There are more than 1,300 "Potentially Hazardous Asteroids" (PHAs) -- objects at least 150 yards (about 140 meters)
across with a very small chance of impacting us someday because their orbital paths take them close to Earth's orbit,
according to NASA's NEO program.
"Asteroids move at an average of 12 to 15 kilometers per second (about 27,000 to 33,000 miles per hour) relative to Earth,
so fast that they carry enormous energy by virtue of their velocity," says Edward Beshore of the University of Arizona,
Tucson, deputy principal investigator for NASA's OSIRIS-REx asteroid sample return mission.
"Anything over a few hundred yards across that appears to be on a collision course with Earth is very worrisome."
The main difficulty is obtaining sufficient observations to be able to predict their orbits with enough certainty to
find out if they could hit us at some point.
The OSIRIS-REx spacecraft will travel to a near-Earth carbonaceous
asteroid (101955) 1999 RQ36, study it in detail, and bring back a sample (at least 60 grams or 2.1 ounces) to Earth.
"When an asteroid makes a close pass to Earth, the gravitational pull from our planet changes the asteroid's orbit,"
says Beshore. "However, how this change will affect the evolution of the asteroid's orbit is difficult for us to
predict because there are also other small forces continuously acting on the asteroid to change its orbit.
The most significant of these smaller forces is the Yarkovsky effect -- a minute push on an asteroid that happens
when it is warmed up by the sun and then later re-radiates this heat in a different direction as infrared radiation."
"A brick building can feel warm even in the early evening hours, because it is radiating away the heat accumulated
from an entire day of sunshine," says Beshore. In the same way, an asteroid radiates most of its heat from its late
"afternoon side," giving it the small Yarkovsky push which is variable depending mostly on the asteroid's size,
shape and composition.
NASA's OSIRIS-REx mission (Origins, Spectral Interpretation, Resource Identification, Security, and Regolith
Explorer) will make the most precise measurements of the Yarkovsky effect to date by visiting a PHA called "1999
RQ36" or just "RQ36."
"For such a large object, it has one of the highest known probabilities of impacting Earth, a 1 in 2,400 chance
late in the 22nd century, according to calculations by Steve Chesley, an astronomer at NASA's Jet Propulsion Laboratory,"
said Beshore. RQ36 is about 457 meters (500 yards) across.
OSIRIS-REx - which is scheduled for launch in 2016, will arrive at RQ36 in 2018 and orbit the asteroid until 2021.
By communicating continuously with a spacecraft in orbit around RQ36, the team will get a much better idea of the a
"We expect OSIRIS-REx will enable us to make an estimate of the Yarkovsky force on RQ36 at least twice as precise
as what's available now," says Jason Dworkin, OSIRIS-REx project scientist at NASA's Goddard Space Flight Center
in Greenbelt, Md.
Click on image to enlarge
This is an artist's concept of the OSIRIS-REx spacecraft near asteroid 1999 RQ36. Credit: NASA Goddard Space Flight Center
The team will use what it learns about the Yarkovsky effect on RQ36 to help estimate the effects on other asteroids.
"What we want to be able to do is create a model that says okay if you give me an asteroid of this size,
made of this composition, with this kind of topography, I can estimate for you what the Yarkovsky effect will be,"
This is an artist's concept of the OSIRIS-REx sample collector, or "tag head," being deployed to
collect a sample of asteroid 1999 RQ36. Credit: NASA Goddard Space Flight Center
"So now I can probably come up with a better notion of what to expect from other asteroids that I don't have
the good fortune to have a spacecraft around."
"There are several mitigation strategies," says Beshore. "We could explode a small nuclear device close above
the surface on one side of the asteroid. This could be very effective -- it would vaporize the surface layer,
which would then fly off at very high speed, causing a rocket thrust that would shove everything over by a few
centimeters per second. This might be plenty to deflect the asteroid. Other strategies include kinetic impactors,
where you strike an asteroid very hard with a heavy projectile moving at high speed.
NASA | OSIRIS-REx Targets NEO
"In 2005, NASA's Deep Impact mission hit comet Tempel 1 with a 370-kilogram (over 815-pound) copper slug at about
five kilometers per second (over 11,000 miles per hour), not nearly enough to significantly alter the orbit of the
five-kilometer-sized body, but a proof of the technology for this kind of mission. Another idea is to use a gravity
tractor -- station a spacecraft precisely enough near the asteroid which would gradually deflect it with only its
"We don't have a lot of experience doing precise maneuvers near one of these objects with very small gravity,"
says Beshore. "It's not easy to stay in orbit around it -- this asteroid's gravitational pull is so weak, the
push from sunlight on our spacecraft's solar panels will be roughly similar to the amount of force from the gravity
of RQ36 itself. "
"We have to factor in a lot of these forces to navigate and operate around an asteroid. With OSIRIS-REx, we'll generate
a set of techniques and technologies for any mission that would go to an asteroid to characterize it in advance of a mitigation mission."