We left off last episode just as Opportunity was arriving at Victoria Crater, on Sol 951,
sol being the term for a Martian day.
This by itself was a remarkable achievement considering Opportunity was only designed
to last 90 sols.
During those 951 sols it had already achieved so much.
By examining many different rocks in various locations along its journey, it was providing
a host of data to determine if there used to be a water ocean on Mars in the past.
During this journey, it had become stuck in sand dunes on numerous occasions, but each
time it persevered and freed itself.
Some of its parts had begun to malfunction, but engineers had been able to implement
workarounds which meant Opportunity had been able to go on unhindered.
It had also survived dust storms that lasted for days on end, but thankfully it was always
able to charge its batteries up again afterwards.
So, what was so interesting about Victoria crater?
Was it able to provide any valuable information about Mars that Opportunity hadn’t picked
up anywhere else?
I’m Alex McColgan and you’re watching Astrum, and together we will go through everything
Opportunity did and discovered around the Victoria crater.
Victoria was always going to be an important landmark for Opportunity to reach, although
few thought Opportunity would last the 7 km journey from its initial landing site.
At this point, Opportunity had lasted more than 10 times longer than its original designed
lifespan.
Victoria is the biggest crater Opportunity had visited yet, being 730 meters wide.
It was also the deepest, meaning ancient bedrock outcrops along the walls of the crater were
exposed to the surface.
Exposed bedrock contains the clues to Mars’ past that Opportunity was looking for.
Also, as you can see, Victoria made for an impressive sight, especially in contrast to
this otherwise flat plain that Opportunity had landed in.
In preparation for this new exploration site, mission controllers uploaded new software
to the rover, which would enable it to decide whether to send back an image, or to extend
its arm in preparation to sample an interesting rock outcrop, and even improved the intelligence
of the rover to detect obstacles it should avoid, and to make its own path around them.
These features save a lot of time for mission controllers, who would have had to have gone
through hundreds of images and wait through painful latency delay for commands to be sent
to the rover.
At its furthest point, the latency between Earth and Mars can be upwards of 40 minutes.
This is one of the reasons why it took three years to only travel 7 kms up until this point.
To test this new software update, mission controllers had Opportunity drive towards
a patch of rubble, and settings were adjusted to make Opportunity think that this rubble
was a no-go zone.
In reality, the biggest of these rocks is only 10cm tall, meaning Opportunity could
have easily driven over it, but they didn’t want to test the systems on something that
could actually damage the rover if it went wrong, so they chose these rocks instead!
As you can see, the rover adjusted its course around the dangerous area, and took this panorama
so mission controllers could see how the rover responded to the new program.
Opportunity began to circle to outskirts of the crater, imaging it as it went.
It came across a few cliffs, or “Capes” along the way, imaging them with a very clever
technique called a “super-resolution mosaic”.
These images have a much higher resolution than the camera onboard Opportunity is capable
of, and they way they did this is by taking lots of photos of the same thing, and then
in post processing on Earth they combined the images to provide the resolution you see
here.
On a side note, this technique is used in many different fields, including microscopy
and astronomy.
Scientists say that the layers you see in the rocks found in all these cliff faces are
due to a geological process called cross-bedding.
They believe these layers are ancient sand dune deposits, which is why the lines are
inclined upwards and not just horizontally.
From these lines, they predict that this used to be a sand dune field not unlike the Sahara
Desert on Earth.
While this science data was very useful, mission controllers were primarily looking for a passage
down to the crater floor, and they thought they found one by here at the ‘Valley Without
Peril’.
On closer examination though, it was decided that the slope was too steep.
Opportunity investigated a couple of cliffs while it was there, and even came across another
meteorite called Santa Caterina, before mission controllers decided to drive the whole way
back to the original arrival point at the crater to try and get down by there.
This site was chosen because the slope was within the safety limits of the rover, plus
the ground was made of flat, exposed bedrock, which meant it wouldn’t have much wheel
slippage.
On the 28th June 2007, or 1,200 sols into the mission, Opportunity prepared itself for
its descent.
But just as it perched itself on the top of the slope, the biggest dust storm Opportunity
had seen yet rolled in, decreasing the brightness of the Sun by 96%.
That’s not quite as bad as it sounds, as the dust also scatters some of the sunlight
towards the rover too, meaning it was generating roughly 128 watts on the darkest day compared
to its usual 700 watts on a clear day.
But this is bad news for a solar powered rover, that kind of power level isn’t enough to
keep the rover going, in fact anything under 150 watts means the batteries begin to run
flat.
So, operations were cut back substantially until the storm was over, and Opportunity
was only commanded to communicate back to Earth once every 3 days.
If the batteries did run flat, the components on board Opportunity could be damaged due
to the intense cold on the planet during the night.
Typically, the motors have heaters powered by the batteries during the night to keep
the motors warm, if the batteries run flat, there would be nothing to stop there being
extreme temperature differences between night and day, at the equator anywhere up to 20°c
during the day and -75°c at its coldest at night, which would damage the sensitive components.
The reason for this temperature variability is because although Mars does have an atmosphere,
it is 100 times thinner than Earth’s meaning it is not anywhere near as good at retaining
the heat it absorbs during the day.
Going back to the batteries, if they reached a critical level, Opportunity was also designed
to trip a low-power fault.
This basically disconnects the rover’s batteries from all but the most essential systems of
the rover, putting it to sleep as it tries to charge its batteries with whatever available
sunlight there is.
Every sol the rover would check the battery level to see if it could reconnect the other
systems, and if so, it would then re-establish communications with Earth.
This low-power mode could mean the rover stays asleep for only a few days, but also it could
go on for weeks or even months.
Chances are that at that point, even if it did switch back on, too many of its components
would be broken and it wouldn’t be able to function anyway.
Back to the dust storm at the start of July, 2007, mission controllers initially thought
it would only last a week, but by the 15th July the storm had reached its peak.
These are true colours images showing a time lapse of the storm, and as you can see it
does get very dark.
Thankfully, the severity of the dust storm was not quite enough to trigger the low-power
fault this time, although it did get worryingly close for a while.
Eventually, by 21st August, or sol 1271, the storm had cleared enough so that Opportunity
could start to move forward again.
Sol 1291 saw Opportunity finally enter the inner slope of the crater, but like it did
before with the previous crater, just after it entered, it backed out again to assess
the amount of wheel slippage from the slope.
As it happened, wheel slippage did exceed the mission controller’s threshold of 40%,
so the rover stopped with two of its wheels still over the lip of the crater.
After two days of decision making, mission controllers decided to press ahead regardless,
and an extended exploration into the crater began.
As Opportunity descended, it took some time to investigate any interesting looking parts
of the exposed bedrock.
It also took some images of the cliffs on either side of it.
I think these images showcase just how steep the slope is that Opportunity is sitting on,
roughly 20 degrees.
Opportunity didn’t venture too far into the crater, as only sand dunes were found
towards the base.
It spent its time examining this light patch of bedrock, taking various pictures and using
its rock abrasion tool to investigate the rock’s structure.
It was determined from the data Opportunity collected that this area was likely not ever
so hospitable to life, so mission controllers picked the next waypoint on their journey,
Endeavour crater.
If you thought Victoria was big, look at this mammoth.
Endeavour was thought to contain some clays, meaning water was once there, so Opportunity
started its journey out of Victoria.
At this point, on sol 1502, Opportunity encountered another fault with its arm.
No matter how much power they put into the joint, the arm wouldn’t move from its stowed
position under the rover.
Sol after sol passed, and the joint refused to move, until they determined the time of
day the joint had the least electrical resistance, which was just at dawn before the heater for
the motor switched off during the day.
When that time came again, they put as much power as possible into the motor and…
it worked.
Not risking ever stowing it away again, engineers had to determine how to manoeuvre the rover
safely without risking damage to the arm.
Opportunity then began its 2-year trek towards the Endeavour crater.
Which we will save for the next episode!
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