The signal broke a seven-hour wait of agonising intensity and sparked scenes of jubilation at the European Space
Agency’s mission control in Darmstadt. The team in charge of the
Rosetta mission achieved what at times seemed an impossible task by
landing a robotic spacecraft on a comet for the first time in history.
The moment the tension broke came shortly after 1600 GMT when the
Philae called home. “We are there. We are sitting on the surface. Philae
is talking to us,” said a jubilant Stephan Ulamec, Philae lander
manager at the DLR German space centre. “We are on the comet.”
Andrea Accomazzo, the Rosetta flight operations director, added: “We cannot be happier than we are now.”
But celebrations were tempered by the later discovery that the
probe’s two harpoons had not fired to fasten the craft down in the
ultra-low gravity. Scientists now think the probe may have bounced after
first coming into contact with the surface. Ulamec said: “Maybe today
we didn’t just land once, we landed twice.”
The safe, if precarious, touchdown of the lander gives scientists a
unique chance to ride onboard a comet and study from the surface what
happens as its activity ramps up as it gets closer to the sun. The first
images beamed back from the lander’s descent revealed a dramatic
landscape of pits and precipices, craters and boulders. However, there
have been gaps in its radio link with the orbiting Rosetta mothership. The Philae lander on its way to the comet, photographed by the Rosetta spacecraft. Photograph: AP
The £1bn ($1.58bn) Rosetta mission aims to unlock the mysteries of
comets, made from ancient material that predates the birth of the solar
system. In the data Rosetta and Philae collect, researchers hope to
learn more of how the solar system formed and how comets carried water
and complex organics to the planets, preparing the stage for life on
Earth.
Space agencies have sent probes to comets before, but not like this.
In 1986, Nasa’s Ice mission flew through the tail of Halley’s comet. In
2005, the agency’s Deep Impact spacecraft fired a massive copper block
at comet Temple 1. But none before now has landed.
The feat marks a profound success for the European Space Agency
(ESA), which launched the Rosetta spacecraft more than 10 years ago
from its Kourou spaceport in French Guiana. Since blasting off in March
2004, Rosetta and its lander Philae have travelled more than 6bn
kilometres to catch up with the comet, which orbits the sun at speeds up
to 135,000km/h.
“We are the first to do this, and that will stay forever,” said Jean Jacques Dordain, director general of the ESA.
Matt Taylor, a Rosetta project scientist, who had selected an
extremely colourful shirt for the event, revealed an impressive – and
brave – tattoo of the lander on the comet’s surface.
“Comets are the original source of Earth’s water. That wee lander is
now in position, poised to re-write what we know about ourselves,” tweeted Chris Hadfield, the former Canadian astronaut and commander of the International Space Station.
Early data from the lander revealed that it had had a softer landing
on comet 67P than expected. But an hour after the landing signal came
through, Paolo Ferri, the ESA’s head of mission operations, said that
Philae’s twin harpoons, which are intended to secure it in place, had
not fired, raising fears about the lander’s stability and chances of
clinging on to the comet for long.
Touchdown for the lander played out 510m kilometres from Earth,
between the orbits of Mars and Jupiter, on a comet hurtling through
space at more than 18km/s. At so vast a distance, even radio signals
travelling at the speed of light take nearly half an hour to travel from
Earth to the spacecraft, making real-time control of the landing
impossible. Instead, the entire descent was precalculated, uploaded and
run automatically.
Landing Philae on the comet’s surface was never going to be easy.
When ESA managers got their first closeup of the comet in July, its unusual rubber duck shape
left some fearing that a safe touchdown was impossible. The shape was
not the only problem. The comet’s surface was hostile: hills and
spectacular jutting cliffs gave way to cratered plains strewn with
boulders. If Philae landed on anything other than even ground it could
topple over, leaving it stranded and defunct.
Rosetta spent weeks flying around the comet to create a surface map
from which mission controllers could choose a landing site. They faced a
trade-off: the site had to be fairly flat and clear of boulders, but
with a good view of the whole comet and plenty of sunlight to charge the
lander’s batteries. From a shortlist of five potential landing spots,
scientists and engineers unanimously voted for a 1 sq km region on the
comet’s “head” later named Agilkia. Scientists celebrate at the space centre in Toulouse as they learn that Philae has landed.Photograph: Remy Gabalda/AFP/Getty Images
At the start of the mission, ESA officials had assumed the comet
would be potato shaped and rated their chances of a successful landing
at 75%. After seeing the shape and terrain of their target close up,
those odds fell to around 50%, but climbed again as technical staff
learned more about the landing site.
The confidence did not last long though. On Tuesday night, hours
before Philae had left its mothership, the chances of a safe landing
took another dip. Overnight, a thruster on the lander failed to respond
to commands sent from Earth. Engineers tried for hours to correct the
fault but to no avail. The malfunction threatened to abort the mission,
but at 0235 GMT on Wednesday mission controllers decided to go ahead
with the landing regardless.
The nitrogen thruster, facing upwards from the top of the lander, was
designed to fire for 60 seconds as Philae touched down to prevent it
from bouncing off the comet’s surface where the gravitational pull is
several hundred thousand times weaker than on Earth. Philae’s shot of its mothership shortly after separation.Photograph: ESA/Handout/ESA/Handout/Corbis
For the mission team, the seven-hour descent, during which Philae
fell at walking speed towards the comet’s surface, was a nail-biting
experience. The lander separated from its mothership at 0835 GMT with
confirmation received on Earth at 0903. For the early part of the 20km
descent, Philae was expected to be out of contact with Rosetta. Around
1100 GMT the mothership reacquired a signal from the lander, which duly
unfurled its legs and began to take pictures.
The first image taken just 50 seconds after separation revealed a look
back at the mothership, one of its 14-metre-long solar arrays clearly
visible, as Philae fell silently to its destination.
One hour from touchdown, Philae was on the right course. Ulamec of
the DLR German space centre said everything looked “fantastic”.
Telemetry from the lander showed that it was rotating, but not enough to
cause problems. Scientists in Toulouse look at the first picture transmitted by Philae.Photograph: Remy Gabalda/AFP/Getty Images
From its orbit around the comet, the Rosetta probe will take more
high-resolution images and gather information on the body’s density,
temperature and chemical makeup. It will also capture dust and gas
released in ever more violent jets from the comet’s nucleus as it nears
the sun.
From its vantage point on the surface, the lander can drill down 20cm
and collect samples of subsurface material for on-the-spot testing. On
board Philae are 10 instruments, including one from the Open University
named Ptolemy which will bake pieces of comet material and analyse the
gases given off to build up a picture of the comet’s composition.
Another instrument, the Comet Nucleus Sounding Experiment by Radiowave
Transmission (Consert), will reveal the comet’s internal structure by
passing radio waves through the icy body to Rosetta on the other side.
The Rosetta mission is planned to run until December 2015, but if
enough fuel remains in the spacecraft’s tanks, mission controllers may
extend its life by six months and give the mothership more high-risk
tasks, such as flying through one of the gas and dust jets streaming
from the comet. Philae has initial battery power to last 40 hours but
will then switch to rechargeable ones replenished by sunlight.
The lander could continue working until March next year, when the
electronics will become too warm to work properly. Even when Philae
packs up, it may still cling on to the comet, perhaps for several
6.45-year-long laps around the sun, before enough material erodes from
the comet’s surface for the lander to lose its grip. Celebrations at the European Space Agency’s mission control in Darmstadt.Photograph: Arne Dedert/Arne Dedert/DPA/Corbisend quote from:Philae spacecraft makes historic landing on comet
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