China lands a rover on Mars

AT 17:17 GMT on May 14th Tianwen-1, a Chinese mission which had been orbiting Mars since February 10th, made a subtle adjustment to its trajectory—one that put it on course to hit the planet’s surface six hours later. After three hours, however, it broke itself in two. One part readjusted its path so as to skim past the planet and stay in orbit. The other, a sealed shell with a heatshield on the outside and a precious cargo within, plummeted on towards the surface at 17,000km an hour.

It entered the atmosphere about 125km above the ground, blazing across the alien sky like a meteor. Once friction with the air had bled off most of its kinetic energy it deployed a parachute. The shell broke open, revealing a landing platform with four legs, a rocket engine and a six-wheeled rover fastened to its top. The engine ignited. When the platform had just 100 metres left to go it paused briefly, hovering as its sensors looked for obstacles that would impede a safe landing. Then it set itself down in a cloud of red dust on Utopia Planitia, one of the great flat plains of Mars’s northern hemisphere.

Touch down

Entry, descent and landing (EDL) is historically the riskiest part of any mission to the Martian surface. Every engineering system has to work perfectly. And it all has to happen entirely on the basis of onboard data processing and programming, unsupervised by any human being. Mars is currently 320m kilometres from Earth, meaning radio signals between the planets take 18 minutes to travel each way. By the time the engineers, researchers and bigwigs gathered at the Beijing Aerospace Control Centre knew for sure that the spacecraft was entering the atmosphere, the dust had long since settled.

Once news of its arrival reached mission control, Chinese media lost little time in announcing the triumph to a waking nation which had, for the most part, been blissfully unaware of the drama playing out in the heavens. Aware of EDL’s risks, the authorities had given little advance warning of the landing attempt. The details of Tianwen-1’s orbital manoeuvres were worked out by amateurs monitoring Chinese telemetry using an Apollo-era radio dish in Germany.

The announcement stressed not just the landing itself, but the complete success of the mission it capped. By orbiting and landing on a planet China had never previously visited, Tianwen-1 had become the most successful first mission to Mars in history. America did not land on Mars until five years after first orbiting it.

That said, both America’s first orbiter and its subsequent Viking landers made their trips in the 1970s. The Soviet Union managed a landing then, too. But the European Space Agency (ESA) has twice failed at the task, in 2003 and 2016—the second of those attempts a partnership with the Russian space agency, Roscosmos. Getting it right first time definitely ranks as an achievement, even half a century on. It is, moreover, one achievement among many. In January 2019 China became the first country to put a rover on the far side of the Moon. And last month it launched the first part of a new space station. A second part is due up shortly.

China still has some way to go, though. The capabilities of Perseverance, the one-tonne lander which America’s National Aeronautics and Space Administration, NASA, deposited at a precisely chosen location in Jezero crater on February 18th, far outstrip those of the Chinese rover, Zhurong, which is a quarter of the size. And Perseverance has the benefit of established orbital infrastructure in the form of the Mars Relay Network, five satellites (three American, two European) that can send high-bandwidth data back to Earth. One of the reasons given for Zhurong’s failure to send back pictures until May 19th was that the Tianwen-1 orbiter had to refine its orbit yet again in order to pass on messages.

When Zhurong does trundle off its platform and on to the plain, attention will focus on data from its ground-penetrating radar, which is designed to be able to detect ice at depths of up to 100 metres. The distribution of ice is of consuming interest to those who study Mars, defining as it does the limits of the planet’s potential habitability both in its less-arid past and, perhaps, its human-settled future.

The Mars Subsurface Water Ice Mapping project (SWIM), an attempt to synthesise results from many different approaches to the question, suggests that when Viking-2 scraped the surface at its landing site in another part of Utopia Planitia in the 1970s, its robotic arm may have been within centimetres of permafrost. But that was at 48°N. Zhurong’s landing site, at 25°N, is within the Martian tropics, where underground ice is much less likely to persist close to the surface. Unlikelihood, though, is not impossibility—and it would make any icy discovery even more exciting.

Wild rovers

How far Zhurong will be able to go in search of ice is hard to say. It is similar in size and design to Spirit and Opportunity, two rovers America landed in 2004, and like them it has an official life expectancy of 90 sols (a sol is a Martian day, 40 minutes longer than an Earthly one). Spirit ended up lasting six years, Opportunity 14, over which it travelled 45km. If Chinese engineering is of a similar calibre and its operation teams similarly canny, Zhurong may still have quite a journey ahead of it.

It may even last until the next landmark in Mars exploration: the return of samples to Earth. It is a goal NASA has spoken of for decades and now intends to realise. Part of Perserverance’s mission is to assemble a cache of samples to be picked up later by a joint NASA-ESA mission. Some years hence, the plan goes, America will land a package close to that cache. This will contain both a small European rover to retrieve the samples and a rocket capable of getting them into orbit, whence another European spacecraft will scoop them up and bring them back to Earth. It is the most ambitious planetary-science mission currently being planned.

China is reported to be planning a sample-return mission, too, for launch around the end of the decade. It showed some of the capabilities required for this by returning samples from the Moon last year. If it were to content itself with bringing back any old sample that could be reached from a lander with a rocket on board, that mission could conceivably be accomplished at about the same time as the more sophisticated NASA-ESA attempt. That really would be an interesting space race. ■

This article appeared in the Science & technology section of the print edition under the headline “Welcome to Utopia”