There’s no ‘Mars curse’ – it's just very hard to land there

Hopes of another successful landing on Mars were dashed last week when the Schiaparelli probe went missing in action during its descent onto the Red Planet.

Pictures released over the weekend from NASA’s Mars Reconnaissance Orbiter appear to show a parachute and a disturbed area of the Martian landscape. Early investigations suggest that the lander’s parachute opened earlier than planned, and caused the lander to plummet to the surface from as high as 4km, possibly exploding on impact.

The good news to be salvaged from this wreckage is that the lander was watched very closely during its descent, both by the Mars Express orbiter and by a radio telescope in India, so engineers should be able to fully understand the chain of events that led to the crash.

The Schiaparelli lander was only one part of the ExoMars project, a joint venture between the European Space Agency (ESA) and Russia to determine whether life has ever existed on Mars. Its landing was partly meant as a dress rehearsal for the ExoMars rover, which will hopefully touch down in 2020.

Although Thursday’s landing went awry, the overall mission was not without success. The Trace Gas Orbiter, which had transported the lander from Earth, entered into Martian orbit successfully before releasing its ill-fated payload. This satellite will play a crucial role in the future ExoMars rover mission, relaying communications between Earth and the Martian surface.

The ‘curse’ strikes again?

This was ESA’s first attempt to land on Mars, and its failure has perhaps predictably prompted fresh references to the supposed “Mars curse”.

Over the decades we’ve been exploring the Red Planet, several missions have gone astray, starting with the very first landing attempt by the Soviet Mars 2 mission in 1971. More recently, NASA’s Climate Orbiter missed its orbit insertion in 1998, and the UK’s plucky Beagle 2 lander failed to send back a signal to Earth on Christmas day 2003.

All in all, 44 missions have been sent to Mars, of which 26 have been full or partial failures, and 18 successful. So if we’re keeping score, our crude Martian success rate is 41%.

But how bad is that really, compared with our other planetary targets? Take our nearest planet, Venus. So far, 40 missions have set off with the prime mission of studying our cloudy neighbour, of which 22 (or 55%) were successes.

Bear in mind too that most failures on both Venus and Mars were during the early era of planetary exploration – and since then our knowledge, experience and computing power have all increased vastly.

Bumpy landings

What if we look specifically at planetary landings, rather than all missions? Does the “curse” stack up there?

Nine spacecraft have successfully landed on Venus (admittedly, they didn’t last long), compared with the seven that have made it onto Mars intact. What’s more, the most recent Venusian probe touched down in 1984, compared with the 2013 arrival of the Curiosity rover on Mars. So Venus seems to have rather fallen out favour, despite it being apparently easier to land there.

What about other worlds? Our track records for Venus and Mars both pale in comparison with Saturn’s moon Titan, which has a 100% success rate – albeit from one mission, ESA’s Huygens lander, which arrived in 2004.

These examples bring us closer to understanding the real “curse” of Mars, which is down to a combination of the two things that make it extra hard to land there: the planet’s very thin and variable atmosphere, and its gravitational pull.

Assuming that any Mars lander survives its voyage across space, the first thing it encounters is Mars’s atmosphere. Although much, much thinner than Earth’s, it is thick enough to require a substantial heat shield to protect the probe from being scorched. As on Earth, the angle of atmospheric entry needs to be perfect – and this is made harder by Mars’s unpredictable weather. A global dust storm, as often rage on Mars for weeks, could lead to a landing being called off before it even touches the atmosphere.

Should your lander get through the atmosphere intact, it will still be falling at a breakneck speed towards the surface. Mars’s gravity, although only one-third of Earth’s, is still a substantial tug towards a rocky surface. Parachutes won’t slow your lander down enough to survive on arrival on the surface, so a different tack is needed. The Mars Exploration Rovers, Spirit and Opportunity, both cushioned their landings with giant inflatable balls – but these are at the mercy of the rocky terrain. Curiosity’s “Sky Crane”, a part of the lander that hovered on rockets long enough to lower the car-sized rover onto the surface, was an audacious piece of engineering. We’ll see if this will work a second time when NASA attempts to repeat the feat with its 2020 rover.

Contrast that with Venus, where the thick atmosphere makes a soft parachute landing an extremely effective option. The real challenge is keeping your spacecraft working in the planet’s hideously hot and corrosive conditions.

On Titan, meanwhile, the weaker gravity and thick atmosphere work together to make landing a much safer bet – once you’ve managed to get your craft safely to such a distant outpost of the solar system.

Given recent big successes like ESA’s Rosetta and NASA’s Curiosity rover, it is all the more galling when things don’t quite come off. There will certainly be some crestfallen people at ESA headquarters this week – from the engineers whose efforts have been dashed, to the researchers left with no data to crunch.

But we (and those who fund the missions) shouldn’t shy away from difficult things. The aim of the ExoMars mission is to find evidence of life, which would revolutionise our understanding of our own place in nature. That prize has to be worth a shot.