Or: How to Get Yourself Stuck Between a Rock and Outer Space
I'm sure a few of you may have realised over the past couple of days the words: Rosetta, Philae, comet, landing, ESA and the sentence: "Holy shit! We just landed a space craft on a comet!" have been thrown around a fair amount.
But just who, what, when, where, why and how is all this about? And why should you care?
Just a heads up - this is a text heavy post and I'm going to be linking to images rather than posting them directly as they're obviously not mine and are protected by copyright against republishing. Sorry!
Who
Let's start nice and gentle. So who the hell just had the audacity to go and fire a hunk of metal about the size of a classic Mini around 6.4 billion (6,400,000,000) km at an object less than 4km across travelling at 135,000 km/h (37,500 m/s!!!) and actually hit the damn thing? Well the ESA, that's who. You may polish your knuckles now ladies and gents of ESA, you've deserved it.
The ESA is the European Space Agency. An international organisation of 20 Member States whose mission is to provide Europe's gateway to space as well as plan and perform Europe's space program. Founded in 1975, ESA aims to provide, promote and advance space research and technology for exclusively peaceful purposes.
Alright, that's pretty nifty. Somehow more than 20 country's statesmen and women are able to cooperate enough for a department dedicated to space research to actually run successfully. And that's the simple part - onto the fun nitty-gritty stuff now!
Where
Next up we'll discuss where, another nice and simple one to ease us into it. Well the ESA is based in Europe, funnily enough, with its headquarters in Paris. However, the actual flight control and cool stuff is happening (on Earth anyway) at the European Space Operations Centre (ESOC) in Darmstadt, Germany.
The other half of this mission is occurring in, you guessed it, space! But, "That's a big place!" I hear you all collectively shouting in earnest at your screen. It's in earnest, as I can't actually hear you through the screen. Geez, we can land a craft on a comet but I can't hear someone shouting at me through the internet, someone has got their priorities mixed!
Within space there are many comets. I'm sure most of you have heard of the very well-known Halley's Comet. Well, we're not aiming for that, you took a wrong turning somewhere if you ended up there. Where this mission is aiming for is a particular comet by the name of 67P/Churyumov-Gerasimenko. Catchy, no? From here on out I think we'll stick with the common shortening of Comet 67P or 67P.
Discovered on the 20th of September, 1969 by the comet's namesake's Soviet astronomers Churyumov and Gerasimenko, (and whoever said that scientists couldn't come up with original naming schemes?), 67P has an irregularly shaped nucleus of two distinct lobes. The larger of these two lobes is ~4.1x3.2x1.3 km and the smaller is ~2.5x2.5x2 km. There are various ways the comet could have formed this way, potentially through a collision of two other comets, through a gravitational affect or through asymmetric sublimation of its surface ice. It is currently orbiting the sun every 6.45 years or so and performing one rotation every 12.4 hours. A comet's orbit will be regularly changing due to the gravitational effect of other objects, particularly when they pass nearby Jupiter, a rather large mass of a planet that also happens to be a major reason why we exist at all! But more about that another time. The current orbital characteristics of 67P are ~8.5x108 km and ~1.86x108 km aphelion and perihelion respectively with its orbit passing between Earth and Jupiter. Aphelion is the comet's furthest distance from the sun and the perihelion is its closest distance to the sun. And the comet is currently about 510 million km away from Earth
The specific landing site is designated 'J' and is located on the 'head' of the comet and was selected on the balance of scientific potential, nearby activity and minimising risk to the lander. Images and more details can be found here.
Phew, OK, there were a fair few number there but they were necessary. And numbers are fun anyway! I know elephants are often used as analogues to help form a relative scale and image in ones head so', to help out, the Comet 67P is approximately the size of 1360 stacked elephants and is ~170 billion stacked elephants away or ~40,000 Earth's away - these scales are mind boggling I know!
When
The scales of numbers involved here are considerably smaller thankfully but still a little complicated. The most timely event occurred yesterday (12 November, 2014 1535 UTC) when the landing craft touched down on the comet. The mission and the whole sequence of events that led up to this point however, started more than 20 years ago.
The mission was approved in November 1993, ignoring all the of the planning, research, development and building that then occurred, the Rosetta mission launched on 2 March, 2004 from Kourou in French Guiana. The space craft then spent the next 10 years travelling to the comet with various key milestones explained below. The total planned mission life time is about 12 years, nominally planned to end in December 2015 once the comet has passed around the sun and is en-route back towards the outer solar system.
Key Milestones
- March 2005 - Rosetta caught up with the Earth for its first gravity assist (more on this later)
- July 2005 - heading to Mars, Rosetta analysed the collusion between Deep Impact's impactor and the comet Tempel-1
- February 2007 - flyby of Mars for its second gravity assist
- November 2007 - flyby of Earth for its third gravity assist in order to reach the asteroid belt
- September 2008 - analysis of asteroid Steins as Rosetta passed within 1700 km
- November 2009 - final gravity assist from Earth
- July 2010 - analysis of asteroid Lutetio as Rosetta passed within 3000 km
- May 2011 - Rosetta goes into hibernation mode to conserve power as it heads into the outer solar system, almost 1 billion km from the Sun, the craft's solar arrays are unable to gather much energy
- January 2014 - Rosetta comes out of hibernation and begins post-hibernation procedures, ground control is able to communicate with the satellite again and begins updating software to upgrade the rather slow information transfer rate of 8 b/s
- May - August 2014 - Rosetta performs 10 orbital correction manoeuvres to align itself for touch down, slow down and begin to orbit the comet
- August 2014 - Rosetta begins to orbit Comet 67P
- September 2014 - Rosetta begins its first observation and analysis of the comet
- November 2014 - the Philae lander craft successfully touches down on the surface of the comet
Future Milestones
- March 2015 - lander likely to become too hot to operate due to proximity to the sun
- August 2015 - comet will pass its perihelion to the sun
- December 2015 - nominal mission end
A nice and simple section and lots of what is mentioned here will be explained in further detail below.
What and How
I figured these two questions are probably simpler to group together so the discussion of 'how' can be with its respective 'what'. There's no way I can fully go into and explain the mission and all its juicy details here but I will do my best to give a decent summary of what's happening out there.
Comets
Let's start with what the heck is a comet, besides that thing that we were aiming to land on? Comets are basically big balls of ice, dust formed in the outer reaches of the Solar System some 4.6 billion years ago during the early stages of development of our Sun and Solar System. This is in opposition to asteroids which are generally made up of rock and metal also from ~4.6 billion years ago during the formation of the solar system. However, there is no clear boundary between comets and asteroids and there are many similarities between them and many comets will ultimately become asteroids once the volatile components are gone. Meteors and meteorites are solid piece of debris that can originate from either comets or asteroids and enter the Earth's atmosphere. Meteors burn up completely in the atmosphere (shooting stars) whilst meteorites survive the journey through the atmosphere and actually impact the Earth's surface. They can range in size from tiny to huge (hope you never see a huge one) with the most recent notable event being the Russian 2013 event. This was the event in February 2013 where a 17m, 10,000 ton asteroid exploded in the atmosphere above Chelyabinsk, Russia at 19 km/s producing a very bright fireball and large explosion damaging over 7200 buildings and causing almost 1500 injuries (mostly from shattered glass).
There are billions of comets and asteroids in the solar system, even though they rarely appear in the news. The most famous of these is Halley's Comet and the asteroid from Armageddon. In general, only comets will form those distinctive 'tails' one imagines. This is from the volatiles sublimating (transition from solid directly to gas) in the heat of the sun. The 'tail' of the comet isn't actually dependent on the direction the comet is travelling but is always pointing directly away from the sun.
Rosetta Mission
As you may have guess by this point, the Rosetta Mission is an ESA operation aiming to perform a detailed analysis of Comet 67P as well as several other smaller objectives. Rosetta is the world's 11th cometary mission (10th launched - more details here) but is achieving many historic firsts. These include:
- First spacecraft to orbit a comet's nucleus
- First spacecraft to fly alongside a comet towards the inner Solar System
- First spacecraft to examine how a frozen comet is transformed by the heat of the sun, in close proximity
- First controlled touchdown on a comet nucleus
- First images obtained from a comet's surface
- First in-situ analysis of a comet
- First European close encounter with objects from the main asteroid belt
- First spacecraft to fly close to Jupiter with its main power source being solar cells
Rosetta Probe
The Rosetta Orbiter is a 2.8 x 2.1 x 2.0m aluminium box comprised of scientific instruments, subsystems, communications dish, two large solar panels, the propulsion system as well as carrying the Philae Lander craft. The probe's objective was not merely to navigate to the comet and drop the lander; there is an array of eleven instruments that will actually be performing the primary, most important, long-term scientific investigations of the comet meaning that even if the lander fails, the mission can still be classified as a success. Images here.
The Rosetta probe is named after the famous Rosetta stone. This 762lg slab of basalt was paramount in unravelling the mysteries of ancient Egypt as the stone contained the hieroglyphics and Demotic script used by the Egyptian civilisation and Ancient Greek which was readily understood by modern historians. By comparing the two sets of writings it was possible to piece together an understanding of the glyphs. As the Rosetta stone provided a gateway to an ancient civilisation, the Rosetta Mission will also allow scientists a look back 4.6 billion years to a time before the planets of the Solar System even existed. As an interesting side note, the Rosetta spacecraft is also carrying a Rosetta Disc. This is a nickel allow disc micro-etched with 13,000 pages of text in 1200 different languages, donated by the Long Now Foundation.
Obviously a very important objective of the probe was getting to the comet in the first place. This was achieved through the use of vertical thrust tube and then 24 small thrusters for trajectory and attitude control. Each of these thrusters delivers a force of ~10 Newtons, or about the same force experience by someone holding a bag of apples (1kg). Over half of the payload of the craft is dedicated to fuel (~1670kg of ~3000kg). As discussed earlier, the Rosetta spacecraft had to perform four gravity assist manoeuvres in order to reach Comet 67P. These 'gravity assists' are slingshot manoeuvres to reach the required exit velocity as no existing rocket has the capability to send such a large craft directly to the comet. The journey is beautifully visualised here.
The probe rendezvoused with the comet in August of this year where it performed a series of manoeuvres to place it in orbit around 67P ~30km above the surface where it could thenunceremoniously dump little Philae after analysing the comet's surface. Image of the asteroid from the probe here. And talk about a killer selfie!
Instrumentation
The Rosetta orbiter craft contains 11 instruments with which to carry out scientific analysis:
Rosetta also performed two asteroid flybys as discussed earlier as well as observing an asteroid fragment in conjunction with the Hubble Space Telescope. Rosetta was able to determine that the first asteroid, 2867 Steins, exhibits a loosely bound rubble-pile structure and was also the first E-type asteroid to be observed close-up. E-type asteroids have a surface comprised of the mineral enstatite (MgSiO3) and exhibit a high albedo (measure of reflection coefficient, or how much light is reflected from the surface) of 0.3 or higher. The second asteroid flyby of 21 Lutetia observed that the asteroid had a surprisingly high density and surface composition thought previously to only exist on larger asteroids. This third flyby was of asteroid fragment P/2010 A2 where Rosetta analysed its dust tail in conjunction with Hubble to confirm that it was indeed an asteroid and not a comet and the dust tail is likely due to particles ejected from an impact with a smaller asteroid.
The Rosetta probe is named after the famous Rosetta stone. This 762lg slab of basalt was paramount in unravelling the mysteries of ancient Egypt as the stone contained the hieroglyphics and Demotic script used by the Egyptian civilisation and Ancient Greek which was readily understood by modern historians. By comparing the two sets of writings it was possible to piece together an understanding of the glyphs. As the Rosetta stone provided a gateway to an ancient civilisation, the Rosetta Mission will also allow scientists a look back 4.6 billion years to a time before the planets of the Solar System even existed. As an interesting side note, the Rosetta spacecraft is also carrying a Rosetta Disc. This is a nickel allow disc micro-etched with 13,000 pages of text in 1200 different languages, donated by the Long Now Foundation.
Obviously a very important objective of the probe was getting to the comet in the first place. This was achieved through the use of vertical thrust tube and then 24 small thrusters for trajectory and attitude control. Each of these thrusters delivers a force of ~10 Newtons, or about the same force experience by someone holding a bag of apples (1kg). Over half of the payload of the craft is dedicated to fuel (~1670kg of ~3000kg). As discussed earlier, the Rosetta spacecraft had to perform four gravity assist manoeuvres in order to reach Comet 67P. These 'gravity assists' are slingshot manoeuvres to reach the required exit velocity as no existing rocket has the capability to send such a large craft directly to the comet. The journey is beautifully visualised here.
The probe rendezvoused with the comet in August of this year where it performed a series of manoeuvres to place it in orbit around 67P ~30km above the surface where it could then
Instrumentation
The Rosetta orbiter craft contains 11 instruments with which to carry out scientific analysis:
- ALICE - Ultraviolet Imaging Spectrometer, used to analyse gases in the comet's tail and atmosphere (the coma) and the surface composition
- CONSERT - Comet Nucleus Sounding Experiment by Radiowave Transmission, used to analyse the comet's interior through analysis of reflected and scattered radio waves
- COSIMA - Cometary Secondary Ion Mass Analyser, used to analyse the dust grains emitted by the comet
- GIADA - Grain Impact Analyser and Dust Accumulator, measures dust grain characteristics (number, mass, momentum, velocity distribution) from the comet nucleus as well as other directions
- MIDAS - Micro-Imaging Dust Analysis System, measures dust environment characteristics (particle population, size, volume, shape)
- MIRO - Microwave Instrument for the Rosetta Orbiter, used to determine the abundance of various gases emitted from the comet, the surface outgassing rate and the surface temperature of the comet
- OSIRIS - Optical, Spectroscopic and Infrared Remote Imaging, used to obtain high-resolution images of the comet, equipped with a wide- and narrow-angle camera
- ROSINA - Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, used to analyse the composition of the comet's atmosphere and ionosphere, velocities of electrified gas particles, their respective reactions and as a secondary device for investigating potential surface outgassing
- RPC - Rosetta Plasma Consortium, comprised of five sensors to measure the physical properties of the comet, examine the structure of the inner coma, monitor activity and analyse the interaction of the comet with solar wind
- RSI - Radio Science Investigation, used to determine the density and internal structure of the comet, define the comet's orbit and study the inner coma using radio signals to measure the mass and gravity of the nucleus.
- VIRTIS - Visible and Infrared Thermal Imaging Spectrometer, maps and analyses the solids and temperature of the comet surface as well as identifying gases and physical conditions of the coma to determine the best landing sites
Rosetta also performed two asteroid flybys as discussed earlier as well as observing an asteroid fragment in conjunction with the Hubble Space Telescope. Rosetta was able to determine that the first asteroid, 2867 Steins, exhibits a loosely bound rubble-pile structure and was also the first E-type asteroid to be observed close-up. E-type asteroids have a surface comprised of the mineral enstatite (MgSiO3) and exhibit a high albedo (measure of reflection coefficient, or how much light is reflected from the surface) of 0.3 or higher. The second asteroid flyby of 21 Lutetia observed that the asteroid had a surprisingly high density and surface composition thought previously to only exist on larger asteroids. This third flyby was of asteroid fragment P/2010 A2 where Rosetta analysed its dust tail in conjunction with Hubble to confirm that it was indeed an asteroid and not a comet and the dust tail is likely due to particles ejected from an impact with a smaller asteroid.
Philae Lander
The Philae Lange is a little (1 x 1 x 0.8m) 100kg robot designed to perform the first ever controlled touchdown on a comet nucleus. The structures consists of carbon fibre baseplate, instrument platform and a polygonal sandwich construction with a solar panel hood and three legs to provide a gentle touchdown which can rotate, lift and tilt to return the lander to an upright position. Its primary objectives are to focus on the "elemental, isotopic, molecular and mineralogical composition of the cometary material, the characterisation of the physical properties of the surface and subsurface material, the large-scale structure and the magnetic and plasma environment of the nucleus." source These samples will differ from the orbiter's and previous gathered dust samples in that they will be in-situ and so will still contain its volatile components and their original physical characteristics unaffected by heat or impact. Images of the lander can be seen here.
The lander is named after the Philae Obelisk, one of two obelisks found at Philae in Egypt which was used in conjunction with the Rosetta stone to decipher Egyptian hieroglyphics as it contained bilingual inscriptions in hieroglyphics and Ancient Greek.
Philae deployed from Rosetta on the 12 November this year from its stable orbit (images of departure here). This was a 7 hour journey along a ballistic trajectory that would result in an impact velocity of ~1m/s (image from 40m before touchdown here). As discussed previously, this is the first ever controlled touchdown on a comet as opposed to NASA's Deep Impact probe that effective crashed itself into another comet (on purpose) in 2005 to perform its analysis. So Philae's design concentrated heavily on safely and securely getting this craft onto the comet surface, a feat not to be underestimated as the escape velocity (the velocity needed to break free of the gravity of an object) of the comet is only ~0.5m/s (1.8km/h - you could literally jump off this thing).
First up are the lander's three legs, allowing a landing on a surface up to a 30º angle. These were designed to absorb and dampen the initial impact upon the surface. The three legs each have large pads to distribute its weight in the event of landing on a soft surface and the instrumentation can still operate in the scenario that the surface is so soft the lander doesn't stop until it sinks to the belly of the bulky body. On the bottom of each of these pads is an ice screw designed to be driven into the surface of the comet upon the initial impact. Next up are twin harpoons that would fire into the surface upon touchdown at 70m/s to anchor the craft to the surface. And finally, a thruster on top of the craft would fire to reduce the recoil from both the initial impact and the harpoon firing.
Unfortunately, this landing all went a bit wrong. The harpoons failed to fire and the top-thruster was not functioning. The harpoons were meant to be fired using 0.3g of nitrocellulose which was unfortunately shown to be unreliable in a vacuum environment in 2013, an official reason for the top thruster not firing has not been released as of yet, but unfortunately things do go wrong with pioneering experiments, not to mention 10 years flying through space. So Philae ended up bouncing. Three times! The first ricochet occurred immediately after impact and sent the lander upwards of 800m off the surface. With a bit of luck added into the mix of the excellent design of the lander, the weak gravity of the comet pulled the lander back down... after two hours. Two more gentle bounces occurred after this and the lander ended up over 1km away from its planned landing site. Not only this, but it only has two of its three feet on the ground, is completely un-anchored and is sitting in the shade of a rather large cliff. But it did successfully touch down on the surface!!! First photo from final touchdown here. And image of the target site, rebound sites and final site here (crosshairs - target site, blue lines - bounce sites, red square - final landing. The second image clearly shows the rather shady area where Philae settled.
This shade is a rather large issue as the lander needs to be powered by its solar panels and its battery can only power the lander for 60 hours. This has left the ESA scientists with two options: the first is to let Philae go back into hibernation mode and hope that the rotation of the comet brings it around to a better angle of the sun as it's currently only receiving ~1.5 hours of sunlight rather than the expected 6-7 hours, but this is by no means definitely going to occur. The second option is to try and reposition the lander by using some of the on-board instrumentation to jolt and jump the craft. This is a rather tricky problem having to take into account the lander's angle, its position, the spin of the comet and the force able to be generated from the equipment.
And because scientists don't tend to shy away from challenges (like you know landing a space craft on a comet) and because they're generally just bad-asses, they've elected to try the second option. With only ~20 hours of battery life left, ESA have activated the lander's drill. This could result in the lander being pushed away from the surface as the drill presses down, or if the drill manages to proceed into the surface at all, the resultant torque force could cartwheel the lander into a new position. Either of these could result in the lander being moved into a better position or possibly toppling the craft and ending the mission for Philae right then. If the lander is still alive after the drilling there are a few more radical actions that can be taken. There is a hammer on-board used to measure the hardness of the surface that could be used, the legs could be used to try and hop the lander into sunlight (though this will be very tricky with only two feet on the surface) or the failed harpoons or thruster systems can be attempted to be re-activated. It's going to be an exciting few hours regardless of what happens.
If all of this manages to get the lander into a better position the actual operating time of the lander is anything but certain. The survival of Philae is dependent on many factors, including its initial landing, power supply, temperature and surface activity. Dust may end up covering the solar panels more quickly than anticipated, especially with this bumpy start, and prevent the batteries from charging. But regardless of this, the operational lifetime of Philae will probably not last beyond March 2015 when the proximity to the sun will render the lander too hot to run.
Instrumentation
Philae carries 10 instruments totalling ~21kg, some of which are beneath its hood of solar cells.
Onto slightly more practical reasons. At a cost of 1.4 billion Euros, the Rosetta mission seems pretty damn expensive. But thanks to physicist Andrew Steele who broke down the costs into a nice comparison on his website Scienceogram, we can see that we've actually got one hell of a bargain price on this mission! For the same price as 4 Airbus A380 we've sent a craft rather a lot farther than they're capable of! And over the course of the mission (1996-2015), the total cost per person in Europe is ~€3.50, or €0.20 per person per year. So really not a bad way to spend a few pennies of your tax money.
And whilst it is hard to put a hard value on what we get back out of the mission, I think it's glaringly obviously worth it. To start with there are all the jobs that have been created through the research, development, building and operation as well as all the support jobs that go along with these positions. Another immediate pay-off is the engineering spin-offs and know how. The contribution and advancement of human knowledge has huge relevance to everyday life both practically and philosophically. Directly from the Rosetta project is the advancement in solar technology made possible through the development of the space craft. Since ESA did not possess the nuclear power technology used in other spacecraft, they went the solar route. More broadly speaking, if particle physicists hadn't had a need to share data effectively, there would be no internet!
Next up let's look at the inspirational factor. Technology advancement in general and space travel in particular has always been a huge motivator for dreamers and thinkers to come up with ideas. This event could easily be the motivation for a whole swathe of new young minds to enter into the scientific world, further advancing our knowledge and leading to who knows what?
Now let's look politically. This is an example of global cooperation and success in an ever more fractious time. Scientists and engineers from more than 20 countries have been working together since 1996 to make this possible! Not just within Europe, but ESA has worked with NASA on this operation as well.
Now this next one has nothing to do with science. It's beautiful! Look at the images in the article above, check out all the images here, hell there is even a five minute AUDIO recording from the Comet 67P!!! Check it out here. I'm sure you've heard that in space, no one can hear you scream. But we've managed to record a comet! And even loads of other sounds from outer space, check them out here. Whilst the sounds from the comet wouldn't have been audible to normal human hearing, it's amazing that we can even record this data let alone convert it into something we can hear and feel. The sounds are said to come from oscillations in the magnetic field around the comet and occur in the 40-50 millihertz range.
Another big reason. At some point in the future (I imagine sooner than you may think), we WILL be mining minerals and water from asteroids and comets. That's a whole story in itself but if you're interested then head here. Everyone needs the products of mining to survive. If it can't be grown, it has to be mined! Everything around you is built from the products of mining and need more resources every day as the human population expands and the technology advances. Resources on Earth are getting harder to find and mine and pretty soon, asteroid mining is actually going to be the more cost effective method of obtaining raw materials.
I'm sure a plethora of other reasons could be discussed, but I will finish with this one: the very survival of the human race depends on this mission and others like it. Many notable scientists have stated as such including Stephen Hawking and Carl Sagan. Whilst we will colonise the moon and Mars in the not too distant future, nowhere will be found as hospitable as Earth without leaving the star system. Earth won't last forever. Be it global warming, nuclear war, genetically resistant disease, an asteroid impact or the eventual heat death of the sun in ~5 billion years, if the human species is to survive in the long term we have the responsibility to venture out to other worlds. And the data, research and technology achieved through these programs gets us a step closer to achieving this every time.
Now that last paragraph may seem a bit doomsday and negative and even scary. But just think of how exciting the prospect is of exploring the rest of the universe! I hope the only reason we spread out into the universe is for peaceful reasons, I have slightly more pragmatic views, but regardless it is a necessity as the population continues to expand and I can't wait to see the developments that will happen in my lifetime.
The lander is named after the Philae Obelisk, one of two obelisks found at Philae in Egypt which was used in conjunction with the Rosetta stone to decipher Egyptian hieroglyphics as it contained bilingual inscriptions in hieroglyphics and Ancient Greek.
Philae deployed from Rosetta on the 12 November this year from its stable orbit (images of departure here). This was a 7 hour journey along a ballistic trajectory that would result in an impact velocity of ~1m/s (image from 40m before touchdown here). As discussed previously, this is the first ever controlled touchdown on a comet as opposed to NASA's Deep Impact probe that effective crashed itself into another comet (on purpose) in 2005 to perform its analysis. So Philae's design concentrated heavily on safely and securely getting this craft onto the comet surface, a feat not to be underestimated as the escape velocity (the velocity needed to break free of the gravity of an object) of the comet is only ~0.5m/s (1.8km/h - you could literally jump off this thing).
First up are the lander's three legs, allowing a landing on a surface up to a 30º angle. These were designed to absorb and dampen the initial impact upon the surface. The three legs each have large pads to distribute its weight in the event of landing on a soft surface and the instrumentation can still operate in the scenario that the surface is so soft the lander doesn't stop until it sinks to the belly of the bulky body. On the bottom of each of these pads is an ice screw designed to be driven into the surface of the comet upon the initial impact. Next up are twin harpoons that would fire into the surface upon touchdown at 70m/s to anchor the craft to the surface. And finally, a thruster on top of the craft would fire to reduce the recoil from both the initial impact and the harpoon firing.
Unfortunately, this landing all went a bit wrong. The harpoons failed to fire and the top-thruster was not functioning. The harpoons were meant to be fired using 0.3g of nitrocellulose which was unfortunately shown to be unreliable in a vacuum environment in 2013, an official reason for the top thruster not firing has not been released as of yet, but unfortunately things do go wrong with pioneering experiments, not to mention 10 years flying through space. So Philae ended up bouncing. Three times! The first ricochet occurred immediately after impact and sent the lander upwards of 800m off the surface. With a bit of luck added into the mix of the excellent design of the lander, the weak gravity of the comet pulled the lander back down... after two hours. Two more gentle bounces occurred after this and the lander ended up over 1km away from its planned landing site. Not only this, but it only has two of its three feet on the ground, is completely un-anchored and is sitting in the shade of a rather large cliff. But it did successfully touch down on the surface!!! First photo from final touchdown here. And image of the target site, rebound sites and final site here (crosshairs - target site, blue lines - bounce sites, red square - final landing. The second image clearly shows the rather shady area where Philae settled.
This shade is a rather large issue as the lander needs to be powered by its solar panels and its battery can only power the lander for 60 hours. This has left the ESA scientists with two options: the first is to let Philae go back into hibernation mode and hope that the rotation of the comet brings it around to a better angle of the sun as it's currently only receiving ~1.5 hours of sunlight rather than the expected 6-7 hours, but this is by no means definitely going to occur. The second option is to try and reposition the lander by using some of the on-board instrumentation to jolt and jump the craft. This is a rather tricky problem having to take into account the lander's angle, its position, the spin of the comet and the force able to be generated from the equipment.
And because scientists don't tend to shy away from challenges (like you know landing a space craft on a comet) and because they're generally just bad-asses, they've elected to try the second option. With only ~20 hours of battery life left, ESA have activated the lander's drill. This could result in the lander being pushed away from the surface as the drill presses down, or if the drill manages to proceed into the surface at all, the resultant torque force could cartwheel the lander into a new position. Either of these could result in the lander being moved into a better position or possibly toppling the craft and ending the mission for Philae right then. If the lander is still alive after the drilling there are a few more radical actions that can be taken. There is a hammer on-board used to measure the hardness of the surface that could be used, the legs could be used to try and hop the lander into sunlight (though this will be very tricky with only two feet on the surface) or the failed harpoons or thruster systems can be attempted to be re-activated. It's going to be an exciting few hours regardless of what happens.
If all of this manages to get the lander into a better position the actual operating time of the lander is anything but certain. The survival of Philae is dependent on many factors, including its initial landing, power supply, temperature and surface activity. Dust may end up covering the solar panels more quickly than anticipated, especially with this bumpy start, and prevent the batteries from charging. But regardless of this, the operational lifetime of Philae will probably not last beyond March 2015 when the proximity to the sun will render the lander too hot to run.
Instrumentation
Philae carries 10 instruments totalling ~21kg, some of which are beneath its hood of solar cells.
- APXS - Alpha X-ray Spectrometer, used to detect alpha particles and x-rays to analyse the elemental composition of the surface
- ÇIVA - six cameras used to take panoramic pictures (check it out here) as well as a spectrometer to analyse composition, texture and albedo of collected samples
- CONSERT - Comet Nucleus Sounding Experiment by Radiowave Transmission, using radio waves from the CONSERT instrumentation on the orbiter, probes the internal structure of the comet through a analysing the penetrating waves to the lander's transponder
- COSAC - Cometary Sampling and Composition experiment), one of two evolved gas analysers used to detect and identify complex organic molecules from their elemental and molecular composition
- PTOLEMY - the second evolved gas analyser used to obtain accurate readings of isotopic ratios of light elements
- MUPUS - Multi-Purpose Sensors for Surface and Subsurface Science, measures density, thermal and mechanical properties of the comet surface using sensors on Philae's anchor, probe and exterior
- ROLIS - Rosetta Lander Imaging System, a CCD camera (an alternative type of sensor to the more common CMOS type allowing higher quality images) used to obtain images during descent and areas analysed by other instruments
- ROMAP - Rosetta Lander Magnetometer and Plasma Monitor, used to study the magnetic field of the comet and its interaction with solar wind
- SD2 - Sample and Distribution Device, Philae's drill that will penetrate more than 20cm into the surface, collect and deliver samples to different ovens or instruments. Now also a big hope to being able to relocate the lander to a landing area with more light
- SESAME - Surface Electrical Sounding and Acoustic Monitoring Experiments, comprised of three instruments in itself: CASSE (Cometary Acoustic Sounding Surface Experiment), PP (Permittivity Probe) and DIM (Dust Impact Monitor). These measure the way sound travels through the surface, the surface's electrical properties and analyses the dust falling back to the surface respectively.
Why Why Why?
OK. Deep breath. You've made it this far (I hope) and you're finally going to be rewarded (besides the lovingly assembled and brilliantly written article above you) with why in all hell are we doing this and why indeed you should care!
So Why Are We Doing This?
Why did we send a 3 tonne hunk of metal hurtling nearly 6.5 billion km to land on a piece of dirty ice? Alright, so specifically speaking, how life began on Earth is one the biggest questions in modern science. Previous observations showed that comets contain complex organic molecules (molecules rich in carbon, hydrogen, oxygen and nitrogen - the pre-requisites for life as we know it) as well as nucleic and amino acids - the building blocks of our DNA. Whilst it is unlikely that biological cells evolved on comets, the prevalent theory is that during the formation of Earth, comets bombarded the proto-planet billions of years ago bringing the essential ingredients for not only water, but life. These ancient remnants from the formation of the Solar System will also allow for further understanding and development of models pertaining to the formation of planets, the development of hospitable planets and even potentially life.
If Philae survives it will also test some hypotheses pertaining as to why essential amino acids are predominantly 'left-handed'. This refers to how atoms are arranged around their central carbon core where a 'left-' and a 'right-handed' molecule are mirror images of each other.
Alright, but we've observed comets before why do it again?
Prior to Rosetta, very little is really known about how a comet works and even this mission will only allow us a further step in the understanding. Previous missions may have observed comets at close quarters but Rosetta is much more advanced and ambitious and will not be limited to simple snap-shots from fly-bys. With Rosetta being comprised of an orbiter and a lander it is able to investigate the comet nucleus and coma over a long period of time as well as the development as it transitions towards the sun and goes from inactive to giving off hundreds of kg of debris every second. Operation Stardust was able to capture some black and white imagery and some dust samples from the comets tail - but as mentioned before, these will have already lost their volatile components and the method of capture involves a lot of heat as the captured particle decelerates very quickly and will affects its physical composition. Deep Impact was impact was able to fill in some more gaps when it landed on a comet but its instrument package was nowhere near as comprehensive or advanced as Rosetta's. Basically, Rosetta can investigate the composition and behaviour of a comet in-situ which is a huge development.
Fine. But Why Should I Care!? I'm not a scientist.
OK, let's break it down.
First, it's fucking awesome! It's amazing and cool and interesting and epitomises everything that makes the human race great and unique - curiosity, exploration and innovation.
And whilst it is hard to put a hard value on what we get back out of the mission, I think it's glaringly obviously worth it. To start with there are all the jobs that have been created through the research, development, building and operation as well as all the support jobs that go along with these positions. Another immediate pay-off is the engineering spin-offs and know how. The contribution and advancement of human knowledge has huge relevance to everyday life both practically and philosophically. Directly from the Rosetta project is the advancement in solar technology made possible through the development of the space craft. Since ESA did not possess the nuclear power technology used in other spacecraft, they went the solar route. More broadly speaking, if particle physicists hadn't had a need to share data effectively, there would be no internet!
Next up let's look at the inspirational factor. Technology advancement in general and space travel in particular has always been a huge motivator for dreamers and thinkers to come up with ideas. This event could easily be the motivation for a whole swathe of new young minds to enter into the scientific world, further advancing our knowledge and leading to who knows what?
Now let's look politically. This is an example of global cooperation and success in an ever more fractious time. Scientists and engineers from more than 20 countries have been working together since 1996 to make this possible! Not just within Europe, but ESA has worked with NASA on this operation as well.
Now this next one has nothing to do with science. It's beautiful! Look at the images in the article above, check out all the images here, hell there is even a five minute AUDIO recording from the Comet 67P!!! Check it out here. I'm sure you've heard that in space, no one can hear you scream. But we've managed to record a comet! And even loads of other sounds from outer space, check them out here. Whilst the sounds from the comet wouldn't have been audible to normal human hearing, it's amazing that we can even record this data let alone convert it into something we can hear and feel. The sounds are said to come from oscillations in the magnetic field around the comet and occur in the 40-50 millihertz range.
Another big reason. At some point in the future (I imagine sooner than you may think), we WILL be mining minerals and water from asteroids and comets. That's a whole story in itself but if you're interested then head here. Everyone needs the products of mining to survive. If it can't be grown, it has to be mined! Everything around you is built from the products of mining and need more resources every day as the human population expands and the technology advances. Resources on Earth are getting harder to find and mine and pretty soon, asteroid mining is actually going to be the more cost effective method of obtaining raw materials.
I'm sure a plethora of other reasons could be discussed, but I will finish with this one: the very survival of the human race depends on this mission and others like it. Many notable scientists have stated as such including Stephen Hawking and Carl Sagan. Whilst we will colonise the moon and Mars in the not too distant future, nowhere will be found as hospitable as Earth without leaving the star system. Earth won't last forever. Be it global warming, nuclear war, genetically resistant disease, an asteroid impact or the eventual heat death of the sun in ~5 billion years, if the human species is to survive in the long term we have the responsibility to venture out to other worlds. And the data, research and technology achieved through these programs gets us a step closer to achieving this every time.
Now that last paragraph may seem a bit doomsday and negative and even scary. But just think of how exciting the prospect is of exploring the rest of the universe! I hope the only reason we spread out into the universe is for peaceful reasons, I have slightly more pragmatic views, but regardless it is a necessity as the population continues to expand and I can't wait to see the developments that will happen in my lifetime.
