There have been some pretty spectacular space missions since we’ve had the ability to
get off of planet earth but few have attempted the complexity and precision required to catch
up with a comet traveling at up to 135,000 km/h, enter an orbit around it and then land
a probe on its surface at less than walking speed, so what does it take to land on a comet.
Although the Rosetta mission to comet 67P/Churyumov-Gerasimenko was the most ambitious at the time its certainly
wasn’t the first. The first cometary mission dates back to 1978
with the ICE or International Cometary Explorer spacecraft which flew thru the tail and within
7800km of the Giacobini-Zinner comet core in 1985 and thru the tale of Haley’s comet
in 1986. Since then nine missions have passed close
by several comets culminating in the Deep Impact mission of July 2005 when NASA sent
a two-part probe to Comet Tempel 1. The main spacecraft performed the observations whilst
a smaller impactor was crashed into the core of the comet to eject material from under
the surface which was then analyzed by the main craft.
The Rosetta mission would be the first time that a spacecraft had attempted a soft landing
on the nucleus of a comet. This wasn’t a controlled landing like a martian lander with
rockets to slow its approach but more of a slow fall by the Philae lander from the Rosetta
Orbiter towards the Comet from about 20km away over a period of 7 hours.
But how did we get there in the first place and why comet 67P/Churyumov-Gerasimenko.
The Rosetta mission was a follow up to the 1986 mission by the ESA spacecraft Giotto
to Haley’s Comet which got within 600km of its core, from that mission it became obvious
that there was much more to study and that would need more ambitious future missions.
Comets are amongst the oldest objects in the solar system, leftovers from the material
that formed the planets and are thought to have brought water and organic compounds to
the early earth and possibly could have been instrumental in the formation of life. Finding
out what comets are made of is like going back billions of years before the earth formed
and has been of great interest to the scientific community.
In the late 80s and early 90s, ESA and NASA cooperated to build two spacecraft based on
the Mariner Mark II designs, which were themselves were updated versions of the Voyager and Galileo
space probes. The NASA project would be the “Comet Rendezvous
Asteroid Flyby” or CRAF which would visit an asteroid before making and encounter and
flying alongside a comet for three years. The ESA project would be the “Comet Nucleus
Sample Return” mission or CNSR which would land on a comet and return a sample of it
back to earth. But by 1992 NASA had to cancel its CRAF project
due to budget cuts, so ESA was left to carry on with the CNSR but by 1993 it became obvious
that the technical difficulties of a sample return mission were just too great for the
budget. The project was redesigned and left out the
sample return and opted for an in-situ examination by a lander on the comet's surface.
------------------ Originally Rosetta was due launch in 2003
and to rendezvous with the short period comet 46P/Wirtanen in 2011 but due to the failure
of the Ariane 5 rocket in December 2002 and the delay due to establishing the cause of
the failure, comet 46/P was now poorly positioned and a new comet had to be found that would
be suitable to rendezvous with as it came into the inner solar system.
Comet 67/P would be the new target, it too was a short period comet with an orbit of
6.45 years and would be making its closest approach to the Sun in the summer of 2015.
It was larger than comet 46P at approximately 4.3 by 4.1 km and was traveling at around
135,000 km per hour. This speed meant that there was no rocket
available that could catch it directly without a huge and impractical amount of fuel. So
In order to match its speed and orbit, Rosetta would have to use the gravity assist method
of slingshotting around Mars and the Earth to get there.
From the launch in 2004, Rosetta would slingshot around the earth, then mars, then back around
the earth for a second time, then back around the earth for the third time. In the process,
Rosetta would fly close by asteroid 2867 Šteins and asteroid 21 Lutetia. These flybys would
part of the science mission and provide valuable testing of the onboard equipment and cameras.
As a result of doing all these slingshot maneuvers and almost 4 complete revolutions around the
sun, it would take 10 years to get alongside the comet. After the last slingshot, Rosetta
would be on an orbit that would take it far away from the sun before it returned to the
inner solar system and but that also would create a new problem.
Normally a deep space mission like this would have used a nuclear thermoelectric generator
like the ones on the Voyager probes to create a reliable and long term supply of electricity.
This is because the sunlight available out at the distance of Jupiter and beyond is too
little to power a spacecraft. But the nuclear fuel used by RTG’s is Plutonium 238, a non-naturally
occurring element its self is created from Neptunium 237 a by-product of the cold war
nuclear weapons program and something that was only produced in quantity by the US and
Soviet Union. After the fall of the Soviet Union, production
of nuclear weapons dropped dramatically and the facilities in the US that made them shut
down, leaving a dwindling stockpile of Plutonium 238 for space missions. This shortage and
the political issues around releasing this nuclear fuel for use outside of the US and
now Russian space programs made RTG’s unavailable for ESA’s Rosetta mission.
So Rosetta would be the first spacecraft to use solar power past the distance of Jupiter
and where there is just 4% sunlight compared to that on Earth. The solar panels on Rosetta
were an advanced very high-efficiency design and very large in comparison to the spacecraft,
each one being 14m long and with a total combined size of 64sq meters. These would produce a
maximum of 1500w power at the closest point of Rosetta’s orbit to the sun and 400w at
the farthest point. The final leg of the journey would take Rosetta
out on an orbit far from the sun before returning to make its final approach to comet 67P. This
would take over 2-1/2 years and as the craft was in low power mode it was decided from
a budgetary and manpower point of view to place Rosetta into hibernation until it was
on the return journey.
In January 2014 Rosetta was brought out of hibernation but it was now traveling 2800
km/h faster than the comet and had to use much of its fuel reserves to slow down, something
that would tax the mission more because earlier it has suffered a fuel leak and had to work
at a lower pressure than it was designed for. Now all this time no one actually knew exactly
what the comet's nucleus looked like or what the surface was like for landing on but they
have an approximate idea of its dimensions from near earth observations.
As Rosetta started to approach the comet and sent the first close-up images, it showed
that it was far from a smooth potato-shaped object that many had expected but instead
it had a rough jagged duck shaped body with two distinct lobes connected together by a
thinner neck, with areas that looked like they could have from a mountainside escarpment
anywhere on earth In order to find a suitable landing site,
Rosetta used the OSIRIS camera system to map the entire surface of the Comet using a series
of triangular shaped maneuvers over a period of 60 days. Two different methods called stereo-photogrammetry
and stereo-photoclinometry were used to look at the shadows cast by boulders, cliffs and
other surface features from 100’s of images of the comet and turn them into a topographic
map and then a 3D model of the comet. The landing site had to somewhere that was
in the sunlight for a long as possible as the comet rotated in space to power the lander
but also smooth enough to land on with the least risk of the lander flipping over on
to its back. It also had to avoid any outgassing vents that might start to eject material as
the comet got nearer to the Sun. As was mentioned earlier the landing of the
Philae lander was a passive event and not guided. Instead, it had 2 harpoons with grappling
lines that would be fired out to anchor it to the surface and screws in its feet to hold
it down. Another major issue was that the gravity of
the comet would be very weak, it was estimated to be about 1/10,000th of that of the earth.
If Philae had a hard landing it could just bounce off the surface back into space before
it would have a chance to attach itself. A number of landing sites were selected but
eventually, they settled on a “Site J” which they believed would be scientifically
favorable as well as a good place to land. The area was given the name “Agilkia”,
after the island where the temples of the island of Philae were relocated too after
the construction of the Aswan dam. In order for Philae to make a successful landing, the
Rosetta orbiter had to fix its position above the chosen landing point and match the rotational
speed of the comet to within 1mm per second. If the rotational speed of the comet and orbiter
was off by even a small amount, over the 7 hours it would take to descend the 20km to
the surface, Philae would no longer be over the chosen landing site and could end up on
a cliff or in a canyon. Once Philae was on its way to the surface
the orbiter had to turn and face Philae in order to watch it’s decent to the surface.
Although the landers impact speed was just 1m/s, things didn’t quite go to plan. Philae
bounced off the surface at 38cm/s and up to a height of approximately 1km. If the rebound
speed had been more than 44cm/s it would have escaped the comet's gravity and gone into
back to space. Philae did have a cold gas thruster on the
top of the lander that would be used to help reduce any bounce or recoil from the firing
of the harpoon lines on landing. However, it was discovered that there was an issue
with the thruster it wasn’t expected to work during landing. The harpoons that were
meant to fire into the surface also failed to operate so the combination of these failures
greatly contributed to the bounce on landing. When Philae detected it had landed, it shut
down its internal reaction wheel but as it had bounced off the surface the momentum of
the reaction wheel was transferred to the lander and it started to tumble once every
13 seconds. It also appears to have caught one leg on a crater wall which slowed the
tumbling to once every 24 seconds. Philae landed over an hour later and bounced
once more before ending up on rough terrain in the shadow of a cliff at an angle of about
30 degrees but its exact location was unknown. In this position, the solar panels were only
in direct sunlight for about 90 minutes every 12-hour rotation of the comet which didn’t
allow the secondary batteries to charge fully. This meant that Philae had only enough power
for about 2 days to carry out experiments before going into hibernation mode and contact
was be lost with the lander. Because of the lack of direct sunlight, it was not known
if Philae would wake up again as it required at least 5.5W of power and a temperature of
above -45C to do so. Seven months later and after repeated attempts
by Rosetta, on June 13th, 2015, a signal was received from Philae for just 78 seconds but
it enough to show that was still working. Because of the angle which Philae was tilted
over by, its antenna was pointing into comet rather than out into space. Rosetta was also
200km way because of the increased dust from the out-gassing of the comet. This dust could
obscure Rosetta’s star trackers and cause it to lost its orientation if it were too
close. Because Philae was receiving only small amounts
of solar energy to charge it’s batteries, the few communications which did occur in
the following weeks were very short-lived and intermittent.
Even after Rosetta was moved to 155km above the comet, the connection was still sporadic
and data from Philae revealed that one of its two radio receivers was no longer working.
Whether this damage was caused by the bouncing across the surface we don’t know but certainly
didn’t do it any good. The last communications with Philae were on the 9th of July and on
the 25th July it was decided to move Rosetta to a new orbit to over the Southern Hemisphere
of the comet to continue its planned science investigations.
This new position meant that it would no longer be in a position to contact Philae, although
over the next few months Rosetta was back over the northern hemisphere a few times and
listening, no further contact with Philae was made.
Over one year and three months later, on 2nd Sept 2016, Philae was finally found again
after an exhaustive search by the ground teams and rosettas narrow field camera, wedged in
a dark crevice on an area of the comet called Abydos, which explained the lack of solar
energy to power the lander. By now Comet 67/P and Rosetta were heading
back out to the asteroid belt and away from the sun which would reduce the power generated
from the solar panels. Unlike the first hibernation, Rosetta’s new orbit alongside the comet
would take it farther from the sun to over 850 million km at the maximum distance. Here
there was no guarantee that there would be enough power for the onboard heaters to keep
Rosetta warm enough to survive this period in deep space.
So the Rosetta team had to make a decision as to what to do next. Rosetta was a spacecraft
that had endured 12 harsh years in space, with two of those close to a dusty comet.
There was also an upcoming month-long solar conjunction – when the Sun would be between
the Earth and Rosetta and the comet. This would severely limit communications and the
downlinking of science data. So the team came to the conclusion that the
aging spacecraft and payload were reaching the end of their natural lifetime and the
30th Sept 2016 would be the end of the mission. It was decided that Rosetta would perform
a controlled impact in the Ma’at region of the small lobe at a landing site called
”Sais”. During the main mission, Rosetta hadn’t
been able to get as close to the comet as expected due to the dust from the outgassing
so this would be one final chance to get close up images and data from the comet.
The proposed impact area had a number of active gas and dust pits which would not only be
very interesting scientifically, it also allowed the spacecraft to be in the line of sight
with the earth for communications and in sunlight to illuminate the scenes.
Rosetta would be taking images right up to the point of impact, after which its radio
would shut down to render it inert and the mission declared over but the investigation
of the data sent back by will keep the scientists busy for many years to come.
I would just like to thank Monica Pascanu, Maria Bennet and Matt Taylor at the European
Space Agency for their help in making this video and for checking its accuracy.
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