[meteorite-list] Spitzer Spies an Odd, Tiny Asteroid (2011 MD)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 19 Jun 2014 10:46:50 -0700 (PDT)
Message-ID: <201406191746.s5JHko0Z025539_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.php?release=2014-193

Spitzer Spies an Odd, Tiny Asteroid
Jet Propulsion Laboratory
June 19, 2014

Astronomers using NASA's Spitzer Space Telescope have measured the size
of an asteroid candidate for NASA's Asteroid Redirect Mission (ARM), a
proposed spacecraft concept to capture either a small asteroid, or a
boulder from an asteroid. The near-Earth asteroid, called 2011 MD, was
found to be roughly 20 feet (6 meters) in size, and its structure
appears to contain a lot of empty space, perhaps resembling a pile of
rubble. Spitzer's infrared vision was key to sizing up the asteroid.

"From its perch up in space, Spitzer can use its heat-sensitive infrared
vision to spy asteroids and get better estimates of their sizes," said
Michael Mommert of Northern Arizona University, Flagstaff, lead author
of a new study appearing today, June 19, in the Astrophysical Journal
Letters. David Trilling, also of Northern Arizona University, leads the
team of astronomers.

The Spitzer results confirm that asteroid 2011 MD has characteristics
suitable for the ARM proposal, elevating it to the "valid candidate"
level. Valid candidates are those asteroids with the right size, mass
and rotation rate to be feasibly captured by the robotic spacecraft. Two
other valid candidates have been identified so far. (The proposal to
capture a boulder from an asteroid involves a different set of
criteria.) NASA continues to search for and find new potential
candidates using its ground-based asteroid survey programs.

Prior to the Spitzer study, the size of 2011 MD was only very roughly
known. It had been observed in visible light, but an asteroid's size
cannot be determined solely from visible-light measurements. In visible
light alone, for example, a white snowball in space could look just as
bright as a dark mountain of cosmic rock. The objects may differ in size
but reflect the same amount of sunlight, appearing equally bright.

Infrared light, on the other hand, is a better indicator of an object's
true size. This is because an object's infrared glow depends largely on
its temperature, not its reflectivity.

>From the new Spitzer data, the team was able to measure the size of
asteroid 2011 MD. When the infrared and visible-light observations were
combined, the asteroid's density and mass could also be measured. The
density of 2011 MD is remarkably low -- about the same as water, which
agrees with a separate analysis of observations taken in 2011. Since
rock is about three times more dense than water, this implies that about
two-thirds of the asteroid must be empty space.

What does an asteroid with that much empty space look like? The team
doesn't know, but proposes two possible solutions: it might be a
collection of loosely bound rocks, like a fleet of flying boulders, or a
solid rock with surrounding fine debris.

A similar "rubble-pile" type of composition was also found for asteroid
2009 BD, another valid candidate for ARM. Trilling and colleagues used
Spitzer to help pin down the size of that asteroid to roughly 10 to 13
feet (3 or 4 meters).

In both studies, Spitzer stared at the asteroids for about 20 hours.
Such long observations are scheduled more often in Spitzer's "warm"
mission, a phase that began in 2009 when the spacecraft ran out of
coolant, as planned. Spitzer, which still has two infrared channels that
operate without coolant, now specializes in longer, targeted observing
campaigns.

"With Spitzer, we have been able to get some of the first measurements
of the sizes and compositions of tiny asteroids," said Trilling. "So
far, we've looked at two asteroids and found both of them to be really
weird -- not at all like the one solid rock that we expected. We're
scratching our heads."

The team says the small asteroids probably formed as a result of
collisions between larger asteroids, but they do not understand how
their unusual structures could have come about. They plan to use Spitzer
in the future to study more of the tiny asteroids, both as possible
targets for asteroid space missions, and for a better understanding of
the many asteroid denizens making up our solar system.

Other authors of the Spitzer paper are: D. Farnocchia, P. Chodas and S.
R. Chesley of NASA's Jet Propulsion Laboratory, Pasadena, California; J.
L. Hora, G. G. Fazio and H.A. Smith of the Harvard-Smithsonian Center
for Astrophysics, Cambridge, Massachusetts; M. Mueller of the SRON
Netherlands Institute for Space Research, Netherlands; and A. W. Harris
of the DLR Institute for Planetary Research, Germany.

JPL manages the Spitzer Space Telescope mission for NASA's Science
Mission Directorate, Washington. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology in
Pasadena. Spacecraft operations are based at Lockheed Martin Space
Systems Company, Littleton, Colorado. Data are archived at the Infrared
Science Archive housed at the Infrared Processing and Analysis Center at
Caltech. Caltech manages JPL for NASA.

Through its Asteroid Initiative, NASA is developing a first-ever mission
to identify, capture and redirect a near-Earth asteroid to a stable
orbit around the moon with a robotic spacecraft. Astronauts aboard an
Orion spacecraft, launched by a Space Launch System rocket, will explore
the asteroid in the 2020s, returning to Earth with samples. Experience
in human spaceflight beyond low-Earth orbit through this Asteroid
Redirect Mission will help NASA test new systems and capabilities needed
to support future human missions to Mars. The Initiative also includes
an Asteroid Grand Challenge, which is seeking the best ideas to find all
asteroid threats to human populations and accelerate the work NASA
already is doing for planetary defense.

JPL manages the Near-Earth Object Program Office for NASA's Science
Mission Directorate in Washington. JPL is a division of the California
Institute of Technology in Pasadena.

More information about asteroids and near-Earth objects is available at:

http://neo.jpl.nasa.gov

http://www.jpl.nasa.gov/asteroidwatch/

More information about Spitzer is at:

http://spitzer.caltech.edu

http://www.nasa.gov/spitzer

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin at jpl.nasa.gov

2014-193
Received on Thu 19 Jun 2014 01:46:50 PM PDT