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Sentry: Earth Impact Monitoring

Introduction
Object Table
VI Table
FAQ
Operational Notes
Removed Objects
Object Details

Object Details

The following summary tables includes basic information about the hazard for this object. The maximum Torino and Palermo Scale values are listed, as well as the number of tabulated potential impacts and their corresponding cumulative Palermo Scale value and cumulative impact probability (shown in the the first table). Certain parameter values depend upon the specific impact event in question, but they change little among the various table entries. For this reason we tabulate only mean values for these parameters (shown in the second table). The observation set used for the analysis is also listed.

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Table Column Descriptions  |  Show Legend
Coloration
The color of the table row gives a rough interpretation of the severity of the threat. White or gray colors indicate a Torino scale of 0 or undefined. All other colors (green, yellow, orange, and red) represent their respective Torino scale.
Date (yyyy-mm-dd.dd)
Calendar date (UTC) of the potential impact.
Sigma VI
Sigma VI is a metric that quantifies how well the impacting orbit found by the orbit-determination filter fits the observations. Zero indicates the best-fitting, central (nominal) orbit and the further from zero, the less likely the event: For an orbit defined by six orbital parameters, roughly 83% of the uncertainty region is within 3-sigma.
Distance (rEarth)
Minimum distance on the target plane (scaled b-plane) from the LOV to the geocenter, measured in Earth radii. For these purposes the radius of the Earth, 6420 km, includes some allowance for the thickness of the atmosphere.
Width (rEarth)
One-sigma semi-width of the LOV uncertainty region, measured in Earth radii.
Sigma Impact
Lateral distance in sigmas from the LOV to the Earth's atmosphere. Zero indicates that the LOV intersects the Earth. It is computed from (Distance - 1)/Width.
Sigma LOV
Coordinate along the Line Of Variations (LOV). This value is a measure of how well the impacting orbit fits the available observations. Zero indicates the best-fitting, central (nominal) orbit and the further from zero, the less likely the event: Roughly 99% of all the uncertainty region lies between -3 and +3. Sentry explores out to Sigma LOV = +/-5.
Stretch LOV (rEarth)
Stretching is the semimajor axis of the local linear uncertainty region. It describes how fast one moves across the target plane as Sigma LOV changes, and is measured in Earth radii per sigma. The local probability density varies inversely with the stretching, and thus larger stretching values will generally lead to lower impact probabilities.
Impact Probability
Probability that the tabulated impact will occur. The probability computation is complex and depends on a number of assumptions that are difficult to verify. For these reasons the stated probability can easily be inaccurate by a factor of a few, and occasionally by a factor of ten or more.
Impact Energy (Mt)
Kinetic energy at impact, based upon the computed absolute magnitude and impact velocity for the particular case, and computed in accordance with the guidelines stated for the Palermo Technical Scale. Uncertainty in this value is dominated by mass uncertainty and the stated value will generally be good to within a factor of three.
Palermo Scale
Hazard rating according to the Palermo technical impact hazard scale, based on the tabulated impact date, impact probability and impact energy.
Torino Scale
Hazard rating according to the Torino impact hazard scale, based on the tabulated impact probability and impact energy. The Torino scale is defined only for potential impacts less than 100 years in the future.
The maximum hazard rating according to the Torino impact hazard scale, based on the tabulated impact probability and impact energy. The Torino Scale is defined only for potential impacts less than 100 years in the future.The maximum hazard rating according to the Palermo technical impact hazard scale, based on the tabulated impact date, impact probability and impact energy.The cumulative hazard rating according to the Palermo technical impact hazard scale, based on the tabulated impact date, impact probability and impact energy.The cumulative probability that the tabulated impact will occur. The probability computation is complex and depends on a number of assumptions that are difficult to verify. For these reasons the stated probability can easily be inaccurate by a factor of a few, and occasionally by a factor of ten or more.Total number of potential impacts resulting from this analysis.Technique used to search for potential impacts. The technique can be based on a Monte Carlo (MC) sampling of the orbital uncertainty or can analyze the trajectories of virtual astroids along the Line of Variations (LOV)Velocity at atmospheric entry.Relative velocity at atmospheric entry neglecting the acceleration caused by the Earth's gravity field, often called the hyperbolic excess velocity. (Vinfinity2 = Vimpact2 - Vescape2, where Vescape = ~11.2 km/s is the Earth escape velocity.)Absolute Magnitude, a measure of the intrinsic brightness of the object.This is an estimate based on the absolute magnitude, usually assuming a uniform spherical body with visual albedo pV = 0.154 (in accordance with the Palermo Scale) but sometimes using actual measured values if these are available. Since the albedo is rarely measured, the diameter estimate should be considered only approximate, but in most cases will be accurate to within a factor of two.This estimate assumes a uniform spherical body with the computed diameter and a mass density of 2.6 g/cm3. The mass estimate is somewhat more rough than the diameter estimate, but generally will be accurate to within a factor of three.The kinetic energy at impact: 0.5 * Mass * Vimpact2, measured in Megatons of TNT.Calendar date (UTC) of the potential impact.Sigma VI is a metric that quantifies how well the impacting orbit found by the orbit-determination filter fits the observations. Zero indicates the best-fitting, central (nominal) orbit and the further from zero, the less likely the event: For an orbit defined by six orbital parameters, roughly 83% of the uncertainty region is within 3-sigma.Sigma MC is a metric that quantifies how well the impacting orbit fits the observations. Zero indicates the best-fitting, central (nominal) orbit and the further from zero, the less likely the event: For an orbit defined by six orbital parameters, roughly 83% of the uncertainty region is within 3-sigma.Minimum distance on the target plane (scaled b-plane) from the LOV to the geocenter, measured in Earth radii. For these purposes the radius of the Earth, 6420 km, includes some allowance for the thickness of the atmosphere.One-sigma semi-width of the LOV uncertainty region, measured in Earth radii.Lateral distance in sigmas from the LOV to the Earth's atmosphere. Zero indicates that the LOV intersects the Earth. It is computed from (Distance - 1)/Width.Coordinate along the Line Of Variations (LOV). This value is a measure of how well the impacting orbit fits the available observations. Zero indicates the best-fitting, central (nominal) orbit and the further from zero, the less likely the event: Roughly 99% of all the uncertainty region lies between -3 and +3. Sentry explores out to Sigma LOV = +/-5.Stretching is the semimajor axis of the local linear uncertainty region. It describes how fast one moves across the target plane as Sigma LOV changes, and is measured in Earth radii per sigma. The local probability density varies inversely with the stretching, and thus larger stretching values will generally lead to lower impact probabilities.Probability that the tabulated impact will occur. The probability computation is complex and depends on a number of assumptions that are difficult to verify. For these reasons the stated probability can easily be inaccurate by a factor of a few, and occasionally by a factor of ten or more.Kinetic energy at impact, based upon the computed absolute magnitude and impact velocity for the particular case, and computed in accordance with the guidelines stated for the Palermo Technical Scale. Uncertainty in this value is dominated by mass uncertainty and the stated value will generally be good to within a factor of three.Hazard rating according to the Palermo technical impact hazard scale.Hazard rating according to the Torino impact hazard scale, based on the tabulated impact probability and impact energy. The Torino Scale is defined only for potential impacts less than 100 years in the future.