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Interplanetary landers which never come back to the Earth Beneath the toxic petrochemical clouds of Saturn’s moon Titan lies one of mankind’s greatest achievements – the tiny Huygens space probe, packed with instruments and electronics but destined never to return to Earth. Other spacecraft are stranded on the dusty plains of the Moon and Mars, while Philae is resting in a gully on comet 67P/Churyumov–Gerasimenko currently speeding at 55,000km/h (34,000mph) towards the Sun. “These space probes are monuments to humanity,” says John Zarnecki, Director of the International Space Science Institute in Bern, Switzerland. Emeritus Professor at the Open University, Zarnecki is probably the only scientist in the world to have worked on three spacecraft now resting on a comet, Mars, and a moon of Saturn. Like other lonely landers – our avatars at the frontiers of knowledge – these probes took years to develop and build, survived perilous journeys around the solar system and some only lasted a matter of minutes before shutting down. It is hard not to feel affection and, maybe even, a sense of guilt that we abandoned them. “You spend years of your life living with these things – you eat, sleep and breathe them,” says Zarnecki. “You do become emotionally attached.” Here is what happened to some of them: “Huygens” On 14 January 2005, the European Space Agency’s (Esa) biscuit-tin–shaped, 2.7mdiameter Huygens probe began its descent through the hydrocarbon smog that envelops Saturn’s moon, Titan. Suspended from the strings of a parachute – a landing system that had somehow survived a seven-year 3.5 billion kilometre (2.2 billion mile) journey – the European spacecraft captured the first-ever views of the alien world. The Huygens spacecraft made the most distant landing of any human-built object Two and a half hours later, Huygens was on the surface – relaying data back via its Cassini mothership, in orbit around Saturn. And 72 minutes after that, the lander was dead – its contact with Earth lost and its battery depleted. “We think Huygens probably transmitted for another 10 minutes after Cassini disappeared over the horizon but there was no-one there to listen to it,” reveals Zarnecki. “Its last bit of data is still winging its way across the galaxy for someone else to pick up.” Ten years on we can only guess at what state the lander is in now. “I do think about it sitting there,” says Zarnecki. “On Titan there’s this mist and rain – an organic goo that settles on the surface – so it’s probably got this covering of goo but is still recognisable.” Zarnecki is optimistic that we will see the little lander again. “We’ll probably go back to Titan with a balloon flying around doing a survey,” he says. “So it’s not beyond the bounds of possibility that during my lifetime I will get images of Huygens.” “Beagle 2” Mars is littered with landers, with two – Nasa’s Opportunity and Curiosity – still trundling across the surface doing their bit for human knowledge. With the exception of the probes that missed the planet completely, such as the Soviet Union’s Mars 7 lander, or crashed into the surface, including the US Mars Polar Lander, most have worked for several days, weeks or months sending back valuable scientific data before succumbing to the harsh conditions. There is, however, one lander that never got to perform: the UK-built Beagle2. Recent images show it reached the Martian surface intact but the panels enclosing its transmitter failed to open. It may even have started to take pictures and record data. “Someone just needs to kick it to open it up,” says Zarnecki, a scientist on the mission. “It would be possible to send a mission to fix it – although I can’t see the space agencies approving that.” Still, with all eyes on Mars, and a future human mission a distinct possibility, it is certainly conceivable that people will one day visit these remnants of Earth technology. And maybe an astronaut will be tempted to give it a kick. Zarnecki says we should be cautious. The Soviet Lunokhods pioneered the use of rovers away from Earth “These are similar to archaeological sites,” he says. “So if we go back and start strip mining Mars or whatever, the Beagle landing site should be given protection to stop Martian settlers plundering landers for spare parts.” “Lunokhod 2” Many people have similar concerns about preserving the historic missions scattered across the surface of the Moon. From the first successful lunar lander, the Soviet Luna 9 of 1966, to the Apollo landers and China’s latest robotic rover, there is no shortage of history littered across the Moon’s surface. Remarkably one of the oldest rovers – the 40-year-oldLunokhod 2 – is still in use. This tubby, eight-wheeled Soviet machine was one of a pair of vehicles that pioneered the successful use of robotic rovers on alien worlds, sending back images of the landscape and data on the composition of lunar soil. However, four months after landing in January 1973, Lunokhod 2 overheated and shut down. In 1993, despite no-one knowing exactly where it was on the lunar surface, the rover was put up for auction. It was bought for $68,500 by video game developer Richard Garriott (who would go on to become a space tourist flying to the International Space Station in 2008). Then, in 2010, Nasa spotted the rover from its Lunar Reconnaissance Orbiter spacecraft. Today, astronomers use Lunokhod 2’s reflective surface to measure the distance between the Earth and Moon. Controversially, Garriott claims that area of the Moon as his. “My lunar rover not only tilled the surface of the Moon, surveyed many kilometres during its travels but it’s still in active use,” he said last year. “My location on the Moon is still in active use and I’m the owner and so I think there’s some legitimacy in claiming that area of the Moon.” “Philae” Another lander that may yet live to fight another day is currently 322 million kilometers away, somewhere on 67P/Churyumov–Gerasimenko. After bouncing across the comet’s surface, Esa’s Philae probe settled in a gully – transmitting 56 hours and 15 minutes of data, including a spectacular selfie, before shutting down. Philae is not expected to survive the extreme cold as the comet heads away from the Sun towards Jupiter As the comet nears the Sun, and sunlight and temperature increase, there is a chance that the lander could awake to continue its research. In fact Philae’s technical manager Koen Geurts recently told the Space Boffins podcast that the odds are looking good. “We have a good chance that Philae will be able to boot,” says Geurts. “Up until mid-August, everyday our chances are increasing.” After that, as the comet’s elliptical orbit takes it away from the Sun and out towards Jupiter, things do not look good for the lander’s long term survival. Although the comet returns to the Sun every seven years, Philae is unlikely to recover. Voyager I, launched in the 1970s, has now left the Solar System and is in interstellar space “Unfortunately,” says Zarnecki, who worked on one of Philae’s instruments, “it’s going to get so cold that it will suffer catastrophic damage.” But when it comes to total isolation, all these missions pale in comparison to the Voyager probes. Voyager 1 is currently 19.5 billion kilometres from Earth. It is unlikely we will ever get it back. Zarnecki suggests it is worth thinking about how are remarkable these spacecraft, and the people who built them. Our solar system: are we alone? The solar system is familiar. It's home. You've probably learned about all the planets in school, memorising their names and their order of distance from the sun. The four planets closest to the sun are rocky, with solid surfaces you can walk or land a spacecraft on. Then you have the four outer planets (excluding Pluto), huge spheres of gas surrounded by rings. In between lies the asteroid belt like a cosmic moat. It's a tidy configuration, and for about a century and a half, it was all we knew about planets. Then, in 1995, everything changed. That's when astronomers discovered the first planet orbiting another star, a Jupiterlike gas giant named 51 Pegasi B. Over the next two decades, astronomers would discover thousands more worlds. According to estimates, as many as hundreds of billions of planets populate the Milky Way galaxy. The solar system, we now know, is far from alone. The multitude of planetary systems seems to be yet another fact of our cosmic inconsequence, in which our corner of the universe is just like any other. But while planetary systems abound, astronomers are finding that in some respects, the solar system stands out. "It's increasingly seeming that the solar system is something of an oddball," says Gregory Laughlin, a planetary scientist at the University of California, Santa Cruz in the US. It's still too soon to know for sure how odd the solar system is (odd like your quirky uncle, or odd like a leprechaun riding a unicorn?), but scientists are already trying to explain why it might be so. If it turns out to be a cosmological anomaly, then so might be Earth - and life. Maybe, we really are special. A space oddity Once you get over the fact that planets are as common as stars, you're faced with their startling diversity. "We kind of always vaguely hoped and expected planets to be common," Laughlin says. "And that's absolutely right - they are common. But they are weirder than our own solar system would lead us to expect." With the help of the Kepler space telescope, which has discovered thousands of planets around other stars, astronomers are finding planets and planetary systems of all shapes and sizes. They've found miniature systems - cute little guys comparable in scale to Jupiter and its four biggest moons. In other systems, planets orbit at large angles compared to the rotational axis of its star. A few systems even orbit two stars at once, getting Star Wars fans excited about a real-life Tatooine planet with two suns. Asteroids have disappeared from the inner Solar System In the solar system, planets are either small and rocky or big and gassy. But now, astronomers have found that most other planets don't fit in either category. Instead, they're in between: smaller than Neptune but bigger than Earth. The smallest of these, sometimes called super-Earths (a somewhat misleading term, since a superEarth isn't necessarily like Earth at all – it’s just a planet that’s a bit larger) might be rocky. But bigger ones, dubbed sub-Neptunes, are puffier and mainly made of gas. What's stranger is that a lot of these planets orbit extremely close to their stars closer than Mercury is to the sun. When astronomers first discovered these tightly orbiting planets in 2009, most were sceptical. "It seemed so crazy that people really didn't believe it," Laughlin says. But then Kepler, which launched in 2009, confirmed that they do indeed exist - and they're everywhere. In fact, half of the stars in the galaxy might host super-Earth-type planets on close orbits. A close up of Jupiter and a few of its moons That, Laughlin says, is one of the biggest differences about the solar system. "We really have nothing interior to Mercury's orbit," he says. "There's zilch. There aren't even any asteroids down there." The other oddity about the solar system is Jupiter itself. Big planets aren't that common, and most of them are on orbits similar to Earth's or Venus's. Only about a couple percent of stars have a Jupiter-sized planet at a distance comparable to Jupiter's orbit. "Having nothing interior to Mercury's orbit and having Jupiter itself - a massive planet on a Jupiter-like orbit - combine to make us unusual," Laughlin says. No one knows why the solar system might be unusual, but Laughlin has an idea. The explanation involves an elaborate scenario in which a nomadic Jupiter swooped in and destroyed infant planets, altering the fate of the solar system and clearing the way for the Earth we know and love. That's weird The bigger question, though, is just how weird is the solar system. "Every indication right now looks like we might be rare," Walsh says. But at the same time, the planetary census is far from complete. "The jury is still out," he says. Astronomers simply haven't detected many planets like those in the solar system yet. "It's more difficult to see the systems like our own by any of the existing planet finding methods," saysJim Kasting, a planetary scientist at Penn State Universityin State College, US. "The fact that we haven't seen many systems like our own doesn't mean they're not common. It just means they're not that easy to see." Even finding other bigger Saturn-sized planets will take time In particular, planets smaller than Earth are still just beyond the reach of current telescopes. Not even TESS will be able to detect Earth-sized planets on Earth-like orbits around sun-like stars. And finding bigger planets like the ones in the outer solar system will require more time. One of the main techniques to detect planets-which Kepler and TESS use, for example - is to look for the slight dimming of starlight when a planet passes in front of it. But because planets with wide orbits take so long to go around their stars (Saturn, for example, takes 29 years), astronomers will have to look for decades before seeing such a transit. But data isn’t lacking when it comes to super-Earths on orbits tighter than Mercury's - or super-Earths in general. "We know those are extremely common," Laughlin says. Astronomers also know that gas giants on Jupiter-like orbits are not common. And the sun, while not rare, is only similar to about 10 percent of the galaxy's stars. So to some degree, at least, the solar system is weird. Of course, "weird" is subjective. Some estimates say that up to a fifth of sun-like stars in the galaxy have planetary systems like ours, which puts the total fraction at a couple percent. A percent or two sounds small, but remember that the galaxy could have hundreds of billions of planetary systems. One percent still leaves tens of billions of other solar systems. Obviously, the Milky Way could consist of hundreds billions of planets "I would be very surprised if the solar system were really strange," says Jack Lissauer, a planetary scientist at NASA Ames Research Center in California, US. "There are so many stars out there. Even if it's only one percent, it's still not really rare." Whether other solar systems give rise to a truly Earth-like planet - a pleasant place for life - is a question with even fewer answers. "There's zero evidence that Earthlike environments are common," Laughlin says. "There's zero evidence that life is common." But for some, the huge numbers raise hopes of an Earth twin. "I think there are planets like that," Lissauer says. "There are planets on which life, if it were able to start, could flourish." A more familiar world… Kasting is also optimistic. "I think our solar system is not unique," he says. "There are likely other planetary systems that are not much different. But, of course, we don't know that - that's why you need to build telescopes and make the observations." And instead of weirdness, you might very well see something familiar.