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| Mars (Credit: Hubble Space Telescope) |
Yesterday’s space news was mixed. The Trace Gas Orbiter
(TGO) has completed its orbital insertion and is now in position over Mars.
However contact was lost with the Schiaparelli lander in the final moments of
its descent towards the red planet. It is still uncertain what happened and
whether this means that the lander has crashed, or whether it made it to the
surface. Regardless of whether contact with the lander can be re-established this
mission is a success. The Trace Gas Orbiter will send back valuable data
regarding the composition of the martian atmosphere, as well as providing a new
communications platform for future landers. The European Space Agency will
learn from the Schiaparelli descent, and the information which this gives them
about landing on Mars will prove invaluable in designing future landers.
Exomars is just the latest in a long line of Mars missions,
some of which have been more successful than others. Let’s look back at the history
of robotic missions, which have told us so much about this distant world. The
first part of this article will cover the early years of Mars exploration, and
will be followed by a second post next week.
Getting to Mars
Relatively speaking Mars is one of the easiest solar system
bodies to get to, but for much of the history of space exploration it has been
a consistent challenge. The closest mars comes to earth is around 75.3 million kilometres
(1). This happens about every two years, and these are the best times to send
spacecraft to the red planet. Because both Earth and Mars are constantly moving
around the sun a spacecraft cannot be sent straight from one planet to the
other. Instead it has to be launched into what is called a Hohmann Transfer
Orbit. This means launching the spacecraft into its own orbit around the sun, which
will later cross the orbit of Mars. The intersection with Mars’ path has to be
carefully calculated. After all, you need to work out where Mars will be in a
few hundred days time, not where it is at the time of launch. Fortunately we’ve
spent hundreds of years making very precise measurements of the movements of
the different planets in the solar system, so working out the correct transfer
orbit with the aid of powerful computers is straightforward. I would say that
it isn’t rocket science, but of course it literally is.
So how many missions
have been sent to Mars and how many have actually succeeded?
Early Soviet attempts
The first attempts to reach Mars came at the height of the
space race. Within a few years of putting sputnik into orbit the USSR were
already aiming even higher. The first Marsnik probe launched in 1960 and others
soon followed. Unfortunately their first few probes didn’t meet with much
success. Marsnik 1 and 2 didn’t make it into Earth orbit, let alone to Mars.
Sputnik 22 achieved orbit of the Earth in 1962, unfortunately its upper stage exploded
while trying to get into a Mars transfer orbit. This came at an unfortunate moment
during the Cuban missile crisis, and actually triggered the US military’s early
warning system, being briefly mistaken for the start of a nuclear attack (3). Several
more Soviet mars missions followed throughout the early 1960s, but were lost
enroute.
The Mariner probes
Meanwhile the United States of America had been developing
the Mariner probes. Mariner 3 was destroyed during launch, but In 1965 Mariner
4 became the first spacecraft to perform a successful flyby of Mars. It sent
back the first 22 images from the red planet and began a new era of space
exploration. Mariner 6 and 7 were also successful and sent back 75 and 126 images
of Mars respectively. However these successful missions were accompanied by
just as many failures. The Soviet Union tried to reach Mars several more times
during the late 1960s but none of these spacecraft made it. The American Mariner
8 also suffered a launch failure in 1971. By this time NASA were frequently
sending pairs of spacecraft, so that if one failed the mission wouldn’t be a
complete failure. The USSR had a similar policy and so their missions were also
quite closely spaced. Whenever a close approach occurred both space agencies
were ready to go. This meant that when they did meet with success it all
happened more or less at once.
1971: A new view of
Mars
It wasn’t until 1971 that the Soviet Union succeeded in its
attempts to reach Mars. The Mars 2 spacecraft entered orbit in November of 1971.
However the Americans just beat them across the finishing line, putting their
own spacecraft, Mariner 9 into orbit shortly before the Russian probe’s arrival.
These orbiters were a step up from the previous flyby missions. They would stay
in proximity to Mars for far longer, and thus be able to send back more data.
In the case of Mariner 9, this resulted in a vast number of new observations. Mars
2 wasn’t so lucky and didn’t return usable data to Earth. Mars 3 had been
launched at around the same time as Mars 2, and met with substantially more success.
It operated in orbit for eight months.
The Mars 2 and 3 missions might have missed out on getting
to orbit first, but they would still be a milestone, as they featured the first
landing attempts. The Mars 2 lander crashed, but the Mars 3 lander became the
first spacecraft to successfully reach the surface. Mars 3 didn’t survive very
long after landing. Contact with the probe was lost after just 20 seconds. However
it demonstrated that it was possible to put a space craft on the martian
surface.
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| The Mariner 9 Spacecraft (Credit: NASA) |
Mariner 9 continued to operate for longer than its Russian counterparts (4).
The American spacecraft kept going for almost a year and returned over three thousand
images. In doing so it fundamentally changed how we viewed Mars. Mariner 9 arrived
during a global dust storm. This meant that it was a while before it was able
to image the surface. However confirming that such storms did occur was a
valuable observation in its own right. When it did get a clear look at the
surface, it would provide a very different view of Mars than the flyby missions
had. All of the images sent back so far had shown a barren, cratered landscape
similar to that of the Moon. Mariner 9 observed a very different Mars. It
imaged the vast volcanoes around Olympus Mons, and the huge valley system which
now bears its name; Valles Marineris. Mariner 9 was the first probe to image
the layers within the martian icecaps, and report evidence that liquid water
had flowed across the surface in the past, carving out valley systems.
If there had once been flowing water on the red planet then
could there also once have been life?
A new generation of Mars probes would need to be sent to
find out...
Tune in next Wednesday for part two of this article on the
Viking Era of Mars exploration
References
1) Mars close approach 2016: http://mars.nasa.gov/allaboutmars/nightsky/mars-close-approach
2) Hohmann transfer orbits https://solarsystem.nasa.gov/basics/bsf4-1.php
4) Mariner 9: http://www.space.com/18439-mariner-9.html
Image credits: View of Mars: NASA, ESA, the Hubble Heritage Team, Hubble Space Telescope (STScI/AURA), J. Bell (Cornell University), and M. Wolff (Space Science Institute, Boulder)
Mariner 9 Probe Via NASA: http://nssdc.gsfc.nasa.gov/image/spacecraft/mariner09.jpg
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