Thanks to MagellanTV for sponsoring today’s
video. Comets. Throughout history, these beautiful celestial
visitors have made spectacular appearances in our skies. They have long been subject to superstition,
and they’ve been seen as harbingers of change. But as science has advanced, we began wondering:
What are they? Where did they come from? How were they formed? And what can they tell us about the origins
of life in our solar system? In 2005, the Deep Impact mission sought to
gain a better understanding of comets by doing something that no-one had ever done before
- launching a spacecraft specifically to crash into one. I’m Alex McColgan, and you’re watching
Astrum. And today we will be learning all about this
incredible mission, how it fared, and what it taught us about cometary science. By the end of the video, I hope to have earned
your like and subscription. By 1999, 8 different spacecraft had been launched
to investigate comets in our solar system, with 5 of them having flown by Halley’s
Comet in 1986. But beyond that, only 2 other comets had been
visited – Comet Giacobini-Zinner in 1985, and Comet Grigg-Skjellerup in 1992. And while some fascinating photos and dust
samples had been taken as close as 200km from some of these incredible celestial bodies’
comas and tails, comets still held many mysteries. What was their internal structure like? What were they made from? And how had they been formed in the first
place? In 1999, NASA scientists proposed a plan to
hopefully answer some of these questions. It would be difficult to understand the internal
structure of comets by simply looking at their surface. To know what was going on, scientists would
need to dig a little deeper. Their plan was to create a crater in a comet
using an impactor spacecraft, which would collide with the comet at high speeds. As they would know the mass of the impactor
and the speed it was travelling at, they could calculate from the size of the impact crater
valuable information about the comet – whether its surface was a loose aggregate of dust
and ice, or whether it had a hard, frozen shell, for instance. The comet they wanted to target was a short
period comet called Tempel 1, which had a nucleus 8km long and 5km wide. Scientists weren’t exactly certain what
would happen when the impactor hit – perhaps the impactor would punch straight through,
like hitting a snow-drift, and not really create a crater at all. There were many theories. But scientists were eager to find out which
was correct. NASA approved the project, giving it a budget
of $330 million, and named it Deep Impact. You might have thought that this was a reference
to the 1998 Hollywood film of the same name, but apparently the names for both the project
and the film had been come up with independently, around the same time. Quite the remarkable coincidence if so, as
Deep Impact (the film) was about scientists trying to blow up a meteor that was on a collision
course with the earth by flying a spacecraft to it carrying nuclear warheads. There certainly seem to be some similarities
to the NASA mission. Especially as NASA scientists worked on the
film. I don’t entirely buy NASA’s claim of a
coincidence. Although, fortunately for the Earth, there
were some differences between the film and the mission too. Tempel 1’s orbit was nowhere near the Earth’s,
and given the small size of the impactor compared to the comet, there was no chance of knocking
it off its current trajectory by more than a centimetre or so. It would be more like a fly hitting the front
windscreen of a large vehicle. Additionally, nukes would not be necessary
to create a crater on Tempel 1, or any kind of explosives for that matter. The sheer speed and kinetic force the impactor
would have when it collided with the comet’s surface would be enough to create the crater,
which some predicted would be roughly 100m across and 30m deep. With the mission going ahead, scientists began
work on the Deep Impact spacecraft. The spacecraft was actually made with two
parts. The payload, and another larger mothership
to carry it and record the result of the impact. This second section was called the Flyby. It weighed 601 kg, was 3m long, and housed
scientific devices, solar panels, a debris shield, and two powerful cameras – the High
Resolution Imager (HRI) and the Medium Resolution Imager (MRI). These would take photos of the comet after
the impact, as well as help with navigation. The impactor itself was smaller, only 372
kg, but it was still smart and housed a camera of its own. This camera, the Impactor Targeting Sensor
(ITS), would take photos of Tempel 1 right up until the moment of impact, streaming back
the information it collected to its parent Flyby, which would then relay the images to
Earth. There was considerable public interest in
the mission, which NASA encouraged in 2003 by getting members of the public to submit
their names to be recorded on a CD which was placed on the impactor. Roughly 625,000 names were collected in this
way, to be carried directly to Tempel 1’s surface. On top of that, NASA timed the impact to take
place on the 4th of July – American Independence day. While this may have been because it was one
day before Tempel 1’s perihelion, and its proximity to the sun may have produced clearer
images, I suspect that the more likely reason for this date was that American scientists
liked the idea of a large cosmic firework. Deep Impact launched on January 12th, 2005
on a Delta II rocket. But then, a problem hit. Within a day of leaving the Earth’s orbit,
Deep Impact’s onboard computers switched itself to safe mode, which it would only do
if there was a fault. Something onboard was apparently overheating. This gave scientists a bit of a scare, but
fortunately, the cause of the problem was quickly found to be a minor programming issue. Acceptable heat tolerances had been set too
low, so Deep Impact thought its thrusters were overheating when in reality they were
just fine. Engineers corrected the issue, and Deep Impact
was able to properly begin its mission. The spacecraft spent the next 6 months travelling
to its rendezvous point with Tempel 1. In that time, it travelled 429 million km. It had to course correct twice on the journey,
but this was actually impressive as it had originally been planned for there to be 3
course corrections. One was so precise that another was deemed
unnecessary. On April 25th, 2005, Deep Impact caught its
first glimpse of Comet Tempel 1. Of course, NASA scientists couldn’t manually
guide Deep Impact as there was a several minute signal lag. Deep Impact and Tempel 1 were now roughly
130 million km away from earth, more than twice the closest distance between Earth and
Mars. Deep Impact’s smart on-board programming
would have to guide it in for the final leg of the journey. On June 29th, the Impactor successfully released
from the Flyby, and positioned itself into the comet’s flightpath to crash into it
head on. This was done for a few reasons. First, the front of the comet was in sunlight,
which would allow for better pictures to be taken. Second, it would allow a greater accumulated
speed to be reached, resulting in greater kinetic force. And on July 4th, 2005, just one second out
from the anticipated arrival time, the impactor hit. And what a magnificent spectacle it produced. Scientists were thrilled that they had struck
so accurately. Deep Impact’s payload had been travelling
at 37,000 kmh, and had struck with a force of 1.96×1010 joules of kinetic energy. This produced the bright flash you see here,
the energy of which is roughly equivalent 5 tons of TNT. This flash was much brighter than scientists
expected. It up the surface of Tempel 1. However, ironically, the success of the first
part of the mission caused an unexpected negative side effect. A large dust cloud was kicked up by the impact,
which obscured the Flyby’s view of the impact crater. Dust outgassed from the comet for the next
13 days, peaking 5 days in, which made it hard to see the results of this interstellar
bullseye. Although it did offer interesting insights
into the internal pressures going on inside the Comet. Around 5 million kg of water and between 10
and 25 million kg of dust were ejected from Tempel 1 in that time. Fortunately, scientists were able to rely
on other eyes to finish up Deep Impact’s mission. The collision had been observed through numerous
other telescopes on or around Earth, including Hubble, Swift, and even many amateur astronomer
telescopes. Furthermore, another spacecraft called Stardust
that had already finished its mission was repurposed and redirected to fly past Tempel
1 and to take photos of the impact crater. It was able to do so a few years later, in
2011. From the images it took, scientists were able
to spot the crater, and calculate that it was approximately 150m across, so 50% larger
than they were predicting. From this they learned that the surface of
Tempel 1 was a very fluffy material, made from more dust than was expected, and finer
in substance than a powdered snowbank. The surface was incredibly porous. In fact, they were able to estimate that 75%
of the comet was actually empty space, the whole thing held loosely together by gravitational
forces. From analysis of the plume that had been ejected
from Tempel 1 after the impact, scientists were able to identify several interesting
material components, including clays, silicates, sodium, and even organic material. While not life itself, these heavily carbon-rich
materials may have been carried to earth by comets in the past, providing the vital materials
that make up life here. So, there we have it, a recap of the Deep
Impact mission to Tempel 1. All in all, the mission was a big success. But that was not the end for Deep Impact. Flyby was later given a new mission, titled
EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), which in 2007
saw it heading off to investigate other comets, and taking hundreds of thousands of photos,
before ultimately dropping out of contact in 2013. But by then, Deep Impact had already done
significant amounts to advance our understanding of comets and our solar system. Learning about minor planets like comets and
asteroids might have a very real impact on humanity in the future too, as companies are
already setting their sights on mining asteroids, due to them being rich in various metals. On MagellanTV there is a really well put together
documentary called Asteroid Mining: The New El Dorado, where it explores the prospect
of mining asteroids, the reason why this may end up being preferred to mining on Earth,
and how far away we are from seeing this becoming a reality. If you want to check it out, you can use my
link in the description to get a trial month of MagellanTV. MagellanTV is a subscription streaming service
which you can use on pretty much any device, focused on the best science documentaries
out there. They have a lot of stuff on space, but also
a lot more beyond that too, and with that trial month I definitely recommend checking
it out. Thank for watching! And a big thanks to my patrons and members
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