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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.