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When mankind first started sending spacecraft out to explore the solar system, the first
planet to be visited was Venus - our closest neighbour - in 1962.
Next was Mars in 1965, and then Jupiter in 1973.
Only then came Mercury in 1974.
And already this order might seem a little odd.
The closest distance between the Earth and Mercury is 77 million km.
In fact, it is the closest planet to us on average.
The closest distance between the Earth and Jupiter is 588 million km, almost 8 times
that.
And Jupiter was visited again in 1974, twice in 1979, in 1992, in 1995, in 2000, in 2007.
Multiple missions were launched to Saturn, Uranus, Neptune, to comets and asteroids…
while Mercury got nothing for 30 years.
Is this because it was deemed uninteresting?
Did we discover everything there was to discover about it with that single, first mission?
No.
That first mission was a flyby, and only mapped about 40-45% of Mercury’s surface.
Actually, the real reason is that Mercury is one of the most challenging planets to
visit in our entire solar system.
Why?
Well, Mercury exists in a furnace.
Due to its proximity to the Sun, its surface temperature reaches highs of 430°C, so any
probe visiting it would need to be highly heat resistant.
But that same proximity to the Sun means that any probe launched towards it will accelerate
faster and faster due to the immense gravitational pull from our star, using rocket fuel against
that would be like swimming up white-water rapids.
Combatting the Sun’s gravity required too much fuel for a discovery-class spacecraft
to carry, slowing down the spacecraft enough to be caught up in Mercury’s orbit seemed
impossible.
But all that changed with the MESSENGER mission.
I’m Alex McColgan and you’re watching Astrum.
And today we will be learning about mankind’s first and only attempt to place a probe into
orbit around one of the hardest to reach planets in the solar system.
Weight is a challenging limitation when it comes to spacecraft.
The heavier a craft, the larger a rocket needed to get it out of Earth’s orbit, and the
more expensive everything becomes.
Scientists try to keep everything as light as possible to reduce this cost.
As fuel takes up precious weight allocations that could go towards scientific instruments,
scientists try to take only what is necessary to help them complete their journey.
However, for about 30 years, scientists could think of no way to put enough fuel on a probe
to get it to slow down enough to enter Mercury’s orbit, especially if they wanted scientific
equipment onboard too.
So, after the success of Mariner 10’s flyby missions of Mercury in 1974-1975, Mercury
exploration was put on hold.
But in 1985, an orbital mechanics expert named Chen-Wan Yen realised that there was a way
of getting a probe into orbit around Mercury that didn’t need new technology.
Instead, he had worked out a particular route an orbiter could take around the solar system
that would slow it down enough to enter Mercury’s orbit with only a few course corrections.
Rather than going straight to Mercury, the orbiter would need to take a longer way.
How long?
Under Chen-Wan Yen’s model a craft would orbit the Sun about 15 times, flying past
Earth once, Venus twice, and Mercury 3 times before finally slowing down enough to enter
its orbit on the 4th pass.
All these planetary flybys would be essential – by skimming the planets’ atmospheres,
vital speed could be shaved off, from atmospheric drag and due to the gravity of the planets.
The entire route would cover a mammoth 7.9 billion km and would take 6 and a half years.
Chen-Wan Yen’s findings were not immediately picked up, but in 1998 NASA began to take
an interest in the idea and after seeing the feasibility of the route, they launched the
MESSENGER probe in 2004.
MESSENGER, or the Mercury Surface, Space Environment, Geochemistry, and Ranging probe was only 1.8m
long and 1.3m wide and weighed 1,100 kg.
This is small and light for a typical NASA mission.
Just a comparison, Juno is 20m long.
MESSENGER came equipped with a powerful thruster, a ceramic heat shield to protect it from the
Sun, two solar panels and a whole suite of scientific equipment for imaging and measuring
data from Mercury.
Scientists hoped to take advantage of this opportunity to learn as much as they could
about the chemical composition of Mercury’s surface, its geological history, its magnetic
field, and its core (among other things).
MESSENGER spent its first year in space making one orbit around the Sun, before meeting back
up again with Earth.
This gave scientists a chance to test its equipment on a known astronomical body, to
make sure there weren’t any errors and to make any adjustments as needed.
MESSENGER took some photos of Earth and the Moon, and also tested its other equipment
to take readings of our atmosphere and magnetosphere.
Fortunately, everything was working perfectly.
As it began to head further sunward, MESSENGER employed a clever technique to help reduce
its acceleration towards the Sun.
It used its solar panels to “catch” solar radiation, like sails on a ship might catch
wind.
Solar radiation hitting an object actually pushes it very slightly.
While this force is very tiny, because MESSENGER’s journey was so long, it really added up.
Making the most of this phenomenon was one of the ways MESSENGER saved propellant and
decelerated naturally.
The next notable landmark in MESSENGER’s journey came in 2006, when it did its first
flyby of Venus.
Sadly, for scientists, this moment came at a time where Venus was exactly on the opposite
side of the Sun from Earth, which meant MESSENGER was not in radio contact!
It did take some photos of the planet which it later sent, but otherwise performed no
science.
However, in 2007, it passed Venus again.
At that time another spacecraft was orbiting Venus – ESA’s Venus Express.
MESSENGER and the Venus Express took the opportunity to work together, performing the first ever
simultaneous measurements of particle-and-field characteristics of the planet.
But then it was on to the main event: Mercury.
MESSENGER made its first flyby of Mercury on 14th January 2008, with everything going
smoothly.
The same was true of the second flyby.
But during the 3rd flyby in 2009, something went wrong.
MESSENGER went into safe mode, which was designed to protect systems on the craft in the event
of an error.
How disappointing to have come so far, only for the mission to potentially fail during
one of the final stages!
MESSENGER remained in safe mode for what must have been 7 hours of stress for all of the
scientists involved.
You see, MESSENGER had to pass through Mercury’s shadow during this flyby, meaning it had to
rely on its batteries for 18 minutes.
Something wasn’t configured right in the power management part of the software.
Fortunately, MESSENGER’s computer reset once power from the panels charged the battery,
and it was able to continue with its mission, swinging around the Sun one more time before
finally entering orbit around Mercury on the 11th of March 2011.
MESSENGER took up an elliptical orbit around Mercury, alternating between as close as 200
km and as far away as 15,000 km.
This is because Mercury acts sort of like a giant sun mirror, radiating heat back into
space.
Remaining too close to Mercury was too hot for MESSENGER, even with its heat shield (which
was more designed to protect it from the Sun – 7 times brighter by Mercury than it is
on Earth), so moving further away every 12 hours gave it a chance to cool off.
MESSENGER spent the next 4 years in Mercury’s orbit, far exceeding scientists hopes and
expectations for the mission, as they had originally been planned for it to only last
1 year.
Before launch, scientists had hoped that MESSENGER would take at least 1,000 photos over the
course of its lifetime.
However, MESSENGER took over 200,000 photographs, giving us a complete map of Mercury’s surface
in high resolution and colour, as well as photographing nearby comets, and other planets.
On 25th December 2014 MESSENGER’s propellant – so carefully saved up until that point
– was finally about to run out.
By this point, MESSENGER was orbiting a mere 25km from the surface of the planet.
Scientists gave the thrusters one last burst to extend its orbit for as long as possible,
but on April 30th, 2015, MESSENGER crashed into the surface of Mercury.
After a journey that had lasted over a decade, and had covered literally billions of kilometres,
MESSENGER’s journey had come to an end.
Now, if the science findings of this mission would be something you’d be interested in
for a future video, please let me know by commenting or leaving a like!
I definitely think it’s an interesting topic and I hope you will too.
And the adventures around Mercury don’t end with MESSENGER.
In 2025, the ESA - JAXA joint Bepi-Colombo Mission will arrive at Mercury, taking over
the reins from NASA.
If you want to know more about this mission before I end up doing a video on it, there’s
a great documentary on MagellanTV’s streaming platform called “Mercury or Bust” which
goes into detail about what makes this mission special, and what ESA/JAXA hope to achieve
while there.
They claim the Bepi-Colombo spacecraft is one of the most complex scientific missions
ever launched, a single spacecraft that will split into two spacecraft upon arrival at
Mercury, so it is definitely worth learning more about it.
And with my link in the description, you can watch it for free because you’ll get a one-month
free trial of MagellanTV’s entire catalogue of over 3000 science documentaries!
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