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Over the sixty years, humans have explored the solar system
in an amazing number of ways.
We’ve sent landers, rovers, orbiters, and flybys.
We’ve used rockets, airbags, sky cranes -- you name it, we’ve probably done it.
But there’s one mission that stands out, not just because it was the first of its kind,
but because it was downright aggressive.
I’m talking about Deep Impact, which didn’t just visit a comet:
it blew a hole in the side of it.
And out of that hole came not just a ton of dust,
but also a new understanding of how comets are put together.
Excitement around the arrival of Halley’s Comet in 1986 led countries around the
world to start sending spacecraft to fly past these icy objects in the ‘80s and ‘90s.
These early flybys provided our first glimpse of what comets looked like up close,
but they painted an incomplete picture.
For one, they passed by at a distance of hundreds or even thousands of kilometers,
so even their best images were missing a lot of detail.
But more importantly, flyby missions can only really study
what’s on a comet’s surface.
That left key questions unanswered
like the composition and strength of the stuff inside the comet.
Since comets and asteroids represent the ancient building blocks
of the solar system, the implications of these questions could be far-reaching.
To find the answers, NASA chose a proposal
to study a comet orbiting between Mars and Jupiter called 9P/Tempel 1.
That mission became Deep Impact.
The spacecraft consisted of two really simple parts.
The flyby probe contained a pair of cameras and the equipment needed
to communicate with mission controllers on Earth.
Then there was the impactor, a 372-kilogram slug designed to...impact.
Okay, that’s not totally fair
it wasn’t just a blunt instrument.
It also had a targeting camera and maneuvering thrusters
to help pinpoint the impact site.
And to add mass, it also carried a huge cylindrical hunk of copper.
Mission scientists specifically chose copper because it’s not reactive
with many elements, and they didn’t expect to find any copper on the comet itself,
so it wouldn’t get in the way of what they were trying to see.
On January 12, 2005, the combined pair launched onboard a Delta rocket
for one of the shortest missions in modern spaceflight.
Less than six months later -- on July 4th -- the impactor made its final approach,
slamming into comet Tempel at more than 36,000 kilometers per hour.
At that speed, it hit with the energy of 4.7 tons of TNT.
In fact, it worked too well -- so well it kind of screwed up the whole plan.
The collision blasted so much material into space that when the flyby spacecraft
photographed the impact site a few minutes later, it couldn’t see anything!
In fact, scientists wouldn’t even know how big the crater was until
nearly six years later, when NASA redirected the Stardust spacecraft to take a peek.
That followup found the crater to be about 150 meters across.
Fortunately, the flyby had plenty of backup.
A vast array of space-based observatories tuned in for the collision,
including Hubble, Spitzer, Swift, and XMM-Newton.
These telescopes, plus many more on the ground,
collected data in a huge number of wavelengths.
And ESA’s Rosetta mission,
which was already on its way to another comet, got in on the action, too.
This data helped scientists estimate that the impact excavated
about 5 million kilograms of ice and another 10 to 25 million kilograms of dust.
Which turned out to be more dust and less ice than researchers had expected.
So while comets are often called “dirty snowballs,”
the results from Deep Impact suggest that “icy dirtballs” might be a better name.
And all that dust contained the kinds of clays and organic molecules
that may have been key to the formation of life here on Earth.
And since scientists think comets represent material left over from the formation
of the solar system, this suggests that the ingredients for life
may have been around from the get-go.
But maybe Deep Impact’s most important contribution is in helping us understand
the consistency of a comet’s material.
The collision showed that Tempel’s outer layers were extremely porous,
containing up to 75% empty space.
In fact, the mission’s principal investigator described the surface as having
“the strength of lemon meringue.”
Probably doesn’t taste as good, though.
After the impact, NASA sent the flyby craft, which was still fully operational,
to study comet 103P/Hartley.
It got to within 700 kilometers of that one, and returned a collection of images.
So, with not one but two comet close encounters under its belt,
the Deep Impact mission was a resounding success.
In fact, it was so successful that NASA gave this mission design another go
four years later with LCROSS, which slammed into the surface of the Moon.
Many of Deep Impact’s scientists joined that project, too.
But Deep Impact was also a reminder that we’ve still got a lot of work to do
to protect Earth from interplanetary impacts.
The mission left a big crater on Tempel,
but it was still tiny compared to the overall size of the comet.
In fact, one scientist described it as being “like a mosquito hitting a 747.”
Comets are big, apparently.
And if we ever want to, say, direct one away from Earth…
we’ve got some stuff to figure out.
So this mission taught us a ton-- from reminding us of our place in the universe,
to showing us the pie-like consistency of a comet.
If you liked learning about this spacecraft, you might be interested to know
it’s available in pin form… for the month of July only!
That’s right, our Pin of the Month is Deep Impact,
and you can pre-order it right now at
It’ll be available to order through the end of the month,
at which point we’ll retire it and have a new pin for you guys.
So keep your eyes peeled for that!