[meteorite-list] Predicting Apophis' Earth Encounters in 2029 and 2036

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 20 Nov 2007 14:03:14 -0800 (PST)
Message-ID: <200711202203.OAA03521_at_zagami.jpl.nasa.gov>

http://neo.jpl.nasa.gov/apophis

Predicting Apophis' Earth Encounters in 2029 and 2036

SUMMARY

Researchers at NASA/JPL, Caltech, and Arecibo Observatory have released
the results of radar observations of the potentially hazardous asteroid
99942 Apophis, along with an in-depth analysis of its motion. The
research will affect how and when scientists measure, predict, or
consider modifying the asteroid's motion. The paper
<http://neo.jpl.nasa.gov/apophis/Apophis_CORRECTED_PREPRINT.pdf> has
been accepted for publication in the science journal "Icarus
<http://dx.doi.org/10.1016/j.icarus.2007.09.012>" and was presented at
the AAS/DPS conference in Orlando, Florida in October of 2007. The
Apophis study was led by Jon Giorgini, a senior analyst in JPL's Solar
System Dynamics group and member of the radar team that observed Apophis.

The analysis of Apophis previews situations likely to be encountered
with NEAs yet to be discovered: a close approach that is not dangerous
(like Apophis in 2029) nonetheless close enough to obscure the proximity
and the danger of a later approach (like Apophis in 2036) by amplifying
trajectory prediction uncertainties caused by difficult-to-observe
physical characteristics interacting with solar radiation as well as
other factors.

BACKGROUND

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.

[Arecibo Radar Image of Apophis]

[Apophis Position Uncertainty]

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

(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.

[Present era through 2029]

[Small factors 2029-2036]

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.

(2) Mitigation

Mitigation was not specifically studied, but the team found small
variations in the surface properties of Apophis 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 - can change its
position in 2036 by at least one Earth radius.

[Apophis Trajectory Change]

For Apophis, distributing as little as 250 kg (550 pounds) of material
(such as carbon fiber mesh) across the surface could 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 result 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 key
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 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.
    * 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.

FUTURE

The future for Apophis on Friday, April 13 of 2029 includes an approach
to Earth no closer than 29,470 km (18,300 miles, or 5.6 Earth radii from
the center, or 4.6 Earth-radii from the surface) over the mid-Atlantic,
appearing to the naked eye as a moderately bright point of light moving
rapidly across the sky. Depending on its mechanical nature, it could
experience shape or spin-state alteration due to tidal forces caused by
Earth's gravity field.

This is within the distance of Earth's geosynchronous satellites.
However, because Apophis will pass interior to the positions of these
satellites at closest approach, in a plane inclined at 40 degrees to the
Earth's equator and passing outside the equatorial geosynchronous zone
when crossing the equatorial plane, it does not threaten the satellites
in that heavily populated region.

Using criteria developed in this research, new measurements possible in
2013 (if not 2011) will likely confirm that in 2036 Apophis will quietly
pass more than 49 million km (30.5 million miles; 0.32 AU) from Earth on
Easter Sunday of that year (April 13).

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.
Received on Tue 20 Nov 2007 05:03:14 PM PST