If you’ve ever looked up at the night sky
and watched a comet streak by, you will know of the wonder of watching one of these celestial
visitors. Often seen with awe or fear throughout humanity’s history, these bright-tailed objects
in our skies were known as harbingers of change. And yet, in the last 50 years, the
tables turned on these icy wanderers, and it went from them coming to visit
us, to us being able to visit them. Scientists had long wondered about the nature and
origins of comets. Where had they come from? How were they formed? In 1986, the first probe was
launched to image Halley’s comet, and scientists began to find answers. But to truly understand
comets, it would take more than photographs. A more physical approach would be needed. Between
1999 and 2005, two probes were launched. Their mission was to interact with comets in
ways that had never been attempted before. One would bring collection equipment that would
allow it to scoop stardust right from the comet’s icy tail, to help scientists analyse the chemical
makeup of these frosty harbingers. The second would take a more forceful approach. Rather than
quietly collecting a smattering of space dust, the second probe would crash head-first into the
surface of the comet itself, exploding with the force of 5 tons of TNT, to see what could be
learned from the resulting crater and debris. And yet, although these two missions
were to different comets, through chance there was one comet that unexpectedly
brought them both together: Tempel 1. I’m Alex McColgan, and you’re watching Astrum.
Join with me in today’s supercut as we explore the explosive story of Tempel 1, and how
the Stardust and Deep Impact probes both were needed to help discover what lay
at the heart of this space-borne nomad. In the early 90’s, comets were still a bit of
an enigma. 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 lit 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, at least to capture images of the explosion. The collision had been observed through
numerous other telescopes on or around Earth, including Hubble, Swift, and even
many amateur astronomer telescopes. Still, this was a serious problem. Although
this outgassing was fascinating to record, the primary purpose of the Deep Impact Mission was
to take photographs of the crater caused by Deep Impact. Without images of the result, many of the
questions about Tempel 1 would remain unanswered, like about its structure and composition. Like
a partially unwrapped gift, Tempel 1 had been opened, but it had not yet been seen what
lay inside. Some other craft would be needed to complete Deep Impact’s unfinished mission.
Fortunately, another craft capable of doing so had already been launched, and, having completed
its own previous mission, was now drifting serenely through space. It was about to receive
another task. It’s time we talked about Stardust. Let’s go back to the late 1990’s, when
cometary science was even more patchy. Although by this point we had sent 6 probes
up to visit these enigmatic celestial bodies, not very much was known about their origins.
It was believed at the time that comets were foreign visitors to our solar system, older
than the sun, having been formed from the loose pre-solar grains of dust that orbit other
stars before drifting through space towards us, only to be caught up in the sun’s gravitational
pull. It was believed that this theory could be confirmed by travelling to one of these comets
and picking up some of this loose dust – or “stardust” – that surrounds them in space. By
examining the isotopic composition, scientists would be able to tell if it was unusual when
compared to the dust given off by our own star. However, this was a challenging mission. As
is often the case, it came down to a question of speed and energy. Comets travel through the
inner solar system at speeds reaching 160,000 kmph. While it was possible for a probe to try
to match that speed and come up alongside it, this had to be done without needing too
much fuel, or the weight of the craft would be too heavy and thus too expensive to
get into space in the first place. Initially, Stardust had nothing to do with Tempel 1. For this
mission, scientists selected a comet known as Wild 2. They believed that they would be able to get
Stardust alongside Wild 2 at a relatively low velocity. However, this velocity would still
be around 6.5km per second, or 23,400kmph. As you can imagine, catching even particles
at that speed would be extremely challenging. Although particles would likely not do too much
damage to Stardust, being too small to really impact it, it would do irreparable harm to the
particles themselves. When an object crashes at 23,400 kmph into a surface, the odds of it
keeping its original shape and structure are incredibly small. Scientists would not learn
much about the structure of these particles if they smashed those particles into pieces, not
to mention the warping effect all that kinetic energy being suddenly converted into thermal
would have on the molecular bonds involved. So, what was their solution? What was their mechanism
for catching objects travelling at those speeds? Well, much like how an airbag softens the blow
for you if you are involved in a car crash, scientists realised that they would need an airbag
of their own. Something that would not halt the particle all at once, but would reduce its speed
over a longer distance, thus reducing the amount of crushing deceleration involved. For this, they
found an incredible material that was basically air. Solid air. They decided to use aerogel.
Aerogel is a fascinating substance that was discovered in 1931 by Samuel Kistler when he made
a bet with fellow scientist Charles Learned about jelly. As you have probably seen if you have
ever made it yourself, jelly is formed of two parts. Firstly, a relatively solid structure that
acts kind of like a sponge, and secondly, water. When you add water to solid cubes of dense
jelly, it absorbs the water and expands into the wobbly substance we are all familiar
with. If you were to extract the water, the solid part of jelly would normally contract
again. Kistler’s bet with Learned was to be the first one to remove all of the liquid from
the jelly without making it shrink. In short, to make a jelly that was entirely filled with air.
An air jelly. Without going into all the details, Kistler won his bet, and at the same time invented
the first aerogel. Aerogel is a fascinating substance, as it is usually over 99% air, and yet
has the structural strength to support bricks. Nowadays it tends to be made from silica
composites, rather than jelly, but can be made from a wide range of materials. It is incredibly
light, and is strangely enough an even better insulator than regular air. And importantly for
Stardust, when particles hit it, it would offer just the right amount of resistance to slow down
the particle without denaturing or destroying it. The trails left behind in the aerogel would
also be useful for scientists to spot where a particle had been captured. Stardust was fitted
with a tennis-racket sized aerogel collector tray made up of 90 blocks of aerogel 3cm thick, with
over 1000 square centimetres of surface area, which would be deployed from inside the main body
whenever sampling was to take place. Stardust would also capture dust from the interstellar
medium, to allow comparisons and to learn more about the dust in our own solar system. Once it
had collected these samples, it would store them on a Sample Return Capsule, which would be fired
back towards the Earth for reentry and collection. This SRC was 0.8x0.5m, weighed 45kg, and came
fitted with aeroshield, navigation recovery aids, and a parachute. Also onboard Stardust was a
navigation camera, a cometary and interstellar dust analyser, and a dust flux monitoring
system, among other scientific devices. The probe launched on 7th February 1999, and spent
the next 5 years travelling through space, passing the asteroid 5535 AnneFrank along the way, which
it took some photos of. But on 2nd January 2004, it finally arrived at its target Comet Wild 2.
And what it found was immediately extraordinary. Scientists had not expected much from
Wild 2. Some NASA scientists described their expectation of it to be “a rather bland
object looking somewhat like a black potato”. However, this is not what they found. Instead, the
surface of Wild 2 was covered with spiky pinnacles hundreds of metres tall, cliffs, massive
holes jetting dust and gas out into space, even on parts of the comet that were pointing
away from the Sun and thus were expected to be less reactive. In short, the surface of the
comet was unexpectedly alive, and self-renewing. Something else was just as notable for its
absence. Craters. Unlike almost every other body in our solar system with surfaces exposed
to space, there were no craters on the surface of Wild 2. This puts it in stark contrast to places
like Mars, or our own moon. Given the period of time Wild 2 is thought to have existed, it surely
must have encountered other objects which impacted with it. So where had these craters gone? It shows
that a comet’s surface can be either self-renewing or active, reducing signs of visible craters
over short time frames, astronomically speaking. And of course, during this flyby, Stardust had
its aerogel collector exposed, and it was rapidly collecting dust samples. Just listen to the
frequency in which dust struck the spacecraft. The samples were carefully stowed away, and upon
reaching the vicinity of Earth, Stardust ejected the SRC. The angle of approach had to be just
right as it was travelling at tremendous speed. If the approach angle was too low, it would just
skim off the atmosphere and fly back into space. If the angle was too high, the heat would
disintegrate the capsule. So it was with great relief that the DC-8 NASA airplane
monitoring the sky saw it approaching at just the right second at just the right
angle. The SRC landed in the Utah desert, where it was recovered, everything having
worked and deployed just as it was designed to. And taking the samples back to the lab, scientists
learned another completely unexpected fact about Comet Wild 2. It was not a visitor to our solar
system at all. Unlike what had previously been believed, Comet Wild 2 had not originated from
another star. It had been born from our own. By comparing the isotopic composition of the
particles Stardust collected with samples from our own solar system, it was proven that
Comet Wild 2 originated from the solar system. And contrary to what all the ice on its surface
might lead you to believe, the rock at its centre was formed under white-hot conditions. Chondrules
and Calcium Aluminium Inclusions were both found among the samples Stardust collected. These are
structures that only form under incredibly hot conditions and can be found in other asteroids
between Mars and Jupiter. So, scientists had to rethink their theory that comets formed in
cold conditions at the edge of solar systems, even if they do spend some time there.
Both fire and ice go into making comets. And thanks to the careful, delicate way
that the particles had been collected, scientists were able to find one last,
surprising thing – the amino acid Glycine. Amino acids are the building blocks that make up
proteins, that are vital for all living things. Although this does not mean that there was
anything alive on Comet Wild 2, this does lend weight to the idea that it was from comets such
as this, crashing into our Earth millions of years ago, that life’s first building blocks found their
way to our planet. Which, I’m sure you will agree, offers a tantalising glimpse into our own origins.
Given all these discoveries, you might have been forgiven for thinking that Stardust’s work was
done. But NASA is always reluctant to waste perfectly good spacecraft if they have more to
give, and Stardust still had fuel in the tank. And so, when the question arose in 2006 of how NASA
would capture that close-up image of Tempel 1, Stardust’s name was put forward. This would prove
to be an interesting opportunity. Stardust was calculated to have enough fuel to make a 6-year
journey around the solar system to arrive at Tempel 1. This would represent the first time a
comet was visited and then revisited years later, providing an intriguing chance to see how
Tempel 1 had evolved over the intervening years. Deep Impact only imaged about a 1/3rd of Tempel
1’s surface as it flew past, but even that was enough to identify fascinating geological
features: layered terrains, smooth flows that contrasted sharply with the rougher terrain around
them, crater-like vents and cliff faces. It would be incredibly insightful to see how these had
changed in the time Tempel 1 had orbited around the Sun. Stardust would even be able to take
images of things previously unseen, giving even greater coverage of the rich geological history
of the comet. There were other advantages to using Stardust. It would be significantly cheaper to use
equipment that had already been launched than to develop and then launch something new. Stardust’s
shielding was even designed specifically with cometary exploration in mind, which certainly
came in use, for reasons I’ll go into later. It had all the camera equipment it needed to take
precise images. And so, Stardust was approved and was given a new name to match its new assignment:
the Stardust New Exploration of Tempel 1 mission, or Stardust NeXT. Of course, achieving this goal
wouldn’t be easy. Course corrections had to be made years in advance, to conserve fuel and make
sure Stardust arrived when it was supposed to. This made things complicated, given that Tempel
1 didn’t just remain static as it travelled – it spins, once every 40 hours. So, it wasn’t just
a case of figuring out how to get Stardust to meet up with Tempel 1. NASA had to make sure it
happened when Tempel 1’s impacted side was facing the Sun and facing Stardust once Stardust flew
past. In effect, even though Tempel 1 was not easy to see clearly, they had to calculate all the
spins that Tempel 1 would make a full year ahead, to ensure the arrival time matched up.
With Stardust’s diminished fuel reserves, there would be little room for error. Incredible
precision and excellent models would be required. As such, NASA enlisted the help of dozens of
observatories around the globe. Tempel 1 was little more than a tiny dot in the night sky
– thus it was impossible to track through its surface features, which were indistinguishable at
that distance. However, its asymmetric shape meant that its brightness fluctuated as it travelled,
dimming as a narrower profile was pointed our way, then brightening as the wider profile rotated
into view, in regular intervals that allowed a detailed model to be created with a high
degree of certainty. Scientists counted the spins as Stardust travelled. One. Two. Three.
Knowing that if they missed a single count, it would potentially mean the failure of the
primary mission objective. Their model needed to be perfect. Stardust travelled for years
through space, engaging in one Earth gravity assist and multiple laps around the Sun, before
timing its final manoeuvre a full year before it would arrive at Tempel 1. The burn would alter its
arrival time by a small yet significant 8 hours. Stardust was now locked in. A year later, as
it closed in on the comet, Stardust’s shields began to detect sounds, as tiny particles began
clattering off it. Tempel 1 was still ejecting dust and small rocks into space. Stardust was
hit dozens of times. Although these rocks were tiny – only a millimetre at most - some of these
hits had enough force to go through the front of Stardust; cutting through a graphite-cyanide
honeycomb sheet as thick as your finger. Still, Stardust survived the barrage and on the
14th of February 2011, Stardust made its flyby. It passed at a distance of 181 km, and took
122 images. I find it amusing that scientists waited for another holiday for a Tempel
1 visit. They’d chosen independence day for their initial impact. Here, on a less
violent visit, they chose Valentine’s day. Scientists had to wait for hours for the images
from Stardust to arrive back. But when they did, NASA saw that they’d managed another bullseye.
They’d correctly predicted the rotation of Tempel 1 to an accuracy of a single degree. Right
on Tempel 1’s surface was the crater that had been left by Deep Impact’s payload. The mission
was a success. From the images Stardust took, scientists were able to 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. Not only that, but they were able
to see other changes that had taken place on Tempel 1’s surface. 3 pits that had
formerly existed had merged to become one. A cliff face had eroded back around 20-30
metres. This indicated that Tempel 1’s surface was a dynamically changing place,
leading to interesting questions about how these formations had formed in the first
place that scientists could now puzzle over. So, Deep Impact’s mission finally had closure, and
had been a resounding success. But that was not the end for Deep Impact. Following in Stardust’s
footsteps, Deep Impact’s 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. What about Stardust? After its extended mission,
scientists saw there was still a little fuel left in its tank. So, it ran with it. Firing it
for as long as it could, scientists checked to see if their models of how much fuel
Stardust held matched up with the reality. To its last breath, Stardust
kept doing science until the end. When at last all its fuel was used up,
it sent one last transmission to Earth to acknowledge that it was being turned off for
good. Now, it finally rests amongst the stars. Comets are truly fascinating things, and
it was thanks to the incredible work of the Stardust probe and the Deep Impact mission
that we were able to learn a great deal about their inner composition and workings. While still
retaining their beauty, we have pierced through their layers of enigma. We understand they are
not some foreign visitors, but originate here, from our own solar system, and may even have led
to the blossoming of life itself on this planet. And it was human ingenuity and precision that
allowed these discoveries to be made. So, the next time you see a comet, with its beautiful tail
flaring out across space away from it, it will no longer be quite so mysterious, or foreboding. They
may even be the reason you are here today. And all it took to learn this was to catch the dust from
one, and to punch another really, really hard.
