How do Spacecraft Photograph The Planets and get the Images back to Earth?


Over the past 50 years our scientists have sent robotic probes to explore our solar system and they sent back some amazing close up images of the planets asteroids and even comets but how did they take these images and what sort of issues are there in taking photos in deep space and getting them back to earth. The first spy satellites used film to get the best quality available at the time because electronically transferring images back to earth wasn't of a high enough quality for the reconnaissance purposes. They were in orbit just a few hundred kilometers above the earth so we could drop the film back to earth but for the first space probes going to the moon, Mars, Venus and beyond that wasn't an option. In 1959 Luna 3 became the first spacecraft to photograph the far side of a moon ironically using captured film that was temperature and radiation resistant and had been captured from US Genetrix spy balloons that have been shot down over the Soviet Union. The camera used was a dual lens system with a 200 millimeter wide angle that could image the whole moon in one shot and a 500 millimeter for close up shots of regions of the moon, though close up wasn't really that close as Luna 3 took images of a far side from around 65,000 kilometers. The camera was also fixed at a body of a craft so Luna 3 was the first craft to used three axis rotation to position itself to take the images. Once the images had been taken the film was developed, fixed and dried on board and then scanned electronically by passing it between a flying spot scanner and a light sensitive sensor A dot of light from a flying spot scanner traversed a film at a resolution of lines per 35 millimeters frame this varying level of light was converted to an electronic signal and transmitted back to earth. Here it was shown on a slow scan TV and then that image was photographed and that became what the rest of the world saw. The poor quality of images is hardly surprising once you take into account that each stage of a processing the image quality was reduced more and more. However it was 1959 and it was the first time it had been done and they were still good enough to show that the far side of a moon was very different to the earth facing side. The very first image of Earth from far away a low earth orbit was by Explorer six and also in 1959 from 27,000 kilometers and was part of a test of an all electronic scanning system to measure cloud cover. This images of the sunlit clouds above the Central Pacific though it's hardly what you could call good. Compare that to the "Blue Marble" shot of the entire earth by the crew of Apollo 17 in 1972 whilst on its way to the moon and using a 70 mm Hassel blad with an 80 mm lens millimeter lens things obviously had to Things had change a lot if we were to send space probes tens or hundreds of millions of kilometers to the planets and see something better than the best telescopes on earth. In 1964 Mariner 4 became the first spacecraft to do a flyby of Mars and take close up images. Unlike Luna 3, Mariner 4 used the slow scan vidic on TV tube to gather the images of the Martian surface. The analog signal output of the tube was then converted to a digital format and then stored on a magnetic tape recorder, the predecessor to today's hard drives. After the camera finished taking the pictures they were replayed back from the tape and sent back to earth for processing at JPL. Because it was going to take the computer a long time to create the images, we are talking about 1964 mainframe here and the need to get something out quickly to the press when the first image data was received one of the team at NASA used crayons from a local art store to color strips of data that they saw on the monitor screen, a bit like painting by numbers and this became the very first image of another planet. When the actual monochrome version was assembled and printed out by the computer it was surprising how well the hand drawn version looked in comparison and that is now on display at the JPL labs at Pasadena, California. In 1975 the Soviets became the first to successfully take pictures from the surface of another planet this time it was Venus. They knew the US was planning the Viking Mars Lander but due to budget restraints and issues with long term reliability of their navigation they chose Venus as a closer target. But a Venus landing makes a Mars landing look like a walk in the park. When it landed it recorded an atmospheric pressure 90 times that of Earth and the temperature was 485 degrees Celsius. Although the surface of Venus is completely obscured by thick clouds from above, from below the pictures it sent back of the surface show but it's about the same brightness as an overcast summer's day here on earth and with good visibility and little atmospheric dust with rocks scattered around the lander. To get the images back to earth for lander relayed the pictures back to earth via the orbiter which had also carried the lander and this was also the first craft orbit Venus. The lander had two 180 degree cameras which would have given a full 360 degree view but the lens cover of one failed to detach on landing. The cameras themselves were photographic scanning devices with moving mirrors. The resolution of each was about 70,000 pixels made up of a 500 by 128 pixel frame. Although it was thought that the heat and pressure destroyed the lander after 53 minutes on the surface, a Soviet source later said that the transmission had stopped because he orbiter had moved out of a communication range of the lander. Taking pictures of a planet hundreds of millions or billions of kilometers away poses many problems firstly there is just the lack of light the amount of sunlight compared to here. On earth is about a thousand times less when you get out to the distance of Pluto. High noon on Pluto is equivalent of what that's a called Pluto time on earth that's around dusk and dawn and roughly when you would have to turn on the headlights of your car. Taking images of asteroids is even harder and is akin to photographing a piece of coal in moonlight whilst travelling faster than a bullet. This means that the camera exposures must be much longer to gather enough light but then the speed of the spacecraft, New Horizons for example is traveling at 16.2 kilometers per second almost 60,000 kilometers per hour and would introduce motion blur if the cameras don't exactly track the object they are flying by. And it's not as if you can just press a button here on earth and they will take a picture on a spacecraft the distances are so great but it takes four and a half hours to send a radio signal to Pluto at the speed of light so everything has to be pre programmed and timed to the second in order to turn the camera or the spacecraft at the correct speed and at the correct time to get the exposure without motion blur or missing the object completely and all that is done by the spacecraft by itself. Only hours or days after will the team on earth know if it's work correctly. Then there is the radiation, not only the cosmic rays from deep space but around the planets themselves. Just like the Earth's magnetic field captures and concentrates charged particles from the Sun in the Van Allen belts, Jupiter and Saturn do the same only on a much larger scale. Due to the interaction of Jupiter's rings yes it has rings like Saturn but just much smaller and the volcanic emissions from Jupiter's moon Io, there are areas of intense radiation around Jupiter some 10,000 times that of the Van Allen belts around earth. So the cameras and electronics on spacecraft like the Juno Jupiter orbiter which launched in 2011 and will fly closer to Jupiter than any other craft are especially radiation hardened. The CPU on Juno is rated to withstand 1 Million RAD's and 20 million RAD's over its lifetime. If a human were exposed to a thousand rads for a few hours the result would almost invariably be fatal. The CPU and other electronics in a radiation vault which has up to 25 millimeter thick titanium walls which reduces the radiation by a factor of 800 the camera itself uses a Kodak KAI 2020 sensor with a resolution of 1600 by 1200 pixels with modifications and a special housing to mitigate the intense radiation. The power supply the length of time but it has to operate also limit the size of the cameras and the strength of the transmissions back to earth. Nuclear powered radio isotope thermo electric generators are used and can last for decades but their output is low that just a few hundred watts. The voyagers used two cameras which were modified versions of the slow scan vidic on tubes used on the earlier Mariner missions. One has a low resolution 200 millimeter wide angle lens and aperture of f 3 whilst the other uses a higher resolution 1500 millimeter narrow angle F 8.5 lens. The resolution of the images after they've been digitized was just 800 by 800 pixels. Although we see color images taken by the Voyager probes, they actually only sent black and white ones each camera has eight colored filters in a controllable wheel that rotates in front of a camera so for a color image they would take three monochrome images, one each through a different filter usually red, green and blue. These three images would then be combined to make a full color image when they received back on earth. Using this method they can get a higher resolution and if the camera used a color camera tube. Often they might need to take images as part of a spectrum which we can't see like infrared or ultraviolet so a monochrome camera and filter combination works much better. The cameras on Voyager could take up to 1,800 images per day, far quicker than could be sent back to earth. These were stored on magnetic tape like on the Mariner probes. The digital data would then be replayed back and sent to earth at a speed of around 7.2 kilo bits per second which was at a distance of around about Jupiter. With an individual picture taking up about 5.2 megabits of data and allowing only for a basic compression method available with time and error correction which sent extra data in case some of a signal was lost, it would take about four to five minutes to send each image so for a full day's worth of 1,800 images who would take just over six days to send them back to earth. By the time the voyages were at the distance of the outer planets the bandwidth had dropped to around a 160 bits per second at this speed it now takes nine hours to send one image an 1800 images would take 1.8 years. Even on modern spacecraft the data rates are very slow compared to what we used to. New Horizons data rate from Pluto in 2015 was just 2 kilobits per second not 2 kilobytes, 2 kilobits and it's even farther away now. This is why it will take up to a year to send back all the images of Ultima Thule taken in December 2018. Something that a lot of people don't realize is that the pictures we see are mostly for public consumption and PR but they're not that important from a purely scientific point of view. Most of the real science is done by the other instruments which are carried onboard. Juno cam was put on the Juno Jupiter orbiter primarily for public science greater public engagement and to make all the images available at NASA's website. It's low resolution and fixed mounting to the spacecraft body led to it being referred to by some as Juno's dash cam. It was designed to survive eight orbits around Jupiter but as of 2018 had survived 17 so it's now been tasked with more scientific duties as well. If you are wondering why these billion dollar spacecraft don't use the latest multi megapixel camera sensors it's because they need reliability above everything else so they use tried and trusted technology whichever time of the design is often a decade or more old. It then takes years to build and then launch again working with the original trusted design where possible. When New Horizons was launched in 2006 it was already nearly a 15 year old design and it wouldn't arrive at Pluto for another nine years. Using this method of tried and trusted technology has proven itself many times now with the best known examples of voyagers 1 & 2 working for the best part of 50 years and maybe more if their nuclear power suppliers can hang on in there a little longer. All of the missions we have launched to photograph and find out more about our solar system have had unforeseen issues which have had to be fixed with a craft hurtling through space millions or billions of kilometers from Earth, problems that tax their creators and operators every bit as much as they did their original design all those years ago. 

Thanks For Reading the Article and see you next time.

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