The following table contains operational notes related to out Earth impact monitoring system. Entries are listed in reverse chronological order.

Date | Note |
---|---|

2017-Apr-06 |
We have transitioned to an improved weighting scheme for asteroid astrometry as described in a recently submitted paper by Veres et al. 2017. Moreover, we updated the set of main belt perturbing bodies to account for more recent mass estimates (Folkner et al. 2014). Because of the statistical nature of Sentry’s impact search, these changes will give results that might differ from previous runs. Some new low impact probability cases could be found and some previous low probability cases could disappear. However, higher impact probability cases will not be significantly affected. |

2016-Jan-21 |
We have updated the hazard assessment of (410777) 2009 FD to include astrometry obtained during the latest apparition from October to December 2015. In particular, the new results account for recent radar observations and the revised estimate of the asteroid’s size. The computation includes the Yarkovsky effect as estimated from the fit to the astrometric observations. |

2015-Dec-07 |
We have updated the 2880 impact probability for (29075) 1950 DA to include the debiasing and weighting scheme by Farnocchia et al. 2015 and use the DE431 version of JPL’s planetary ephemerides. The computation was performed with a Monte Carlo sampling of the orbital elements and the Yarkokovsky effect, as estimated from the fit to the astrometric observations. |

2015-Mar-02 |
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 non-principal axis rotation of Apophis (Pravec et al., 2014) and the most recent estimates of diameter and thermal inertia (Mueller et al., 2014). |

2014-Aug-19 |
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 rubble-pile asteroid (29075) 1950 DA”, Nature, vol. 512, pp. 174-176. |

2014-Apr-29 |
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. |

2014-Mar-03 |
We have updated the risk table for 101955 Bennu based on the research described by Chesley et al., “Orbit and Bulk Density of the OSIRIS-REx Target Asteroid (101955) Bennu” (Icarus, in press, 2014) [Preprint]. The new results make use of Arecibo radar astrometry from September 2011, which yields a high-precision estimate of the Yarkovsky effect and in turn the bulk density of Bennu. The new cumulative impact probability is about 1 in 2700. |

2013-Nov-25 |
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. |

2013-Oct-04 |
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. |

2013-Aug-12 |
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 main-belt 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. |

2013-May-01 |
We have updated the risk table for 99942 Apophis based on the recently released radar astrometry as well as optical astrometry through 2013-Apr-26. For the hazard assessment we continue to apply the technique discussed by Farnocchia et al. (Icarus, v. 224, pp.192-200, 2013). The updated Fig. 6 from the Farnocchia et al. paper shows the current estimate for the probability distribution on the 2029 b-plane. 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](http://www.naic.edu/~pradar/sched.shtml> are planned. |

2013-Jan-09 |
We have updated the risk table for 99942 Apophis based on the recent publication by Farnocchia et al. |

2011-Sep-20 |
We have transitioned to the debiasing and weight scheme described in Chesley, Baer and Monet (Icarus, vol. 210, pp. 158-181). 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 2010-Dec-7 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. |

2010-Dec-07 |
As a part of fielding some enhancements to our process we are rerunning all objects in order to bring them up-to-date 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. |

2010-Nov-23 |
Updating our note of 2010-Jul-26 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. |

2010-Jul-26 |
In some cases, investigations into potential impacts are conducted for more than 100 years into the future. Currently, there are two well-observed objects for which long-term analyses have been carried out. -
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. -
The second object, (101955) 1999 RQ36, currently has non-zero 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. 460-471, 2009).
Note the Torino Scale is formally undefined for potential impacts more than one century into the future and so not applicable in such cases. |

2009-Oct-07 |
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 Division of Planetary Sciences meeting. |

2008-May-18 |
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. |