Arecibo Radar Image of Apophis
RESULTS OF THE STUDY
(1) Extending the "Standard Dynamical Model"
Trajectory predictions for asteroids are normally based on a standard model of the solar system
that includes the gravity of the Sun, Moon, other planets, and the three largest asteroids.
However, additional factors can influence the predicted motion in ways that depend on rarely known
details, such as the spin of the asteroid, its mass, the way it reflects and absorbs sun-light, radiates
heat, and the gravitational pull of other asteroids passing nearby. These were examined, along with
the effect of Earth's non-uniform gravity field during encounters, and limitations of the computer
hardware performing the calculations.
One would normally look for the influence of such factors as they gradually alter the trajectory over
years. But, for Apophis, the changes remain small until amplified by passage through Earth's gravity
field during the historically close approach in 2029.
For example, the team found solar energy can cause between 20 and 740 km (12 and 460 miles) of
position change over the next 22 years leading into the 2029 Earth encounter. But, only 7 years
later, the effect on Apophis' predicted position can grow to between 520,000 and 30 million km
(323,000 and 18.6 million miles; 0.0035-0.2 AU). This range makes it difficult to predict if Apophis
will even have a close encounter with Earth in 2036 when the orbital paths intersect.
It was found that small uncertainties in the masses and positions of the planets and Sun can cause
up to 23 Earth radii of prediction error for Apophis by 2036.
The standard model of the Earth as a point mass can introduce up to 2.9 Earth radii of prediction
error by 2036; at least the Earth's oblateness must be considered to predict an impact.
The gravity of other asteroids can cause up to 2.3 Earth radii of prediction uncertainty for Apophis.
By considering the range of Apophis' physical characteristics and these error sources, it was
determined what observations prior to 2029 will most effectively reduce prediction uncertainties.
Observing criteria were developed that, if satisfied, could permit eliminating the 2036 impact
possibility without further physical characterization of Apophis.
Such observations could reduce the need for a visit by an expensive spacecraft and reduce the risk
of Apophis being prematurely eliminated as a hazard under the standard model, only to drift back
into the hazard classification system years later as the smaller, unmodeled forces act upon it.
(1) Extending the "Standard Dynamical Model"
Trajectory predictions for asteroids are normally based on a standard model of the solar system
that includes the gravity of the Sun, Moon, other planets, and the three largest asteroids.
However, additional factors can influence the predicted motion in ways that depend on rarely known
details, such as the spin of the asteroid, its mass, the way it reflects and absorbs sun-light, radiates
heat, and the gravitational pull of other asteroids passing nearby. These were examined, along with
the effect of Earth's non-uniform gravity field during encounters, and limitations of the computer
hardware performing the calculations.
One would normally look for the influence of such factors as they gradually alter the trajectory over
years. But, for Apophis, the changes remain small until amplified by passage through Earth's gravity
field during the historically close approach in 2029.
For example, the team found solar energy can cause between 20 and 740 km (12 and 460 miles) of
position change over the next 22 years leading into the 2029 Earth encounter. But, only 7 years
later, the effect on Apophis' predicted position can grow to between 520,000 and 30 million km
(323,000 and 18.6 million miles; 0.0035-0.2 AU). This range makes it difficult to predict if Apophis
will even have a close encounter with Earth in 2036 when the orbital paths intersect.
It was found that small uncertainties in the masses and positions of the planets and Sun can cause
up to 23 Earth radii of prediction error for Apophis by 2036.
The standard model of the Earth as a point mass can introduce up to 2.9 Earth radii of prediction
error by 2036; at least the Earth's oblateness must be considered to predict an impact.
The gravity of other asteroids can cause up to 2.3 Earth radii of prediction uncertainty for Apophis.
By considering the range of Apophis' physical characteristics and these error sources, it was
determined what observations prior to 2029 will most effectively reduce prediction uncertainties.
Observing criteria were developed that, if satisfied, could permit eliminating the 2036 impact
possibility without further physical characterization of Apophis.
Such observations could reduce the need for a visit by an expensive spacecraft and reduce the risk
of Apophis being prematurely eliminated as a hazard under the standard model, only to drift back
into the hazard classification system years later as the smaller, unmodeled forces act upon it.
ORIGINAL ARTICLE:
INTRODUCTION
Upon its discovery in 2004, Apophis was briefly estimated to have a 2.7% chance of impacting
the Earth in 2029. Additional measurements later showed there was no impact risk at that time
from the 210-330 meter (690-1080 foot) diameter object, identified spectroscopically as an Sq
type similar to LL chondritic meteorites. However, there will be a historically close approach to
the Earth, estimated to be a 1 in 800 year event (on average, for an object of that size).
The Arecibo planetary radar telescope subsequently detected the asteroid at distances of
27-40 million km (17-25 million miles; 0.192-0.268 AU) in 2005 and 2006. Polarization ratios
indicate Apophis appears to be smoother than most NEAs at 13-cm scales. Including the high
precision radar measurements in a new orbit solution reduced the uncertainty in Apophis'
predicted location in 2029 by 98%.
While trajectory knowledge was substantially corrected by the Arecibo data, a small estimated
chance of impact (less than 1 in 45,000 using standard dynamical models) remained for April
13, 2036. With Apophis probably too close to the Sun to be measured by optical telescopes
until 2011, and too distant for useful radar measurement until 2013, the underlying physics of
Apophis' motion were considered to better understand the hazard.
INTRODUCTION
Upon its discovery in 2004, Apophis was briefly estimated to have a 2.7% chance of impacting
the Earth in 2029. Additional measurements later showed there was no impact risk at that time
from the 210-330 meter (690-1080 foot) diameter object, identified spectroscopically as an Sq
type similar to LL chondritic meteorites. However, there will be a historically close approach to
the Earth, estimated to be a 1 in 800 year event (on average, for an object of that size).
The Arecibo planetary radar telescope subsequently detected the asteroid at distances of
27-40 million km (17-25 million miles; 0.192-0.268 AU) in 2005 and 2006. Polarization ratios
indicate Apophis appears to be smoother than most NEAs at 13-cm scales. Including the high
precision radar measurements in a new orbit solution reduced the uncertainty in Apophis'
predicted location in 2029 by 98%.
While trajectory knowledge was substantially corrected by the Arecibo data, a small estimated
chance of impact (less than 1 in 45,000 using standard dynamical models) remained for April
13, 2036. With Apophis probably too close to the Sun to be measured by optical telescopes
until 2011, and too distant for useful radar measurement until 2013, the underlying physics of
Apophis' motion were considered to better understand the hazard.
RESULTS OF THE STUDY
(2) Mitigation
Mitigation was not specifically studied, but the team found small variations in the energy
absorption and reflection properties of Apophis' surface are sufficient to cause enough trajectory
change to obscure the difference between an impact and a miss in 2036. Changing the amount of
energy Apophis absorbs by half a percent as late as 2018 - for example by covering a 40 x 40
meter (130 x 130 foot) patch with lightweight reflective materials (an 8 kg payload) - can change
its position in 2036 by a minimum of one Earth radius.
A change somewhat greater than this minimum would be required to allow for prediction
uncertainties. For Apophis, scaling up to distribute 250 kg (550 pounds) of a reflective or
absorptive material (similar to the carbon fiber mesh being considered for solar sails) across the
surface could use the existing radiation forces to produce a 6-sigma trajectory change, moving at
least "99.9999998" percent of the statistically possible trajectories away from the Earth in just 18
years.
While no deflection is expected to be necessary, the team's research demonstrates that any
deflection method must produce a change known in advance to be greater than all the error
sources in the prediction, including some greater than those considered with the standard model.
(3) Impact probability
The study did NOT compute new impact probabilities. This is because important physical
parameters (such as mass and spin pole) that affect its trajectory have not yet been measured
and hence there are no associated probability distributions. The study characterizes how the
Standard Dynamical Model can over or under-estimate impact probability for those objects having
close planetary encounters prior to the potential impact.
The situation is similar to having 6 apples (the measured Apophis parameters) and 6 boxes whose
contents are unknown (the unmeasured Apophis parameters), then trying to compute the
probability one has a total of 12 apples (impact probability). The result reflects back what is
assumed about the unknown contents of the boxes, but doesn't reveal new information. The
contents of the boxes must be observed (measured) to learn something new.
For similar reasons, the Apophis study instead uses the minimum and maximum range-of-effect in
place of computing impact probabilities to provide reasonable criteria for excluding impact in the
absence of detailed physical knowledge, once new position measurements are obtained at six key
times.
(4) Non-Apophis Conclusions
Aspects of the study relevant to asteroids other than Apophis:
* The Standard Dynamical Model can misestimate impact risk for the more numerous sub-km
objects preceded by close planetary encounter(s). This problem might be addressed by
reassessing impact potential after planetary encounters, given new measurements.
* The minimum-maximum effect of unmeasured parameters can provide enough information to
exclude threats in certain cases, even if a realistic impact probability cannot be computed.
* Amplification of small trajectory offsets makes valid prediction across a close-encounter difficult
without physical knowledge, but offers the potential to redirect the entire uncertainty region and
has significant implications for costly spacecraft missions.
* A deflection effort must be known in advance to produce change greater than predicted
uncertainties due to ALL parameters, not only the Standard Dynamical Model. For example, if a
method produces 10 Earth-radii of change, but prediction uncertainties from all sources are 20
Earth-radii, the deflection would move the asteroid around within the noise, producing an
unpredicted result or even a new hazard.
CREDITS
In addition to Giorgini, co-authors of the report include Dr. Lance A. M. Benner and Dr. Steven J. Ostro of JPL; Dr.
Michael C. Nolan, Arecibo Observatory, Puerto Rico, and Michael W. Busch of the California Institute of
Technology.
Arecibo Observatory is operated by Cornell University under a cooperative agreement with the National Science
Foundation. JPL is managed for NASA by the California Institute of Technology in Pasadena.
(2) Mitigation
Mitigation was not specifically studied, but the team found small variations in the energy
absorption and reflection properties of Apophis' surface are sufficient to cause enough trajectory
change to obscure the difference between an impact and a miss in 2036. Changing the amount of
energy Apophis absorbs by half a percent as late as 2018 - for example by covering a 40 x 40
meter (130 x 130 foot) patch with lightweight reflective materials (an 8 kg payload) - can change
its position in 2036 by a minimum of one Earth radius.
A change somewhat greater than this minimum would be required to allow for prediction
uncertainties. For Apophis, scaling up to distribute 250 kg (550 pounds) of a reflective or
absorptive material (similar to the carbon fiber mesh being considered for solar sails) across the
surface could use the existing radiation forces to produce a 6-sigma trajectory change, moving at
least "99.9999998" percent of the statistically possible trajectories away from the Earth in just 18
years.
While no deflection is expected to be necessary, the team's research demonstrates that any
deflection method must produce a change known in advance to be greater than all the error
sources in the prediction, including some greater than those considered with the standard model.
(3) Impact probability
The study did NOT compute new impact probabilities. This is because important physical
parameters (such as mass and spin pole) that affect its trajectory have not yet been measured
and hence there are no associated probability distributions. The study characterizes how the
Standard Dynamical Model can over or under-estimate impact probability for those objects having
close planetary encounters prior to the potential impact.
The situation is similar to having 6 apples (the measured Apophis parameters) and 6 boxes whose
contents are unknown (the unmeasured Apophis parameters), then trying to compute the
probability one has a total of 12 apples (impact probability). The result reflects back what is
assumed about the unknown contents of the boxes, but doesn't reveal new information. The
contents of the boxes must be observed (measured) to learn something new.
For similar reasons, the Apophis study instead uses the minimum and maximum range-of-effect in
place of computing impact probabilities to provide reasonable criteria for excluding impact in the
absence of detailed physical knowledge, once new position measurements are obtained at six key
times.
(4) Non-Apophis Conclusions
Aspects of the study relevant to asteroids other than Apophis:
* The Standard Dynamical Model can misestimate impact risk for the more numerous sub-km
objects preceded by close planetary encounter(s). This problem might be addressed by
reassessing impact potential after planetary encounters, given new measurements.
* The minimum-maximum effect of unmeasured parameters can provide enough information to
exclude threats in certain cases, even if a realistic impact probability cannot be computed.
* Amplification of small trajectory offsets makes valid prediction across a close-encounter difficult
without physical knowledge, but offers the potential to redirect the entire uncertainty region and
has significant implications for costly spacecraft missions.
* A deflection effort must be known in advance to produce change greater than predicted
uncertainties due to ALL parameters, not only the Standard Dynamical Model. For example, if a
method produces 10 Earth-radii of change, but prediction uncertainties from all sources are 20
Earth-radii, the deflection would move the asteroid around within the noise, producing an
unpredicted result or even a new hazard.
CREDITS
In addition to Giorgini, co-authors of the report include Dr. Lance A. M. Benner and Dr. Steven J. Ostro of JPL; Dr.
Michael C. Nolan, Arecibo Observatory, Puerto Rico, and Michael W. Busch of the California Institute of
Technology.
Arecibo Observatory is operated by Cornell University under a cooperative agreement with the National Science
Foundation. JPL is managed for NASA by the California Institute of Technology in Pasadena.
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| Apophis |
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| News on October 07, 2009 After recalculating their figures, JPL scientists downgrade the odds of Apophis hitting the planet to four in a million (or one-in-250,000) chance of a collision. "We've all but ruled out" a collision in 2036, said Steve Chesley, an astronomer with the Near-Earth Object office at JPL. [Source 2036 asteroid strike on Earth 'all but ruled out' Los Angeles Times, October 7, 2009] |
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