It’s commonly said that roughly half of all missions to Mars have succeeded- while roughly half have failed. But as it turns out, what counts as a successful or failed mission is more complicated than working out a simple percentage.
Why is this? Missions to Mars usually rely on several moving parts. Should one of these parts fail, even though other parts remain intact, whether the mission in sum counts as a success or failure becomes highly subjective.
A good example of this is the British-built Beagle 2 probe. Although it reached the Martian surface intact, it failed to communicate back to Earth as one of its four spacecraft ‘petals’ failed to open.
Surely, this could be considered a failure- as of course, its utility was more limited than expected- but it still reached the planet. Given the difficulty of even making a landing, this alone may be considered a success. After all, many other lander missions have ended up as scrap while trying to land. Could the Beagle 2 probe, therefore, be considered a partial success?
How exactly a Mars mission is defined also impacts success and failure ratios. For example, should missions to the planet’s moons be considered as ‘missions to Mars’ too? And how about missions, like NASA’s ‘Dawn’ and the Rosetta probe by the European Space Agency, that made a flyby of Mars on the way to their final destinations?
It is for this reason- to avoid complications in definition- that many choose to say ‘roughly half’ of Mars missions failed or succeeded. Regardless, out of all possible missions to Mars, so far, NASA has the best record for anything that may count as a ‘success’ in landing missions. Out of a total of 12 landing attempts on the red planet from different space agencies, only eight have been successful- and all of these have been from NASA.
Sources: BBC, Planetary Society