OSIRIS-REx Scientists Measure Yarkovsky Effect



May 24, 2012

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OSIRIS-REx Scientists Measure Yarkovsky Effect

Scientists with the UA-led asteroid sample return mission, OSIRIS-REx,
have measured the mass and orbit of their destination asteroid, 1999 RQ36,
with great accuracy.

Scientists with the University of Arizona-led asteroid sample return
mission OSIRIS-REx have measured the orbit of their destination asteroid,
1999 RQ36, with such accuracy they were able to directly determine the
drift resulting from a subtle but important force called the Yarkovsky
effect ­ the slight push created when the asteroid absorbs sunlight and
re-emits that energy as heat.

The new orbit for the half-kilometer (one-third mile) diameter 1999 RQ36
is the most precise asteroid orbit ever obtained, OSIRIS-REx team member
Steven Chesley of the NASA Jet Propulsion Laboratory said. He presented
the findings May 19 at the Asteroids, Comets and Meteors 2012 meeting in
Niigata, Japan.

Remarkable observations that Michael Nolan at Arecibo Observatory in
Puerto Rico made in September, along with Arecibo and Goldstone radar
observations made in 1999 and 2005, when 1999 RQ36 passed much closer to
Earth, show that the asteroid has deviated from its gravity-ruled orbit by
roughly 100 miles, or 160 kilometers, in the last 12 years, a deviation
caused by the Yarkovsky effect.

The Yarkovsky effect is named for the 19th-century Russian engineer who
first proposed the idea that a small rocky space object would, over long
periods of time, be noticeably nudged in its orbit by the slight push
created when it absorbs sunlight and then re-emits that energy as heat.

The effect is difficult to measure because it's so infinitesimally small,
Chesley said.

"The Yarkovsky force on 1999 RQ36 at its peak, when the asteroid is
nearest the sun, is only about a half-ounce ­ about the weight of three
grapes on Earth. Meanwhile, the mass of the asteroid is estimated to be
about 68 million tons. You need extremely precise measurements over a
fairly long time span to see something so slight acting on something so

Nolan, who obtained his doctorate at the UA, succeeded in a heroic effort
to get a 16-ton power supply for the transmitter from Pennsylvania to
Puerto Rico in six days in time for the observations, which he made on
three separate nights last September. Nolan and his team measured the
distance between the Arecibo Observatory and 1999 RQ36 to an accuracy of
300 meters, or about one-fifth of a mile, when the asteroid was 30 million
kilometers, or 20 million miles, from Earth.

"That's like measuring the distance between New York City and Los Angeles
to an accuracy of 2 inches, and fine enough that we have to take the size
of the asteroid and of Arecibo Observatory into account when making the
measurements," Nolan said.

Chesley and his colleagues used the new Arecibo measurements to calculate
a series of 1999 RQ36 approaches closer to Earth than 7.5 million
kilometers (4.6 million miles) from the years 1654 to 2135. There turned
out to be 11 such encounters.

In 2135, the 500-meter (1,640-foot) diameter asteroid will swing by Earth
at around 350,000 kilometers (220,000 miles), its closest approach over
the 481-year time span. That's closer than the moon, which orbits about
390,000 kilometers (240,000 miles) from Earth. At such close distances,
the asteroid's subsequent trajectory becomes impossible to accurately
predict so close approaches can only be studied statistically, Chesley

"The new results don't really change what is qualitatively known about the
probability of future impacts," Chesley said. "The odds of this
potentially hazardous asteroid colliding with Earth late in the 22nd
century are still calculated to be about one in a few thousand."

But the new results do sharpen the picture of how potentially hazardous
1999 RQ36 could be farther into the future. Scientists now have identified
many low-probability potential impacts in the 2170s through the 2190s
while ruling out others, Chesley said.

"OSIRIS-REx science team members Steve Chesley and Mike Nolan have
achieved a spectacular result with this investigation," said Dante
Lauretta, the mission's principal investigator and professor of planetary
science at the UA. "This study is an important step in better
understanding the Yarkovsky effect ­ a subtle force that contributes to
the orbital evolution of new near-Earth objects."

Lauretta added that "this information is critical for assessing the
likelihood of an impact from our target asteroid and provides important
constraints on its mass and density, allowing us to substantially improve
our mission design."

The final piece to the puzzle was provided by the University of
Tennessee¹s Josh Emery, who used NASA¹s Spitzer Space Telescope in 2007 to
study the space rock's thermal characteristics. Emery¹s measurements of
the infrared emissions from 1999 RQ36 allowed him to derive the object¹s

From there he was able to determine the degree to which the asteroid is
covered by an insulating blanket of fine material, which is a key factor
for the Yarkovsky effect.

With the space rock's orbit, size, thermal properties and propulsive force
(Yarkovsky effect) understood, Chesley could perform the space rock
scientist equivalent of solving for "x" and calculate its bulk density.

³1999 RQ36 has about the same density as water, and so it¹s very light for
its size,² said Chesley. ³This means that it¹s more than likely a very
porous jumble of rocks and dust.²

Asteroid 1999 RQ36 is of particular interest to NASA as it is the target
of the agency's OSIRIS-REx (Origins, Spectral Interpretation, Resource
Identification, Security, Regolith Explorer) mission. Scheduled for launch
in 2016, ORIRIS-Rex will visit 1999 RQ36, collect samples from the
asteroid and return them to Earth.

NASA detects, tracks and characterizes asteroids and comets passing 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 establishes their orbits to
determine if any could be potentially hazardous to our planet.

Finding the bulk density of a solitary space object by combining radar
tracking and infrared observations might once have seemed almost science
fiction, Chesley said.

What OSIRIS-REx scientists are beginning to learn about Yarkovsky drift
strengthens the idea that "the Yarkovsky effect can be used to probe the
physical properties of asteroids that we can't visit with spacecraft," he

OSIRIS-REx stands for "Origins, Spectral Interpretation, Resource
Identification, Security, Regolith Explorer." The OSIRIS-REx spacecraft is
to launch in 2016, reach asteroid (101955) 1999 RQ36 in 2019, examine it
up close during a 505-day rendezvous, then return at least 60 grams of it
to Earth in 2023. More information can be found on the mission's website,
http://osiris-rex.lpl.arizona.edu .

The OSIRIS-REx mission is a project of NASA's New Frontiers Program
managed by NASA's Marshall Space Flight Center in Huntsville, Al., for
NASA's Science Mission Directorate in Washington. The Lunar and Planetary
Laboratory at the UA leads the science mission. NASA's Goddard Space
Flight Center in Greenbelt, Md., is responsible for overall mission
management. Lockheed Martin will build and operate the spacecraft.

# # #

OSIRIS-REx mission: http://osiris-rex.lpl.arizona.edu
UA Lunar and Planetary Laboratory: http://www.lpl.arizona.edu

Dante Lauretta, Principal Investigator, OSIRIS-REx mission
UA Lunar and Planetary Laboratory

Daniel Stolte
University Communications
The University of Arizona