[meteorite-list] NASA Brings Out the Big Gun for Asteroid Impact Science

[Ron Baalke]

http://www.wired.com/wiredscience/2013/08/a-scientist-and-his-gun/all/

NASA Brings Out the Big Gun for Asteroid Impact Science
By Adam Mann
Wired Magazine
August 19, 2013
	
MOUNTAIN VIEW, Calif. - Just before he gets ready to fire a projectile 
down the 14-foot barrel of a vertical gun, planetary scientist Peter Schultz 
turns to me and gives an apologetic smile.

"There's something you have to do," he says, as his graduate student snickers. 
"You have to assume the Gault position."

The Gault position, it turns out, involves crossing your index finger 
over your middle, your ring finger over your pinkie, then crossing your 
two arms over one another and finally crossing your legs (while standing). 
Schultz assumes it, explaining that it serves as a good luck measure, 
as does his graduate student and the other engineers in the gun control 
room. I comply, as does WIRED photographer Ariel Zambelich.

"We're armed," someone calls. "Voltage looks good." A klaxon buzzes and, 
seconds later, there's the sound of a powerful explosion from the next 
room over. A burst of flame and sand appears on the computer screen in 
front of us and, just like that, the NASA Ames Vertical Gun range has 
provided a new data point for science.

The gun is a fantastic tool for studying the effects of meteorite impacts 
on different places in the solar system. You see, Earth is something of 
an anomaly. Most other rocky bodies are covered in countless craters ranging 
from the size of continents down to the size of sand grains. The active 
tectonics of our planet recycle its crust, erasing the long-term scars 
that come from living in a solar system full of debris. But just about 
every other terrestrial planet, moon, asteroid, and comet is coated in 
pockmarks, a testament to how pervasive and important impacts have been 
in our solar system's history.

Over the course of its nearly 50-year career, the gun range been used 
to figure out why the scars of an impact look different on Mars than they 
do on Venus. It has helped explain how the man on the moon could have 
gotten his face. And it has provided key data for many NASA missions, 
in particular the Deep Impact spacecraft, which shot a projectile into 
an asteroid.

Peter Schultz, who teaches geoscience at Brown University, has done much 
of this research. He's worked at the gun range for 33 years, becoming 
its principal investigator in 2012, and he knows a great deal about its 
history and lore.

Though it's called a gun, the facility doesn't look much like any firearm 
you've ever seen. The main chassis is a long metal barrel as thick as 
a cannon mounted on an enormous red pole that forks at the end into two 
legs. The red pole was once used to hold MIM-14 Nike-Hercules missiles 
that served as an anti-ballistic defense against Soviet nuclear warheads, 
Schultz explains. This complex is pointed at a huge rotund cylinder and 
can be moved up and down in 15-degree increments to simulate a meteorite 
strike at different angles. The entire machine is housed in a 3-story 
industrial building here at NASA's Ames campus.

At the far end of the barrel, a gunpowder explosion is used to compress 
hydrogen gas to as much as 1 million times atmospheric pressure. The compressed 
gas gets released and sent down the launch tube, firing a projectile pellet 
at speeds between 7,000 and 15,000 mph. The shot enters the cylinder, 
in which low pressure or even a vacuum is maintained, and hits a dish 
filled with different material that simulates whatever planetary body 
researchers are studying. High-speed cameras mounted on windows around 
the cylinder record the impact aftermath at up to 1 million frames per 
second.

The origin of both the facility and the odd position I was compelled to 
take stem from planetologist Donald Gault, who designed and used the range 
to study impacts on the moon. Built in 1965, the gun range helped interpret 
information returned from the Ranger probes, which crashed into the lunar 
surface during the Apollo era. Scientists weren't sure of the exact composition 
of the regolith at the time and needed to know before attempting to land 
people there.

"There were reports at the time that it was going to be really, really 
fluffy," said Schultz. "There was one document that said the astronauts 
would land and then sink out of sight."

Using data from the gun, Gault helped figure out that the Apollo astronauts 
weren't going to die by lunar quicksand. After NASA finished its goal 
of safely landing and returning astronauts, Gault continued using the 
gun range to study the formation of craters on the moon. When he retired, 
NASA planned to mothball the gun but an outcry from the planetary science 
community re-opened the firing range as a national facility. It was during 
this time that Schultz, who had worked with Gault as a post-doc, was hired 
to take over as science coordinator for the gun range.

The day WIRED visited the gun, Schultz and his graduate student, Stephanie 
Quintana, were simulating meteorite impacts on Mars. Inside the facility's 
vacuum chamber was a large gray dish full of dolomite powder, standing 
in for the Martian surface.

Schultz and Quintana were investigating how a meteorite explosion could 
create a dust and vapor shockwave that would form a vortex with speeds 
three to four times that of a tornado, inflicting serious damage. The 
researchers had already used satellite images to identify telltale scars 
(.pdf) around real impact craters on Mars. Though they had some ideas, 
how exactly these frozen wind streaks formed remained a mystery.

Schultz explained that they would be firing a quarter-inch Styrofoam pellet 
into the dolomite powder and watch the ensuing outburst. He's easy to 
talk to, genial, energetic, and quick to divulge interesting tidbits of 
information on meteorite impacts that reveal his breadth of knowledge 
on the topic.

"The situation on Mars is totally different from what would happen on 
Venus," he said. The thin Martian atmosphere allows for ejecta from an 
impact to spread out far and wide in all directions. But Venus' crushing 
atmospheric pressure holds in the vapor, preventing it from expanding 
and acting "like a pressure cooker," he said. When a meteorite hits Venus, 
the dust and debris condenses under the pressure and rains down as molten 
silica which then flows out from the crater, creating long and beautiful 
deposits that trail away from the impact site.

In the middle of this impromptu interplanetary impact comparison course, 
another one of Schultz's students, Megan Bruck Syal, tells him that data 
from one of their instruments is in. It's the spectrometer, which they 
will use to analyze the ball of gas and vapor created during their simulated 
Mars surface impact.

"Oh, you got it!" Schultz said, rubbing his hands together like a kid 
expecting candy. He glances at the spectra, whoops, and then sings a few 
bars of "We're in the money.' "Hot damn," he said. "Those are nice and 
sharp."

It's clear that Schultz brings this same passion for scientific discovery 
into every experiment he does. He explains one test he conducted years 
ago in which he fabricated transparent spheres and then shot a projectile 
into them to watch how a shockwave evolves inside a planetary body.

The interesting twist came when he simulated meteorite coming in at an 
angle to the surface, a process known as an oblique impact. With a high-speed 
camera, Schultz watched how the shockwave from an impact hitting at a 
tangent of around 30 degrees propagated forward. The vibrations spread 
out from the initial impact site and then converged on the other side 
of the sphere, but not directly opposite the crater.

"I applied this to understanding how you make the man in the moon," he 
said.

On the lunar far side is one of the biggest impact craters in the solar 
system, the South Pole Aitken Basin, which would stretch halfway across 
the U.S. if it were on Earth. Schultz has suggested that the enormous 
rock that hit the moon billions of years ago to form that crater may have 
come in at an oblique angle.

Using computer models, he calculated that the shockwave could have circled 
around to the moon's near side, causing a 10-minute tremor. Cracks would 
have appeared in the surface, opening and closing, and cracking again. 
This could have created something like a pump that allowed magma to rise 
to the lunar surface, which erupted as lava that covered huge areas known 
as the Mare Imbrium and Oceanus Procellarum, major nearside features that 
humans have gazed at for millennia.

A room stationed in the same building as the gun range has shelves piled 
high with miscellania, resembling something like a garage for the facility. 
In here, Schultz showed me some results from his previous experiments. 
A thick flat block of aluminum features a mighty divot. It's basically 
an impact crater than you can hold in your hand, and it was amazing to 
see the details - the depressed crater floor, the raised rim, the bright 
rays extending backwards from the impact site.

You quickly get the sense that Schultz has fun with many of his experiments. 
He showed me a high-speed video from an impact that simulated the explosion 
at Chicxulub crater 65 million years ago, ending the reign of the dinosaurs. 
Small plastic dino toys had been set around the impacting dish. The film 
showed the wave of debris rising up and expanding out past the toys.

"Oh no! Nooo," he said in a high-pitched moan, giving voice to the plastic 
dinos experiencing the brunt of this explosion.

Apart from playtime, it's actually this curtain of dust emanating from 
an impact site that gives Schultz a lot of his information. The Ames gun 
range was critical in interpreting the results from NASA's Deep Impact 
mission, which shot a projectile into the surface of comet Tempel 1 in 
2005 and photographed the ensuing plume.

Schultz used the gun range to conduct many experiments simulating different 
scenarios that could have arisen based on the comet's composition. When 
the first pictures from Deep Impact were beamed back to Earth, he was 
ready, even though scientists had a lot of difficulty looking through 
the debris to see the impact site. Some of his prior experiments predicted 
that the plume would have "an upside-down lampshade type appearance, then 
it would have a vertical column," said Schultz.

"And that's what we saw," he said. "We knew that if the comet had a very 
low but specified density, it would affect the way the material came out 
of the crater."

Deep Impact showed that Tempel 1 was much drier and dustier than scientists 
had previously believed. Researchers were able to interpret the results 
so quickly because of extensive experiments with the gun.

The range has few rivals in the world of ballistics. Ames maintains two 
other facilities, the Hypervelocity Free-Flight Aerodynamic Facility, 
used to test vehicle reentry into the atmosphere, and the Electric Arc 
Shock Tube Facility, which conducts experiments on radiation. A few newer 
gun ranges have been built at other laboratories in recent years but none 
have the large chamber and fast firing speeds of this one.

Considering that it's a half-century-old piece of technology, I asked 
Schultz if the gun range might ever be replaced. Advances in computer 
speeds and processors have made modeling very complex phenomena much easier 
to do in digital form. He looked thoughtful for a moment.

"I don't think so,' he finally said. 'When you do an impact, you have 
complexities at all scales. We're seeing things at one-hundredth of the 
diameter of the projectile, and I don't think you can do that in a computer 
and get all the large scale stuff at the same time."

"Part of my joy is finding things that the computers can't do,' he said 
smiling. 'What's cool is that every time we fire we're always doing something 
a little different. So it's exciting to see what happens."

"I've got to see the sparks fly,' he said. 'It never gets old, it just 
never gets old."