a high-resolution camera, has been snapping pictures of Mars since 2006. Its main task is to take a close look at the geology of the planet, but MRO has also been recruited as a space crash investigator.

The camera is so good that it has photographed landers on the surface. We've seen the Viking landers from 1976 and the two Mars Exploration Rovers that landed in 1994. MRO also photographed the Phoenix lander from 2008. It managed an amazing "action shot" of the lander falling on its parachute, then showed it on the ground.

Phoenix was a short-term mission that was not expected to survive the harsh Martian winter. It didn't. NASA was not surprised when radio calls went unanswered as winter passed.

Later, MRO photographed the lander, and revealed that one of its two large solar panels had been snapped off by the pressure of ice deposits that buried the spacecraft! Of course, Phoenix had also suffered damage to other components, but MRO had shown its value for checking the state of spacecraft.

The Mars Reconnaissance Orbiter has also been used to search Mars for the wreckage of other recently failed missions. Some parts of a Mars lander can show up easily on high-resolution photos. Parachutes are large and white.

They've been spotted on other landers. It's also possible to see parts of the aeroshell that covers landers as they enter the atmosphere. A small crater formed by one of the Viking aeroshells has even been found by a Viking orbiter. Landers can also be spotted, as previous examples have shown.

Surprisingly, MRO has found no trace of the recently lost landers! Not even wreckage.

It would be tempting to deepen the myth of the Great Galactic Ghoul with such stories, but this is possibly less mysterious than some would like. The failure to discover large elements of the spacecraft actually helps to solve the mysteries.

It can confirm that parachutes were not deployed properly for some landers. It confirms that the Mars Polar Lander did not suffer a radio or systems failure after touchdown. It suggests that some missions could have been destroyed during atmospheric entry.

In the future, cameras with even more power than that wielded by MRO will probably be sent to Mars. Surveys may be performed to search for the shipwrecks of Mars.

We may know more of the fate of these missions in the future, but the problems they experienced will possibly still remain unresolved. Mysteries will continue. The story of the Ghoul will continue to haunt the spaceflight community. Hopefully, he won't feed again for a long time.

Dr Morris Jones is an Australian space analyst and author. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.

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