[meteorite-list] How to Target an Asteroid

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 17 Apr 2013 09:07:30 -0700 (PDT)
Message-ID: <201304171607.r3HG7UNL017159_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.php?release=2013-138

How to Target an Asteroid
Jet Propulsion Laboratory
April 16, 2013

Like many of his colleagues at NASA's Jet Propulsion Laboratory,
Pasadena, Calif., Shyam Bhaskaran is working a lot with asteroids these
days. And also like many of his colleagues, the deep space navigator
devotes a great deal of time to crafting, and contemplating,
computer-generated 3-D models of these intriguing nomads of the solar
system.

But while many of his coworkers are calculating asteroids' past, present
and future locations in the cosmos, zapping them with the world's most
massive radar dishes, or considering how to rendezvous and perhaps even
gently nudge an asteroid into lunar orbit, Bhaskaran thinks about how to
collide with one.

"If you want to see below the surface of an asteroid, there's no better
way than smacking it hard," said Bhaskaran. "But it's not that easy.
Hitting an asteroid with a spacecraft traveling at hypervelocity is like
shooting an arrow at a target on a speeding race car."

The term hypervelocity usually refers to something traveling at very
high speed -- two miles per second (6,700 mph / 11,000 kilometers per
hour) or above. Bhaskaran's hypothetical impacts tend to be well above.

"Most of the hypervelocity impact scenarios that I simulate have
spacecraft/asteroid closure rates of around eight miles a second, 30,000
miles per hour [about 48,000 kilometers per hour]," said Bhaskaran.

In the majority of our solar system, where yield signs and "right of
way" statutes have yet to find widespread support, hypervelocity impacts
between objects happen all the time. But all that primordial violence
usually goes unnoticed here on Earth, and almost never receives
scientific scrutiny.

"High-speed impacts on asteroids can tell you so many things that we
want to know about asteroids," said Steve Chesley, a near-Earth object
scientist at JPL. "They can tell you about their composition and their
structural integrity -- which is how they hold themselves together.
These are things that are not only vital for scientific research on the
origins of the solar system, but also for mission designers working on
ways to potentially move asteroids, either for exploitation purposes or
because they may be hazardous to Earth."

Hypervelocity impacts by spacecraft are not just a hypothetical
exercise. Scientists have taken the opportunity to analyze data from
used spacecraft and rocket stages that have impacted the moon and other
celestial bodies since the Apollo program. On July 4, 2005, NASA's Deep
Impact spacecraft successfully collided its dynamic impactor with comet
9P/Tempel 1 -- it was the first hypervelocity impact of a primitive
solar system body.

Bhaskaran, who was a navigator on Deep Impact, would be the first to
tell you that not all hypervelocity impacts are created equal.
"Impacting an asteroid presents slightly different challenges than
impacting a comet," said Bhaskaran. "Comets can have jets firing
material into space, which can upset your imaging and guidance systems,
while potential asteroid targets can be as small as 50 meters [164
feeet] and have their own mini-moons orbiting them. Since they're small
and dim, they can be harder to spot."

Along with the size of the celestial body being targeted, Bhaskaran also
has to take into account its orbit, targeting errors, how hard an impact
the scientists want, and even the shape.

"Asteroids hardly ever resemble perfect spheroids," said Bhaskaran.
"What you've got floating around out there are a bunch of massive
objects that look like peanuts, potatoes, diamonds, boomerangs and even
dog bones -- and if the spacecraft's guidance system can't figure out
where it needs to go, you can hit the wrong part of the asteroid, or
much worse, miss it entirely."

The guidance system Bhaskaran is referring to is called "AutoNav," which
stands for Autonomous Navigation. To reach out and touch something that
could be halfway across the solar system and traveling at hypervelocity
requires a fast-thinking and fast-maneuvering spacecraft. It is a
problem that even the speed of light cannot cure. "When it comes to
these high-speed impact scenarios, the best info you get on where you
are and where you need to be comes very late in the game," said
Bhaskaran. "That's why the last few hours before impact are so critical.
We need to execute some final rocket burns, called Impactor Targeting
Maneuvers (ITMs), quickly. With Earth so far away, there is no chance to
send new commands in time.

"So, instead, we have AutoNav do the job for us. It is essentially a
cyber-astronaut that takes in all the pertinent information, makes its
own decisions and performs the actions necessary to make sure we go
splat where we want to go splat."

Currently, Bhaskaran is running simulations that make his virtual
impactor go splat against the furrowed, organic-rich regolith of
asteroid 1999 RQ36. The 1,600-foot-wide (500-meter-wide) space rock is
the target of a proposed JPL mission called the Impactor for Surface and
Interior Science (ISIS). The impactor spacecraft, which looks a little
like a rocket-powered wedding ring, would hitch a free ride into space
aboard the rocket carrying NASA's InSight mission to Mars. The
impactor's trajectory would then loop around Mars and bear down on RQ36.

"One of the things that helps me sleep at night is that we know a lot
about RQ36 because it is the target of another NASA mission called
OSIRIS-REx," said Bhaskaran. "But it also provides some challenges
because the scientists want us to hit the asteroid at a certain moment
in time and at a certain location, so that the OSIRIS-REx spacecraft can
be sure to monitor the results from a safe vantage point. It is a
challenge but it's also really exciting."

The part of the ISIS mission Bhaskaran is most interested in is what
happens after our rocket-festooned, cyber-hero rounds Mars and begins to
close the distance with the asteroid at a speed of 8.4 miles per second
(49,000 kilometers per hour). Over the next several months, the mission
navigators would plan and execute several deep space maneuvers that
refine the spacecraft's approach. Then, with only two hours to go,
AutoNav would take over to make the final orbital changes.

"AutoNav's imaging system and its orbit determination algorithms will
detect the asteroid and compute its location in space relative to the
impactor," said Bhaskaran. "Without waiting to hear from us, it will
plan for and execute three ITMs at 90 minutes, 30 minutes and then three
minutes out. That last rocket firing will occur when the asteroid is
only 1,500 miles [2,400 kilometers] away. Three minutes later, if all
goes according to plan, the spacecraft hits like a ton of bricks."

While Bhaskaran loves ISIS for the navigation challenge it provides, the
proposed mission's principal investigator likes what the
out-of-this-world equivalent of the release of nine tons of TNT does to
the surface -- and interior -- of an asteroid.

"We expect the crater excavated by the impact of ISIS could be around
100 feet across," said Chesley. "From its catbird seat in orbit around
the asteroid, OSIRIS-REx, at its leisure, would not only be able to
determine how big a hole there is, but also analyze the material thrown
out during the impact."

The data would not only provide information on what makes up the
asteroid, but how its orbit reacts to being hit by a NASA spacecraft.

"While the effect of ISIS on the orbit of asteroid 1999 RQ36 will be
miniscule, it will be measurable," said Chesley. "Once we know how its
orbit changes, no matter how small, we can make better assessments and
plans to change some future asteroid's orbit if we ever need to do so.
Of course, to get all these great leaps forward in understanding, we
have to hit it in the first place."

Which leads us back to Bhaskaran and his hard drive laden full of
hypervelocity impact simulations.

"We have confidence that whenever called upon, AutoNav will do its job,"
said Bhaskaran. "The trick is, we just don't tell AutoNav it's a one-way
trip."

Bhaskaran will present his latest findings on guidance for hypervelocity
impacts on Tuesday, April 16, at the International Academy of
Astronautics' Planetary Defense Conference in Flagstaff, Ariz.

NASA detects, tracks and characterizes asteroids and comets passing
relatively close to Earth using both ground- and space-based telescopes.
The Near-Earth Object Observations Program, commonly called
"Spaceguard," discovers these objects, characterizes a subset of them,
and predicts their paths to determine if any could be potentially
hazardous to our planet.

JPL manages the Near-Earth Object Program Office for NASA's Science
Mission Directorate in Washington. Steve Chesley of JPL is leading the
Impactor for Surface and Interior Science (ISIS) mission proposal. JPL
is a division of the California Institute of Technology in Pasadena.
NASA's Goddard Space Flight Center, Greenbelt, Md., manages the
OSIRIS-Rex project.

More information about asteroids and near-Earth objects is at:
http://www.jpl.nasa.gov/asteroidwatch , and on Twitter: _at_asteroidwatch .

DC Agle 818-393-9011
Jet Propulsion Laboratory, Pasadena, Calif.
agle at jpl.nasa.gov

2013-138
Received on Wed 17 Apr 2013 12:07:30 PM PDT