Robin McKieSun, 7 February 2021, 6:15 am
The skies above Mars will witness some startling aeronautical displays in the next few days when three rival space robot probes reach the red planet after journeying for millions of miles across space.
The United Arab Emirates’ probe Hope orbiter will arrive first, on Tuesday, followed by China’s Tianwen-1 spacecraft the next day. Finally, the US rover Perseverance will make its dramatic descent to the surface of Mars on 18 February.
It is a remarkable armada that reveals the growing desire of many nations to develop their own space technology and explore the solar system. Just how well they succeed when they reach their target this week and next remains to be seen, however. Mars is an unforgiving place to visit..
Of the dozens of Mars missions since 1960, about half have crashed or missed the planet altogether, thanks to component failures, rocket engine mishaps or software errors.
“It can be a heartbreaking business,” admitted UK physicist Colin Wilson, of Oxford University. “I have had instruments on two previous Mars missions – Britain’s Beagle lander and Europe’s Schiaparelli probe – and each time I was in the control room, clutching my seat, during their descents. And on both occasions the probes crashed.”
Mars is a difficult destination for several reasons. First, it is millions of miles away, astrobiologist Susanne Schwenzer of the Open University pointed out. “It is not like going to the moon which is only a quarter of million miles distant. That is the equivalent of a putt in a game of golf. By contrast, Mars is incredibly distant. In golf terms it is the equivalent of a full tee-shot and a lot trickier.”
In addition, Mars has an atmosphere but not a thick one. “That means there is enough air to trigger dust storms and winds that sheer and push your lander off course and into danger,” added Wilson. “On the other hand, it is not thick enough to allow you to use parachutes for a probe’s entire descent.”
In the past, US space engineers have relied on fitting airbags to their probes allowing them to bounce to a standstill after being dropped from a parachute. However, Nasa’s new generation of rovers are too complex and heavy for such manoeuvres and Perseverance will instead rely on a rocket platform called a sky crane to lower it to the Martian surface.
This technique was used once before, in 2012, to land the US rover Curiosity. Now Perseverance, a much heavier rover, will follow suit on a journey that has been dubbed Nasa’s “seven minutes of terror”. That is the time the SUV-sized rover, which weighs more than a tonne, will take to reach the surface of Mars after striking the planet’s upper atmosphere at more than 13,000mph.
Atmospheric friction will bring about the first cut in speed. Then a huge parachute will be released automatically and this will cut the probe’s speed to a few hundred miles an hour. Then the sky crane’s rocket engines will fire and the probe will slow down until it hovers about 20 metres above the surface of the red planet.
The crane will lower the rover on cables until it touches the surface, the cables will be cut and the sky crane will fly off to make its own uncontrolled landing a safe distance from Perseverance. Only then will a message be relayed to Nasa engineers to let them know the good news.
By contrast, the UAE’s spaceship Hope, the Arab world’s first interplanetary spacecraft, will have a relatively simple time this week. It is designed merely to orbit Mars, which it will achieve by carrying out a 30-minute burn of its main engine.
If the burn is successful that will slow the spaceship sufficiently for it to be captured by Mars’ gravitational field and enter orbit round it. Hope will then spend the next two years studying Mars to better understand how, over billions of years, it lost a thick atmosphere that was capable of sustaining water vapour on its surface but which was slowly transformed into a cold and arid world.
China’s Tianwen-1 is also scheduled to enter Martian orbit this week. It will study the planet for several months before dropping a lander that will carry a 250kg robot rover on to the planet. If it works, China will become only the second nation in the world to successfully land a robot vehicle on another world, after the US.
“China has already safely landed rovers on the moon, but this will be a far greater achievement and will really show what their space scientists can do today,” said Schwenzer.
Crucially, the three probes form part of a spearhead of missions that in coming years should transform our knowledge of the planet, by returning Martian rock and soil samples to Earth for study. This task will be started by Perseverance, which is scheduled to pinpoint promising geological sites, extract soil samples and leave caches of them at selected locations. Future missions, involving Europe and the US, will then retrieve these samples and return them to Earth.
“When we do that, we will hopefully get answers to the simple question: is there, or was there, life on Mars,” added Schwenzer.
“It is a crucial issue – for if life did evolve on Mars, independently of life on Earth, that means life evolved twice, separately, in the same solar system and is likely to be common in the cosmos.”