
Sentry Notes


 2015Mar02
 We have computed an updated impact probability table for (99942)
Apophis. The new table is based on the recent publication
by Vokrouhlicky et al. (2015). The new table relies on a refined
estimate of the Yarkovsky effect that accounts for the nonprincipal
axis rotation of Apophis (Pravec
et al., 2014) and the most recent estimates of diameter and
thermal inertia (Mueller
et al., 2014).
 2014Aug19
 We have updated the 2880 impact probability
for (29075) 1950 DA to account for the additional information on the
asteroid's physical model as described in Rozitis
et al.,"Cohesive forces prevent the rotational breakup of
rubblepile asteroid (29075) 1950 DA", Nature, vol. 512, pp. 174176.
 2014Apr29
 The 2009 FD potential impact tabulation has
been updated to incorporate uncertainties due to the Yarkovsky effect,
which dominates over present day position uncertainties. Therefore the
current posting may not be updated until enough new observations or
other information are available to warrant a recomputation.
 2014Mar03
 We have updated the risk table for 101955
Bennu based on the research described by Chesley et al., "Orbit and
Bulk Density of the OSIRISREx Target Asteroid (101955) Bennu"
(Icarus, in press, 2014) [Preprint]. The new results
make use of Arecibo radar astrometry from September 2011, which yields
a highprecision estimate of the Yarkovsky effect and in turn the bulk
density of Bennu. The new cumulative impact probability is about 1 in
2700.
 2013Nov25
 We have updated the 2880 impact probability for
(29075) 1950 DA based on recently reported 2012 radar
astrometry. The results are based on Farnocchia and Chesley
(2013), which has been revised to account for the 2012
observations and is now in press on Icarus.
 2013Oct04
 We have now computed an impact probability
for (29075) 1950 DA in 2880 and posted the results to the risk page. This is based on the recent
publication by Farnocchia
and Chesley (2013), now accepted by Icarus. As in the case for
Apophis, below, the new hazard assessment for 1950 DA accounts for the
Yarkovsky effect, despite the fact that it has so far not been
definitively seen in the orbital motion. This requires us to account
for potential orbital variations due to uncertainties in the physical
properties of the asteroid, such as spin axis orientation, thermal
inertia and bulk density.
 2013Aug12

We are nearing completion of a recomputation of all risk tables with
an updated planetary ephemeris, designated DE431, which will better
model the gravitational perturbations of the planets. Our asteroid
perturber model has been updated to be consistent with DE431, and we
are now using perturbations from the 16 most massive mainbelt
asteroids, rather than only the largest three as was done in the
past. Note that many objects with very low impact probabilities are
only detected on a statistical basis, and so this recomputation can
yield different results than those obtained before for these low
interest cases. In particular, we will find some new potential impacts
(and potential impactors) and will not identify some that were found
in previous searches. Cases of higher interest will not change
significantly between runs.
 2013May01
 We have updated the risk table for 99942 Apophis
based on the recently released radar astrometry as well as
optical astrometry through 2013Apr26. For the hazard assessment we
continue to apply the technique discussed by Farnocchia et
al. (Icarus, v. 224, pp.192200, 2013). The updated Fig. 6 from the Farnocchia et al. paper shows the current
estimate for the probability distribution on the 2029 bplane. The
2036 keyhole, which was previously of some interest, is situated at
approximately 1600 km on the abscissa (i.e., outside the plot
boundaries). The hazard assessment is now quite stable and we do not
intend to update again until there is significant new observational
information for Apophis, which could come as early as June, when the
next radar
observations are planned.
 2013Jan09
 We have updated the risk table for 99942
Apophis based on the recent publication by Farnocchia et
al.
 2011Sep20
 We have transitioned to the debiasing and
weight scheme described in Chesley, Baer and Monet (Icarus, vol. 210,
pp. 158181). This means that we are treating the asteroid
observational data in a way that is more consistent with the
statistical uncertainties and that has been shown to produce better
fits and more reliable predictions. As explained in our 2010Dec7
note below, such a recomputation necessarily leads to minor changes in
the listings, as well as some new additions and removals to the object
list.
 2010Dec7
 As a part of fielding some enhancements to
our process we are rerunning all objects in order to bring them
uptodate with our current software and dynamical models. Note that
many objects with very low impact probabilities are only detected on a
statistical basis, and so this recomputation can yield different
results than those obtained before for these low interest cases. In
particular, we will find some new potential impacts (and potential
impactors) and will not identify some that were found in previous
searches. Cases of higher interest will not change between runs.
 2010Nov23
 Updating our note of 2010Jul26 below,
another object has been found to have potential impacts in the far
future, beyond 100 years. 2009 FD is roughly 130 m in diameter with an
estimated 1 in 435 chance of impact in 2185. The current analysis
assumes only gravitational accelerations and does not incorporate the
potentially important Yarkovsky (thermal) accelerations. Thus the 2009
FD Risk Table may be refined by future analyses that attempt to
incorporate a more complete dynamical model.
 2010Jul26

In some cases, investigations into potential impacts are conducted for more
than 100 years into the future. Currently, there are two wellobserved
objects for which longterm analyses have been carried out.
1. Asteroid (29075) 1950 DA, has a significant possibility of impact on
March 16, 2880. A careful computation of the impact probability, which is
less than 0.33%, is challenging because the orientation of its spin pole is
poorly known. Giorgini et al. (Science, Vol. 296. no. 5565, pp. 132  136,
2002) analyzed this object's motion, which is discussed here:
http://neo.jpl.nasa.gov/1950da
2. The second object, (101955) 1999 RQ36, currently has nonzero impact
probabilities on numerous occasions during the years after 2165. This is
analyzed in a paper published by Milani et al. (Icarus, Vol. 203, pp.
460471, 2009), which is available as here:
http://arxiv.org/abs/0901.3631 .
Note the Torino Scale is formally undefined for potential impacts more than
one century into the future and so not applicable in such cases.
 2009Oct07
 The risk assessment for Apophis has been
updated to reflect new astrometry released by Tholen et al. (DPS 2009)
and dispersions due to the Yarkovsky effect. Results reported by
Chesley et al. at the 2009 Div. of
Planetary Sciences meeting.
 2008May18
 Sentry has switched to a new server and management
architecture. As a part of this transition, all objects in the NEA
catalog were reanalyzed with the new system. This recomputation leads
inevitably to minor differences in the results due to the statistical
nature of the impact monitoring algorithms.


