There’s a couple iconic photographs from space that everyone is familiar with. The most recognizable is probably this one I used a couple years ago during the 40th anniversary celebration of the Apollo missions showing Buzz Aldrin standing on the dusty surface of the moon. A few other notables are ones like Earthrise, The Pale Blue Dot and the STS-1 mission liftoff (note the white external fuel tank, one of only 2 to have it) but above them all stands the Blue Marble, an incredibly breath taking view of our earth as seen by the Apollo 17 crew on their mission to the moon.
It’s a beautiful photo and one that changed my, and certainly many others, view of the world. I don’t know why I used to think this but before seeing this picture I imagined the world being mostly cloudless, not covered in the swaths of thick cloud that you see in the picture above. It also puts your entire life in perspective, much like the Pale Blue Dot does, knowing that in the end we’re all clinging to this giant water covered rock shooting through space.
Over the years NASA has set about recreating the Blue Marble as technology progressed, mostly just as an aside to one of their many Earth sensing programs. The big difference between the original and these subsequent releases is that the newer ones are composite images, I.E. they’re not a single photograph. You can see this quite clearly in the 2005 version which shows how the Earth would look like if there was no cloud cover, something that’s simply impossible to photograph. The most recent addition to this lineage of whole Earth pictures is the Blue Marble 2012 and it’s quite spectacular:
The original picture is some 8000 x 8000 pixels large (64 megapixels) and gives you an incredible amount of detail. The resolution is high enough for you to be able to pick out topographical details with relative ease and you can even see the shadows that some of the clouds are casting on the ground below them. The original article that was linked to me had a lot of interesting comments (a lot on how the Americas appear to be somewhat distorted) but one that caught my attention was a question about one of the differences between the two pictures.
Why, they asked, is there no thin blue halo in the original picture?
The halo they were referring to is clearly visible if you view the larger version of the new Blue Marble picture and seems distinctly absent in the original. The planet hasn’t radically changed (geologically, at least) in the time between the pictures so the question is a curious one. To figure this out we have to understand the differences in how both these images came to be and in there is where our answer lies.
The original Blue Marble was taken by a single 70mm Hasselblad camera with a 80mm lens at a distance of approximately 45,000KM away from the Earth. The newer version is a composite reconstruction from several images taken by the Suomi NPP satellite which orbits at around 500KM above the Earth’s surface. Disregarding the imaging technology used and the reconstruction techniques on the modern version it becomes apparent that there’s a massive difference in the distance that these pictures were taken. Looking at the halo you’ll notice that it’s quite small in comparison to the size of the Earth so as your distance from Earth increases the smaller that halo will appear. So for the original Blue Marble the halo is pretty much invisible because the resolution of the camera is insufficient to capture it. The newer picture, being much closer and having a higher effective resolution, is able to capture it.
These kinds of images are always fascinating to me, not just for their beauty but also for the story behind what went into creating them. The number of man hours that went into creating something like this that appears so simple is staggering and demonstrates that we, as a species, are capable of great things if we put our minds to it. The Blue Marble 2012 might not become the icon that its predecessor was but its still an awe inspiring image to look at and even more interesting one to contemplate.
Whilst the Space Shuttle will always be one of the most iconic spacecraft that humanity has created it’s design was one of compromises and competing objectives. One of the design features, which influenced nearly every characteristic of the Shuttle, was the requirement from the Department of Defense that stipulated that the Shuttle needed to be able to launch into a polar orbit and return after a single trip around the earth. This is the primary reason for the Shuttle being so aeroplane like in its design, requiring those large wings so it has a long downrange capability so that it could return to its launch site after that single orbit. The Shuttle never flew such a mission, but now I know why the DoD required this capability.
It was speculated that that particular requirement was spawned out of a need to capture spy satellites, both their own and possibly enemy reconnaissance craft. At the time digital photography was still very much in its infancy and high resolution imagery was still film based so any satellite based spying would be carrying film on board. The Shuttle then could easily serve as the retrieval vehicle for the spy craft as well as functioning as a counter intelligence device. It never flew a mission like this for a couple reasons, mostly that a Shuttle launch was far more expensive than simply deorbiting a satellite and sending another one up there. There was also the rumour that Russia had started arming its spacecraft and sending humans up there to retrieve them would be an unnecessary risk.
The Shuttle’s payload bay was also quite massive in comparison to the spy satellites of the time which put further into question the DoD’s requirements. It seems however that a recently declassified spy satellite, called HEXAGON, was actually the perfect fit and could have influenced the Shuttle’s design:
CHANTILLY, Va. – Twenty-five years after their top-secret, Cold War-era missions ended, two clandestine American satellite programs were declassified Saturday (Sept. 17) with the unveiling of three of the United States’ most closely guarded assets: the KH-7 GAMBIT, the KH-8 GAMBIT 3 and the KH-9 HEXAGON spy satellites.
“I see a lot of Hubble heritage in this spacecraft, most notably in terms of spacecraft size,” Landis said. “Once the space shuttle design was settled upon, the design of Hubble — at the time it was called the Large Space Telescope — was set upon. I can imagine that there may have been a convergence or confluence of the designs. The Hubble’s primary mirror is 2.4 meters [7.9 feet] in diameter and the spacecraft is 14 feet in diameter. Both vehicles (KH-9 and Hubble) would fit into the shuttle’s cargo bay lengthwise, the KH-9 being longer than Hubble [60 feet]; both would also fit on a Titan-class launch vehicle.”
HEXAGON is an amazing piece of cold war era technology. It was equipped with two medium format cameras that would sweep back and forth to image an area, capturing an area 370 nautical miles wide. Each HEXAGON satellite carried with it some 60 miles worth of film in 4 separate film buckets which would detach from the craft when used and return to earth where they would be snagged by a capture craft. They were hardy little canisters too with one of them ending up on the bottom of an ocean but was retrieved by one of the navy’s Deep Submergence Vehicles. There were around 20 launches of the HEXAGON series of craft with only a single failure towards the end of the program.
What really surprised me about HEXAGON though was the resolution they were able to achieve some 30+ years ago. HEXAGON’s resolution was improved throughout its lifetime but later missions had a resolution of some 60cm, more than enough to make out people and very detailed images of say cars and other craft. For comparison GeoEye-1, which had the highest resolution camera on an earth imaging craft at the time of launch, is only just capable of a 40cm per pixel resolution (and that imagery is property of the USA government). Taking that into consideration I’m wondering what kind of imaging satellite the USA is using now, considering that the DoD appears to be a couple decades ahead of the commercial curve.
It’s always interesting when pieces of a larger puzzle like the Shuttle’s design start falling into place. Whilst it’s debatable whether or not HEXAGON (and it’s sister craft) were a direct influence on the Shuttle there’s enough coincidences to give the theory a bit of credence. I can see why the USA kept HEXAGON a secret for so long, that kind of capability would’ve been down right scary back in the 80’s and its reveal makes you wonder what they’re flying now. It’s stuff like this that keeps me obsessed about space and what we, as a species, are capable of.