How do Spacecraft Photograph the Planets & get the Images Back to Earth?


Over the past 50 years we’ve 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. In a previous video we looked at how 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 farside 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 a
thousand 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 70mm Hasselblad with an 80mm 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 vidicon 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 are
encased 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 radioisotope
thermoelectric 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 vidicon tubes used on the earlier
Mariner missions. One has a low resolution 200 millimeter wide angle
lens and aperture of f3 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 kilobits
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. Junocam 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
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