Outside of earth Europa is probably the best place for life as we know it to develop. Beneath the radiation soaked exterior, which consists of an ice layer that could be up to 20KM thick, lies a vast ocean that stretches deep into Europa’s interior. This internal ocean, though bereft of any light, could very well harbor the right conditions to support the development of complex life. However if we’re ever going to entertain the idea of exploring the depths of that vast and dark place we’ll first need a lot more data on Europa itself. Last week NASA has greenlit the Europa Clipper mission which will do just that, slated for some time in the 2020 decade.
Exploration of Europa has been relatively sparse, with the most recent mission being the New Horizons probe which imaged Europa on its Jupiter flyby on its path to Pluto. Indeed the majority of missions that have imaged Europa have been flybys with the only long duration mission being the Galileo probe that was in orbit around Jupiter for 8 years which included numerous flybys of Europa. The Europa Clipper mission would be quite similar in nature with the craft conducting multiple flybys rather than staying in orbit. The mission would include the multiple year journey to our jovian brother and no less than 45 flybys of Europa once it arrived.
It might seem odd that an observation mission would opt to do numerous flybys rather than a continuous orbit however there are multiple reasons for this. For starters Jupiter has a powerful radiation belt that stretches some 700,000KM out from the planet, enveloping Europa. This means that any craft that dares enter Jupiter’s orbit its lifetime is usually somewhat limited and should NASA have opted for an orbital mission rather than a flyby one the craft’s expected lifetime wouldn’t be much more than a month or so. Strictly speaking this might not be too much of an issue as you can make a lot of observations in a month however the real challenge comes from getting that data back down to Earth.
Deep space robotic probes are often capable of capturing a lot more information than they’re able to send back in real time, leading to them storing a lot of information locally and transmitting it back over a longer period of time. If the Europa clipper was orbital this would mean it would only have 30 days with which to send back information, not nearly enough for the volumes of data that modern probes can generate. The flybys though give the probe more than enough time to dump all of its data back down to Earth whilst it’s coasting outside of Jupiter’s harsh radiation belts, ensuring that all data gathered is returned safely.
Hopefully the data that this craft brings back will pave the way for a potential mission to the surface sometime in the future. Europa has so much potential for harboring life that we simply must investigate it and the data gleaned from the Europa Clipper mission will provide the basis for a future landing mission. Of course such a mission is likely decades away however I, and many others, believe that a mission to poke beneath the surface of Europa is the best chance we have of finding alien life. Even if we don’t that will provide valuable insight into the conditions for forming life and will help point our future searches.
The last decade has seen NASA change tack quite a few times, mostly under the direction of different presidents who had very different ideas about how the venerable agency should function. Much of it came in the form of a lot of hand wringing about whether or not we should return to the Moon or simply go straight to Mars, with the current strategy to put NASA astronauts on our red sister sometime in the 2030s (although they might be too late if SpaceX has their way). This new direction included sending astronauts to a near-Earth asteroid by 2025 in order to vet some of the technology required to eventually send those astronauts to Mars and NASA has just detailed what that mission will be.
The initial mission was going to attempt to capture an entire asteroid, one around 8m in diameter, using an inflatable cylinder that would envelope the asteroid and then return it to a cis-lunar (between the Earth and the Moon) orbit. Now this wouldn’t have been a massive asteroid, probably on the order of 8m or so, but it still would have been a pretty massive endeavour to bring it back to a closer orbit. However there was another potential option for this mission: instead of retrieving the whole asteroid a probe would instead pluck a small boulder from the surface of a much larger asteroid and then return that back to the cis-lunar orbit. NASA announced today that the second option would be the one they’d pursue going forward with the mission timeframe still slated for sometime in the next decade.
Interestingly the second option is significantly more expensive, to the tune of $100 million, however the technology that will be developed to support it was seen as being of much more benefit than the other mission. Once a candidate asteroid has been selected the craft will be launched into orbit around it where it will identify and select a boulder for retrieval. It will then land on the boulder, capture it, and then lift it back off into orbit around the asteroid again. The craft will remain there for some time afterwards to see if the idea of a gravity tractor craft could work to divert a potentially dangerous asteroid from colliding with Earth. Then, depending on how successful that was, the craft will either remain there a little longer or begin the journey back towards earth, it’s newly captured asteroid boulder in tow. Then astronauts from Earth will embark on a month long mission to travel to the asteroid, study it and then potentially bring it (or at least samples) back to Earth.
It’s an ambitious mission but one that will be the proving ground for the vast majority of technologies required to get humans to Mars. Whilst we’ve learnt a lot about long duration spaceflights thanks to the International Space Station there’s a lot more we need to develop in order to support the same duration flights away from the protection of our Earth. Specifically this relates to the radiation shielding requirement (something which still doesn’t have a great solution) but there’s also numerous other questions that will need to be answered before we launch a craft towards Mars. A month to a nearby asteroid fragment might not sound like much but it will be another giant leap forward technology wise.
NASA is stil a far cry from its heydays during the cold war but its starting to rekindle that explorer spirit that drove them to achieve such great things all those years ago. Opting for the more ambitious mission profile means that our understanding will be more greatly increased as a result, hopefully fueling further exploration with a view to us one day becoming a multi-planet species. We’re still a while away from seeing this happen but it’s so good to finally see a light at the end of the tunnel.
Moving things between planets is a costly exercise no matter which way you cut it. Whilst we’ve come up with some rather ingenious ideas for doing things efficiently, like gravity assists and ion thrusters, these things can only take us so far and the trade offs usually come in the form of extended duration. For our robotic probes this is a no brainer as machines are more than happy to while away the time in space whilst the fleshy counterparts do their bits back here on Earth. For sending humans (and larger payloads) however these trade offs are less than ideal, especially if you want to do round trips in a reasonable time frame. Thus we have always been on the quest to find better ways to sling ourselves around the universe and NASA has committed to investigating an idea which has been dormant for decades.
NASA has been charged with the task of getting humans to Mars by sometime in the 2030s, something which shouldn’t sound like an ambitious feat (but it is, thanks to the budget they’ve got to work with). There are several technical hurdles that need to be overcome before this can occur not least of which is developing a launch system which will be able to get them there in a relatively short timespan. Primarily this is a function of the resources required to keep astronauts alive and functioning in space for that length of time without the continual support of launches from home. Current chemical propulsion will get us there in about 6 months which, whilst feasible, still means that any mission to there would take over a year. One kind of propulsion that could cut that time down significantly is Nuclear Thermal which NASA has investigated in the past.
There are numerous types of Nuclear Thermal Propulsion (NTP) however the one that’s showing the most promise, in terms of feasibility and power output, is the Gas Core Reactor. Mostly this comes from the designs high specific impulse which allows it to generate an incredible amount of thrust from a small amount of propellant which would prove invaluable for decreasing mission duration. Such designs were previously explored as part of the NERVA program back in the 1970s however it was cancelled when the supporting mission to Mars was cancelled. However with another Mars mission back on the books NASA has begun investigating the technology again as part of the Nuclear Thermal Rocket Element Environmental Simulator (NTREES) at their Huntsville facility.
NTP systems likely wouldn’t be used for the initial launch instead they’d form part of the later stage to be used once the craft had made it to space. This negates many of the potential negative aspects like radioactive material being dispersed into the atmosphere and would allow for some concessions in the designs to increase efficiency. Several potential craft have been drafted (including the one pictured above) which use this idea to significantly reduce travel times between planets or, in the case of supply missions, dramatically increase their effective payload. Whether any of these will see the light of day is up to the researchers and mission planners at NASA but there are few competing designs that provide as many benefits as the nuclear options do.
It’s good to see NASA pursuing alternative ideas like this as they could one day become the key technology for humanity to spread its presence further into our universe. The decades of chemical based rocketry that we have behind us have been very fruitful but we’re fast approaching the limitations of that technology and we need to be looking further ahead if we want to further our ambitions. With NASA (and others) investigating this technology I’m confident we’ll see it soon.
If there’s any place in our solar system that we’d want to start seriously looking for life it’d be Europa. The dust covered snowball of a moon likely contains a vast subsurface ocean, one that is kept liquid by the giant gravitational forces of its host planet Jupiter. This makes Europa a great candidate for life as we know it as once we find water it’s inevitable that we find life shortly thereafter. The challenge with Europa though is getting to that subsurface ocean to study it as it could be covered in several kilometers of water ice, not something you can simply puncture through. Whilst there are numerous people more qualified than me on this subject, many of them actually working in the aerospace industry, with NASA asking for ideas for a potential mission to Europa I figured I’d throw my 2 cents in.
So the total budget for the potential mission is a cool $1 billion and whilst that sounds like a lot of money projects that I’d consider simpler than my idea (like say Curiosity which was $2.5 billion) but I think there’s potential to build a platform that could fuel further missions. With that in mind this initial mission is likely only to be a scouting mission, one that will give us the most detailed picture of Europa possible so that the follow up mission can choose the perfect site to land on and commence the search for life in its vast underground ocean. My proposal then is to develop a compact nuclear reactor (not a RTG) to power a scouting craft laden with instruments to analyse the gravitic field and surface of Europa. This craft will be able to find the point at which the surface ice is the thinnest and identify potential landing sites for the second generation craft: a cryobot that will punch through to the ocean below.
Putting a nuclear reactor into space might sound like the plan of a crazed sci-fi nerd but there’s actually been dozens of small prototype reactors launched into space with all of them proving to be safe and reliable. The power capabilities of such a reactor are far beyond that of what a small satellite would usually require however attempting to melt through kilometers of ice will require power of that scale. Thus it would make sense to fund research into developing the power supply first and then validating it on the scouting craft. Then, once that mission is successful, the reactor can be scaled to the appropriate dimensions for the cryobot mission and even used in other deep space programs.
Having such a generous amount of power available also opens up the opportunity to using instruments on the scouting craft which would not be feasible, typically. Things like high-power antennas (which could function as a relay for the follow up mission), radar imagers or bigger and better versions of other instruments. My knowledge on the power requirements of various instruments is limited but I know that even the most advanced RTGs, like the one in Curiosity, produce a measly 125W. Being able to draw on several kilowatts, an order of magnitude more power, seems like it would open up many opportunities that just weren’t possible previously.
I’m probably vastly underestimating how much it would cost to develop such technology, especially in today’s nuclear hostile political environment, but if we’re serious about actually digging under Europa’s surface I don’t see what our other options would be. Melting through giant sheets of ice is no small task and one that has requirements that far surpass anything we have currently. Using that $1 billion mission to set ourselves up for future exploration seems like the best bet especially considering how many other applications a safe, small nuclear reactor would have.
Whilst the debate among the space enthusiast community still rages about what the next target for human exploration should be those with the capability seem to have already made a decision: we’re going to Mars. NASA has committed to getting astronauts there some time around 2030 and SpaceX’s founder and CEO, Elon Musk, has long held the dream that he’d be retiring on Mars. There’s also the Mars One which, to my surprise, is still going and garnering attention worldwide even here in my home country. The lack of a return mission to the Moon does raise some questions about the technology that will be used as we don’t have any craft capable of going past low earth orbit, not since the Apollo program ended almost half a century ago.
NASA has been working on a new crew capsule for some time now, dubbed the Orion. Initially this was part of the planned 2020 mission to return to the Moon however the majority of that was scrapped in favour of going directly to Mars. The capsule and the revised launch system were retained however and will form the basis of NASA’s future manned space missions. However if the Moon is no longer the goal for this craft and it’s end goal will be long duration flight there’s a lot of testing that needs to be done before we send one of them to Mars. Interestingly NASA has gone for an incredibly ambitious mission to put the Orion’s long duration flight capabilities to the test: an asteroid capture and analysis mission.
There’s currently two mission profiles being considered, both of them seeming like something straight out of science fiction. The first (and I’ll guess least likely of the two) is a robotic craft will make its way to a large asteroid, break a chunk of it off and then bring it back into orbit around the moon. The second would be a straight up asteroid capture with the craft grabbing an asteroid in its entirety (it would be small, about 7m or so in diameter) and, again, putting it into lunar orbit. Then once the asteroid is in a stable orbit NASA will send crew to it in an Orion capsule to study it, testing out some of the long duration capabilities as well as other rudimentary space activities like EVAs.
Such a mission is actually quite feasible (at least the latter profile) from a technical perspective. Pretty much all the technology required to capture an asteroid of that size is available today and there’s already 6 candidate asteroids identified. The main issue I see with it is time as just getting to the asteroid is planned to take at least 4 years with another 2 to 6 required for it to make the trip back. That means if the mission were to launch today it could potentially take up to 2024 before it returns to us which doesn’t leave a lot of time for NASA to test out the Orion capsule on it, This could be sped up considerably by changing it’s launch profile to include a second stage rocket to boost it rather than relying on the ion thrusters to achieve escape velocity but that would come with additional expense. There’s also the possibility of foregoing the robotic part of this mission completely and just sending humans although that poses just as many challenges as going straight to mars.
I’m glad to see NASA making a return to missions like these, ones that truly push the envelop of humanity’s space capabilities. It’s going to be interesting to see how the mission develops as there’s lots of different variables that need to be sorted out, some that will change the mission dramatically. Still the thought of us being able to capture an asteroid, bring it into lunar orbit and then send humans to study it is just an incredible thing to think about and I truly hope NASA sees this one through to fruition.
As many know my experience with 3D printing has come with mixed results, as the kit I bought with 3 friends required more calibration than I was willing to do and my friend’s Solidoodle proved to be a reliable way to create the objects I needed. I’m still highly interested in the area (I was going to post a review of Microsoft’s 3D Builder but just never found the time to hook it all up) and I strongly believe that the commoditization of manufacturing at the small scale will prove to be revolutionary. One area of particular interest was the idea of a food printer, something that could potentially make a meal out of some base nutritional components.
As it turns out this might be closer than I first imagined (skip to 1 minute in for the good stuff):
NASA stated investigating the idea of 3D printing food a little while ago, investing a small amount of money into research to create a device capable of creating edible foodstuffs on the International Space Station. Primarily this was to fuel a longer term goal to provide food for an interplanetary trip to Mars as its believed that 3D printed food could dramatically reduce waste and improve efficiency with transported materials. Whilst this current demonstration appears to be limited to producing pizza (something which seems a perfect fit for a first run) NASA’s vision is for something far more general and it looks like they’re well on their way to achieving that.
It’s a big step considering that we’ve had printers capable of producing chocolate models for some time, but the leap to other food has proved somewhat elusive. It will likely be quite some time before it gets much more general than your run of the mill pizza however although some of the designs making the rounds are really quite impressive. Time will tell if they’ll ever become mass market devices but I can definitely see themselves finding a home in space stations and high end restaurants looking to create truly unique dishes.
For a country that was barred from ever working with the leader in space technology the progress China has made in the last decade has been incredibly impressive. They’ve quickly gone from humble beginnings in 2003 where their first taikonaut made it into orbit to a fully fledged space station in 2013, showing that they have the technical expertise required to consistently attempt envelope pushing activities. Of course whilst the most interesting aspect of any space program is the manned activities (who doesn’t love seeing people in space!) there’s always the quiet sibling in the robotics departments, attempting missions that few humans will be able to attempt. I must admit that until today I was also ignorant of China’s robotic efforts in space but suffice to say they’re just as impressive as their human based accomplishments.
China’s Chang’e program (the name of the Chinese Goddess of the Moon) is a series of lunar spacecraft tasked with creating highly detailed maps and models of the Moon’s surface with the intent that that data will be used for future manned missions. Chang’e 1 was launched back in 2007 and remained in lunar orbit for 2 years. It created the most accurate and detailed sufrace map of the moon to date and, once it was done, plummeted into the surface it just mapped to send up a spray of regolith that could be studied from here on Earth. It’s successor, Chang’e 2, was launched in 2010 and had similar capability (albeit with higher resolution instruments and a lower orbit) but instead of being plunged into the moon at the end of its mission it was instead sent out to do a flyby of asteroid 4179 Toutatis. Its current trajectory will eventually see it hit interstellar space however its likely it’ll run out of fuel long before that happens and the purpose of the extend mission is to validate China’s Deep Space Tracking network.
Chang’e 3, launched just yesterday, will be the first craft China has ever launched that will land on the Moon’s surface. For a first attempt it’s a fairly ambitious little project consisting of both a lander and a rover, whereas similar missions usually go for a lander first prior to attempting a rover. The lander is an interesting piece of equipment as it contains a RTG as a power source as well as an ultra-violet telescope, making it the first luna based observatory. Whilst it won’t be anything like the Hubble or similar space telescopes it will still be able to do some solid science thanks to its location and it makes the lander’s useful life much longer than it typically would be.
The rover is just as interesting, being roughly equivalent to the Mars Exploration Rovers (Spirit and Opportunity) in terms of size and weight. It can provide real time video back to Earth and has sample analysis tools on board. The most important instrument it carriers however is a radar on its base allowing it to probe the lunar surface in a level of detail that hasn’t been done before, giving us insights into the make up of the regolith and the crust beneath it. It will be interesting to see what its longevity will be like as its power source is its solar panels (unlike its parent lander) and the lack of atmosphere should mean they’ll remain clean for the forseeable future.
As of right now there’s another 2 more missions in the Chang’e line both of which have similar capabilities with the exception of Chang’e 5 which will be a lunar sample return mission. After that it’s expected that China will start to eye off manned lunar missions, starting with the traditional flag planting operations and then quickly escalating to a fully fledged moon base not long after. It’s quite possible that they’ll accomplish that within the next 2 decades as well as their past accomplishments show how quickly they can churn out envelope pushing missions, something that other space fairing nations have been lacking as of late.
Whilst it might not be of the same heights we saw during the cold war there’s definitely another space race starting to heat up, although this time it’s between the private space industry and China. Whilst it’s likely that China will win the race to the Moon and possibly Mars I can’t help but feel that the private industry isn’t too far behind. Heck, combine Bigelow Aerospace and SpaceX and you’ve already got the majority of the Chinese manned program right there! Still this does not detract from the accomplishments the Chinese have made and I only hope that eventually the USA changes its stance on co-operating with them.
Maybe it’s the combination of mission secrecy and close resemblance to the now retired shuttle fleet but the X-37B seems to get more press than any other space craft currently flying in orbit. When I first saw the diminutive shuttle cousin back in April last year I figured it was just a unique experiment that the Department of Defence was carrying out and the rumours about it’s satellite capturing capabilities were greatly exaggerated. Indeed towards the end of the mission I investigated the idea that it was already performing such a task but based on the current trajectories of other satellites it didn’t seem like that was the case. The X-37B blasted off once again at the start of this year again shrouded in mystery as to what its actual mission was and it’s been up there ever since.
That last fact is interesting as the X-37B’s stated capabilities put it at being on-orbit for a maximum of 270 days. The deadline for its return to earth would have then been around November 30th, a date that has well past now. The United States Air Force has stated that its mission has been extended and it should be on orbit for a while to come. This is interesting because it tells us that the X-37B is a lot more capable than they’ve state it is. Whilst this could just be good old fashioned American over-engineering it does lead some credence to the theories that there’s a whole bunch of capabilities hidden within the X-37B that aren’t officially there.
What’s been really interesting however are the discussions surrounding a potential manned variant of the X-37B. As it stands the X-37B is quite a small craft, measuring a mere 10 meters long and a payload bay that’s only got a few cubic meters of storage space. Overall its very similar in size to the Soyuz craft so there’s definitely some potential for it to be converted. Rumour has it that the X-37B would be elongated significantly though, bringing its total length up to 14 metres with enough space to sit 7 astronauts. Granted it wouldn’t be as roomy as the shuttle was (nor could it deliver non-crew payloads at the same time) but it would be a quick path to restoring the USA’s manned flight capability. That would hinge on the man rating Atlas V launch system which is currently under investigation.
It’s not just space nuts that are getting all aflutter about the X-37B either. China has expressed concerns that the X-37B could be used as a orbital weapons delivery system. The secrecy surrounding the actual mission profiles that the X-37B has been flying is probably what has prompted these concerns and it being under the sole purview of the Air Force doesn’t help matters. In all honesty I doubt the X-37B would be used as a weapons platform since it’s more of a generalist/reconnaissance craft than a weapons platform. If there’s someone you want to worry about launching weapons into orbit it would be the Russians as they are the only (confirmed) nation to have launched armed craft. A dedicated weapons platform would also look nothing like the X-37B, especially if it was going to be designed for on-orbit combat (who needs wings in space?).
The next couple months will give us some more insight as to the true purpose of the X-37B. It’s quite likely that these first couple flights have just been complete shake downs of all the systems that make up the X-37B with the first flight being orbital manoeuvring verification and the current flight being an endurance test. Should it stay up there for a significant amount of time it’s more likely that it’s some form of advanced reconnaissance craft rather than something crazy like a satellite capturer or orbital weapons platform. The prospect of a manned variant is quite exciting and I’ll be waiting anxiously to see if the USA pursues that as an option.
It was just over 2 months ago when a Russian Progress craft crashed shortly after lift off. It was a devastating blow for the International Space Station project as the Progress spacecraft and the Proton rocket it rides to space on are the lifeline that keeps the ISS going. The failure of a Progress craft also called into question the man-rated Soyuz craft as they’re quite similar craft and should they be unable to launch that would effectively spell the end of human activities on the ISS. Investigations into the disaster continued and they finally nailed down the cause of the failure.
The cause turned out to be contamination of the fuel lines in the Progress craft. This in turn caused a low fuel supply to the gas generator which the on board computer interpreted as a fault and shut down the engines completely. This left the craft on a sub-orbital trajectory eventually leading it to crash in the Atlai region in Russia. The investigation revealed that this particular fault was of no immediate threat to either the Progress or Soyuz craft however so Roscosmos saw no need to delay any of the following flights further than they already had.
Yesterday then saw the first launch of Progress since the incident back in August, and thankfully it was completely successful:
An unmanned Russian cargo ship launched toward the International Space Station Sunday (Oct. 30) packed with nearly three tons of supplies for the orbiting lab’s crew in what marked the first delivery run to the station since an August rocket crash.
The cargo ship, called Progress 45, lifted off atop a Soyuz rocket at 6:11 a.m. EDT (1011 GMT) from a launch pad at the central Asian spaceport of Baikonur Cosmodrome. It will arrive at the space station early Wednesday.
The successful launch of the Progress craft means that missions using the manned Soyuz craft can continue on without fear of them failing in the same way. This is crucial to the on going ISS mission as prior to this launch the future of the manned crews was in question and could have resulted in the ISS being unmanned for the first time in a decade. The reasoning behind this is simple, if the Progress and Soyuz are grounded then there’s no launch system that can take over their capability. Sure we have things like the JAXA HTV and the ESA ATV which are proven cargo delivery vehicles but they’ve both only launched once and neither could keep up with the rapid launch rate that the Progress offers. The Soyuz is the only means we currently have to get people onto the ISS and it being grounded would effectively end our ability to keep a human presence there.
With the shakedown of the Progress complete and the mission looking to be a success it looks like we’ll be able to reinstate the full crew size of 6 in the ISS. Whilst the station can be run with only a crew of 3 (indeed it was for the majority of its life) there’s a lot more work that can be done when the crew is doubled, especially if EVAs are required. With the SpaceX Dragon demonstration missing rapidly approaching we’re not far off having another means with which to reach the ISS. As these recent events have shown having another launch capability is critical to ensuring that our missions in space can continue uninterrupted and hopefully we’re not too far off a time when there’s more than just 2 manned launch providers.
The time has finally arrived, 30 years after the space shuttle Columbia blasted off into space her sister ship Atlantis roared into orbit just 2 days ago. It is estimated that near 1 million people flocked to the areas surrounding the Kennedy Space Center to watch the final ever launch with millions more tuning in from around the world to witness it online. I have to be honest and say that I missed the live event myself, blame goes to the sources that cited a 70% chance for no-go weather on the day, but I quickly caught up with the events spending hours pouring over the details of the last ever space shuttle mission over my morning coffee the day after.
STS-135 is a very unique mission in many ways. It began as STS-335 a Launch on Need mission designed to be launched if STS-134 had any problems on orbit and was unable to return to earth safely. For such missions a fully stacked orbiter has to be ready to launch within a very short time frame, usually on the order of a couple months. This means the usual shuttle preparations have to already be done in order to launch that quickly and with the shuttle program retiring that meant there would be one fully loaded orbiter that would essentially go to waste. Last year saw the proposal to turn STS-335 into STS-135 approved although without any specific funding for the additional mission. NASA announced in February that the mission would go ahead with funding approval or not, setting the stage for STS-135 to be the last shuttle mission in history.
The final mission of the space shuttle also shares the record of taking up the smallest crew with one of Challenger’s early missions STS-6, bringing only 4 astronauts into space. The reason for this is simple, since there are no other shuttles available to act as rescue boats should Atlantis not be able to return from orbit those stranded astronauts will have to come back down in the regular Soyuz missions. Whilst the International Space Station is quite capable of handling the extra load for a while it will still take almost a year just to ferry those 4 astronauts back down and many of the ISS contingency plans are based around having no more than 6 astronauts on the station at any one time (without additional craft docked). STS-135 then only brings the bare minimum crew required to complete the mission and all signs are pointing to them being able to return safely so far.
The payload of this mission is focused solely on keeping the ISS functioning for the rest of its intended lifetime, another 9 years or so. Atlantis carries in its payload bay one multi-purpose logistics module (named Raffaello) load with 16 resupply racks, the maximum it can carry, with an additional lightweight multi-purpose carrier that will be used to return some failed components back to earth for analysis. The failed components in question are a pump module for the external thermal cooling system and a ammonia pump module, both of which have already been replaced in orbit. STS-135 also carriers with an additional piece of equipment for the ISS, a proof of concept device for on-orbit refueling of satellites. For the demonstration it will be attached to the Dextre robot and should it prove successful the technology could make its way into the commercial sector. There will also be 2 iPhone 4s and 2 Nexus S’s carried up to be used with the ISS’s SPHEREs, basically small satellites that reside inside the space station.
It’s a historic time as this mission marks the beginning of the end to a 30 year endeavor that NASA has undertaken. It might not be the most glamorous end to the program, being basically a supply mission to keep the ISS going, but it’s an important one none the less ensuring that the gap between the shuttle’s retirement and the availability of other craft doesn’t impact on NASA’s goals in space. For youngsters like me it marks the end of the iconic craft that we grew up with and I know its going to be a long time before another one will be able to take its place. For that simple fact the shuttle will always have a special place in my heart, just like I know it does in so many others.