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2009 ST171
Earth Impact Risk Summary 

V_{impact} 
14.86 km/s 
V_{infinity} 
9.84 km/s 
H 
26.8 
Diameter 
0.015 km 
Mass 
4.3e+06 kg 
Energy 
1.1e01 MT 
all
above are mean values
weighted by impact probability 

Analysis based on 19 observations spanning 1.1579 days (2009Sep28.23931 to 2009Sep29.39719) 

Orbit diagram and elements available
here.


These results were computed on Jun 15, 2014
2009 ST171
Earth Impact Table 
Date 
Distance 
Width 
Sigma Impact 
Sigma LOV 
Stretch LOV 
Impact Probability 
Impact Energy 
Palermo Scale 
Torino Scale 
YYYYMMDD.DD 
(r_{Earth}) 
(r_{Earth}) 


(r_{Earth}) 

(MT) 


20660930.16 
1.42 
4.29e01 
0.969 
1.35232 
2.85e+04 
1.0e06 
1.13e01 
6.94 
0 
Summary Table Description
The Summary Table 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. The observation set used
for the analysis is also listed. 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:
 V_{impact}  Velocity at atmospheric entry.
 V_{infinity}  Relative velocity at atmospheric entry
neglecting the acceleration caused by the Earth's gravity field, often
called the hyperbolic excess velocity. (V_{infinity}^{2}
= V_{impact}^{2}  V_{escape}^{2}, where
V_{escape} = ~11.2 km/s is the Earth escape velocity.)
 H  Absolute Magnitude, a measure of the intrinsic brightness
of the object.
 Diameter  This is an estimate based on the absolute magnitude,
usually assuming a uniform spherical body with visual albedo p_{V}
= 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.
 Mass  This estimate assumes a uniform spherical body with
the computed diameter and a mass density of 2.6 g/cm^{3}. The
mass estimate is somewhat more rough than the diameter estimate, but
generally will be accurate to within a factor of three.
 Energy  The kinetic energy at impact: 0.5 * Mass * V_{impact}^{2}.
Measured in Megatons of TNT.

Impact Table Legend
See our Introduction for a more extensive explanation of these terms.
 Date
 The calendar date (UTC) of the potential impact.
 Distance
 The minimum distance on the target plane (scaled bplane)
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
 The onesigma semiwidth of the LOV uncertainty region, measured
in Earth radii.
 Sigma Impact
 The 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
 The 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 bestfitting, 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
 The 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
 The 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
 The 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
 The hazard rating according to the Palermo Technical Impact
Hazard Scale, based on the tabulated impact date, impact probability
and impact energy.
 Torino Scale
 The 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.


