Why it's so hard to land on Mars: Bob McDonald

Itlooksmore and more like the Schiaparelli landercrashed on Mars this week, a huge disappointment for the European Space Agency and the Russian space agency, Roscosmos.

But the incident is only the last in a long history of robot missions to Mars, where almost60 per centhave failedfor one reason or another. That's because our neighbouring planet is not easy to land on.

Why Mars is a difficult target

First of all, Mars is very far away, and a spacecraft has to follow a curving path outwardfrom Earth for about 500 million kilometres, depending on the planet's position in its orbit around the sun. That journey takes more than half a year to complete using current technology and is fraught with hazards at every step.

Before even leaving the ground, the first obstacle is the gravity of the Earth, which can only be overcome with speed, alot of it, 40,000 km/hor 11 km/s.That is 33 times faster than the speed of sound, and 11 times faster than a speeding bullet.The only way to reach those velocities is with our most powerful rockets. That's obstacle number 2.

With so much fire in their tails to reach the incredible velocities of space travel, rockets pack quite a wallop. From the moment the big engines light up, vibrations shake the whole machine, as it roars upward, going from zero to beyond the Earth's atmosphere in under two minutes.The acceleration places tremendous stress on your robot spacecraft and all of its scientific instruments. It took a three-stage rocket and four burns to get ExoMars the kick it needed to get out of Earth's orbit on its journey to Mars.

Once away from the protective magnetic field of the Earth, the spacecraft is exposed to radiation from the sun, plus a stream of electrically charged particles called the solar wind, and ultra-high energetic cosmic rays coming from the galaxy. All of these can fry the electronics that control the entire mission.

When the spacecraft reaches Mars, its lander faces the Red Planet for a straight-in approach. It strikes the atmosphere at more than 21,000 km/hlike a meteor, causing the Martian atmosphereto flare up in a fireball reaching 1,500 C.Hiding behind a protective heat shield, the spacecraft experiences G-forces up to 12timesEarth's gravityas it rapidly slows down.

A huge parachute opens while travelling faster than supersonic speed, causing another jolt, followed by swinging under the canopy, then finally, in the case of Schiaparelli, the parachute is let go and nine rocket engines fire to slow it to a walking pace and gently lower it to the surface.

That's a long sequence of events, and the tricky part is that the spacecraft has to do it all on its own. Mars is so far away from Earth that a radio signal, travelling at the speed of light, takes four to 21 minutes to cross the distance, depending on the position of the two planets.So a robot cannot be driven directly like a radio-controlled airplane or drone.

Everything must be programmed into the flight computers beforehand and happen in the right order at exact times. If any mistake is made along the way, the whole mission can be lost.

Learning from missed attempts

Some historical mistakes have been frustratingly simple. In 1999, NASA's Mars Polar Landerwas lost at the last moment due to a sensor that was supposed to detect the bump of the lander's feet hitting the ground and shut down the rocket engine. Unfortunately, while still high above the surface, the landing legs were unfolded, creating a similar bump, so the sensor shut the engine down prematurely and the spacecraft fell the rest of the way and crashed.

A year earlier, the Mars Climate Orbiterdisintegrated in the Martian atmosphere because of a mixup between metric and imperial units used to calculate how long the engine should run.

These were simple, but costly mistakes.

The silver lining in a crash landing is discovering the weak points in the system. The Schiaparelli lander was a test bed for a future rover mission in 2020. If the scientists are able to determine the cause of the crash, which at this point seems to be a premature shutdown of the engines, then they can ensure that problem doesn't happen again.

Getting to Mars is an enormous challenge, one that will face much larger spacecraft carrying people in the future. The first astronauts attempting to land there would have just over a 50/50 chance of making it based on the historical record so far.

Let's hope those odds can be improved by then.