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! Ever wondered about the process of getting
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