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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.
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