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A ballistics analysis of the Deep Impact ejecta plume: Determining Comet Tempel 1's gravity, mass, and density
In July of 2005, the Deep Impact mission collided a 366 kg impactor with the nucleus of Comet 9P/Tempel 1, at a closing speed of 10.2 km s −1. In this work, we develop a first-order, three-dimensional, forward model of the ejecta plume behavior resulting from this cratering event, and then adjust th...
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Published in: | Icarus (New York, N.Y. 1962) N.Y. 1962), 2007-10, Vol.190 (2), p.357-390 |
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Main Authors: | , , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that cite this one |
Online Access: | Get full text |
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Summary: | In July of 2005, the Deep Impact mission collided a 366 kg impactor with the nucleus of Comet 9P/Tempel 1, at a closing speed of 10.2 km s
−1. In this work, we develop a first-order, three-dimensional, forward model of the ejecta plume behavior resulting from this cratering event, and then adjust the model parameters to match the flyby-spacecraft observations of the actual ejecta plume, image by image. This modeling exercise indicates Deep Impact to have been a reasonably “well-behaved” oblique impact, in which the impactor–spacecraft apparently struck a small, westward-facing slope of roughly
1
/
3
–
1
/
2
the size of the final crater produced (determined from initial ejecta plume geometry), and possessing an effective strength of not more than
Y
¯
=
1
–
10
kPa
. The resulting ejecta plume followed well-established scaling relationships for cratering in a medium-to-high porosity target, consistent with a transient crater of not more than 85–140 m diameter, formed in not more than 250–550 s, for the case of
Y
¯
=
0
Pa
(gravity-dominated cratering); and not less than 22–26 m diameter, formed in not less than 1–3 s, for the case of
Y
¯
=
10
kPa
(strength-dominated cratering). At
Y
¯
=
0
Pa
, an upper limit to the total ejected mass of
1.8
×
10
7
kg
(
1.5
–
2.2
×
10
7
kg
) is consistent with measurements made via long-range remote sensing, after taking into account that 90% of this mass would have stayed close to the surface and then landed within 45 min of the impact. However, at
Y
¯
=
10
kPa
, a lower limit to the total ejected mass of
2.3
×
10
5
kg
(
1.5
–
2.9
×
10
5
kg
) is also consistent with these measurements. The expansion rate of the ejecta plume imaged during the look-back phase of observations leads to an estimate of the comet's mean surface gravity of
g
¯
=
0.34
mm
s
−2
(0.17–0.90 mm s
−2), which corresponds to a comet mass of
m
t
=
4.5
×
10
13
kg
(
2.3
–
12.0
×
10
13
kg
) and a bulk density of
ρ
t
=
400
kg
m
−3
(200–1000 kg m
−3), where the large high-end error is due to uncertainties in the magnitude of coma gas pressure effects on the ejecta particles in flight. |
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ISSN: | 0019-1035 1090-2643 |
DOI: | 10.1016/j.icarus.2007.08.001 |