NASA aren’t the only ones who have performed breath-taking missions to other worlds, and
personally, the mission that really holds its own against some of the impressive stuff
NASA has done is the ESA’s Rosetta-Philae mission to comet 67P/Churyumov–Gerasimenko.
This mission allowed us to see a comet up close, a visitor that originally
came from the Kuiper belt, or the distant belt of comets and asteroids found beyond the orbit
of Neptune.
It was also a ground-breaking mission in that it was able to land a probe on the surface,
and directly interact with the materials found on the comet.
Its discoveries were vast and impressive, and gave us an insight to the far reaches
of our solar system, how it formed - and most importantly - did comets once contribute to
life on Earth?
But not everything went according to plan with this mission.
Philae, the lander part of the mission, was expected to perform scientific experiments
on the surface of the comet for 5 months, but it didn’t manage anywhere near that,
so what went wrong?
I’m Alex McColgan, and you’re watching Astrum, and together we will find out everything
that happened with the Rosetta-Philae mission around Churyumov–Gerasimenko.
As always, let’s start right at the beginning.
The Rosetta-Philae mission launched on the 2nd March 2004 onboard ESA’s own Ariane
5 rocket.
It was initially set to be launched on the 12th January 2003 and had a totally different
target, but the Ariane 5 rocket had a major failure a few months previously with another
mission, meaning all launches were grounded until they could determine and fix the problem.
Once in space, the Rosetta spacecraft had an eventful 7-year journey on its way to the
comet.
As is often the case with missions to the outer solar system, it needed some gravity
assists to get that far out.
It had a flyby of Earth in 2005, and then a flyby of Mars in 2007.
This Mars flyby was pretty risky, it would be just 250km above Mars’ surface.
Also, during this flyby, which lasted 15 minutes, it would be completely in Mars’ shadow,
and because the craft is solar powered, the craft had to go into standby mode or its power
levels would have been dangerously low, meaning it relied on batteries that weren’t designed
for this task to keep the craft powered in order to switch itself back on after the flyby.
It also meant nothing could be transmitted back to Earth as this was happening, so mission
controllers simply had to wait and hope the craft was able to turn itself back on.
They called this the “billion-euro gamble”.
But the flyby was a success, and the mission continued on, even managing to capture some
photos of Mars in the process.
The spacecraft flew by Earth a second time again in 2007 at a distance of 5,700km.
Interestingly, during this flyby it was spotted and presumed to be a near-earth asteroid,
and was added to the Catalina Sky Survey as a 20m potential Earth impactor asteroid.
It wasn’t until a few days later that the mistake was found out!
Heading back into the solar system with increased speed from the flyby, its trajectory took
it past 2867 Šteins, a small 5km asteroid in the asteroid belt beyond the orbit of Mars.
Rosetta did one final Earth flyby in 2009, which finally gave it the boost it needed
to reach the comet.
During this last leg of the journey in 2010, it flew by another asteroid called 21 Lutetia,
which is 100km in diameter.
I think seeing both of these asteroids gives an interesting comparison between asteroids
and comets, because as you are about to see, they look remarkably different.
After visiting Lutetia, the spacecraft went into hibernation mode until 2014 until it
started to approach the 4.5 km wide comet Churyumov–Gerasimenko.
After a number of burns with the onboard thrusters, they aligned the trajectory of Rosetta with
the trajectory of the comet.
As Rosetta approached, these fantastic images started to come in, and they surprised everyone.
The comet seemed to actually be a contact binary, full of jagged edges and sharp sides,
cliffs, valleys and crevasses.
It’s a breath-taking sight to behold, and is really a lot more interesting looking than
anyone had previously thought.
But, upon seeing these images, mission planners collectively thought “how are we going to
land on this?!”
You see, the Rosetta-Philae mission has two parts, the orbiter, Rosetta, or the mothership
of this mission, and a much smaller lander probe, Philae.
Mission planners wanted Philae to land of the surface in order to closely examine the
materials making up the comet with a variety of different scientific instruments.
They couldn’t afford to have it fall over on its back upon landing, not providing any
scientific data about this special object.
So, the first thing Rosetta did upon entering orbit was to map the unusual surface of the
comet to find the ideal landing site.
Apart from the non-spherical nature of the comet, another thing to consider was the amount
of outgassing at any particular site.
You see, some parts of comets are very active, with lots of water being ejected into space,
especially as the comet approaches the Sun.
This water outgassing into space is what forms the defining characteristic of a comet, its
unique coma.
Landing on an outgassing site was seen as too dangerous for the health of the Philae
lander, so a less active site was chosen.
So, on the 12th November 2014, Philae was ejected from the Rosetta mothership, and started
its descent towards this section of the comet.
Philae was equipped with two harpoons which were due to shoot into the surface to stop
the probe bouncing on impact, as the escape velocity of the comet is only 1m/s.
To give you some perspective, if you were standing on its surface, you could probably
jump off it altogether, its gravity too weak to pull you back.
There was eager anticipation and nervousness at mission control as Philae descended, as
due to all the many different factors, there was a lot that could go wrong.
Philae eventually touched down on the surface, and as the spacecraft landed, it sent back
a landing signal to mission control to say job well done.
What mission control didn’t realise at that moment though was that their worst-case scenario
had just happened, the onboard harpoons hadn’t fired and although the probe had landed, it
immediately bounced off the comet’s surface.
Commands were sent to it to start performing scientific experiments, but Philae was actually
floating in space at that moment.
There was a serious amount of confusion for a while until they realised it hadn’t properly
come to a rest, and was still in the process of bouncing.
Luckily, the bounce’s velocity was only 38cm/s, meaning that it only rose 1km from
the surface, before eventually coming back down again.
Over the course of 2 hours it bounced 3 times, before eventually coming to a rest.
Sadly though, it was well away from its initial landing site, and came to rest here, in a
region that was often obscured from the Sun, meaning it would quickly run out of power
due to its solar panels not getting much light.
On the other hand, it was extremely fortunate that it landed the right way up so it could
still collect at least some valuable scientific data.
Mission controllers suddenly had a rush on their hands, to collect as much scientific
data as possible before the batteries ran out.
Their estimated five-month mission suddenly shorted to less than 60 hours!
By the 14th November, battery power had run out, and communication was lost.
Amazingly, in June 2015, with increased exposure to the Sun due to the comets closer approach
during its orbit, the little Philae lander was able to turn on again and sent a signal
back to Rosetta.
Although the signal was intermittent, the lander was able to perform one last science
experiment, which was to drill into the surface of the comet, and it sent back its results.
As time went on though, the comet became more active as it got closer to the Sun.
It originally only outgassed 500g of water and dust per second, but at this point it
was outgassing 300kg per second, which meant in order to keep Rosetta safe, it had to orbit
further and further out.
By the 9th July, Philae transmitted its last signal, and then went quiet forever.
It wasn’t until September that mission controllers eventually found the final resting place for
Philae, you can see its leg sticking out just by here.
Knowing where the Philae landed was important, as it would put the scientific data it collected
into perspective.
So, what did Philae discover while on the surface?
Did it put those 60 hours to good use?
And what about Rosetta from orbit?
Was it able to make some ground-breaking discoveries?
Well, you’ll have to wait for the next episode!
And considering I’m saving the discoveries for another video shows that they did uncover
a lot.
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