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 ago.
If you like to challenge yourself then our sponsor for this video brilliant.org
has something that will be right up your street.
Brilliants daily challenges provide you with a challenge and a context and
framework that you need to work them out so you learn the concepts by applying
them. If you like the daily problem then there are more like it so you can
explore a concept in greater detail and develop your framework if you are
confused and need more guidance then you can join the community in discussing
these problems. These thought-provoking challenges will lead you from curiosity
to mastery one day at a time so what are you waiting for go to brilliant.org/curiousdroid
and finish your day a little smarter and the first 200 of
you to do so will get 20% off of the annual subscription and to view all the
problems in the archives.