v
j j ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
2GM ⨀
r h
r
:
ð1:16Þ
This can be used to calculate the heliocentric speed of a comet on a parabolic orbit
at the Earth’s distance from the Sun. This is around 42 km s
À1 which, when
combined with the Earth’s orbital speed (~30 km s
À1 ), gives the worst case impact
velocity at the Earth for a comet entering the Solar System on a parabolic, retrograde,
orbit and illustrates the potential danger from such an object. In order to imagine the
consequences, a 67P-sized comet hitting the Earth at this velocity would be equivalent to around 6 Â 10
12 tons of TNT. For comparison, the “Fat Man” atomic bomb
that was exploded over the Japanese city of Nagasaki at the end of the Second World
War was equivalent to 2 Â 10
4 tons of TNT. This also gives an estimate of the
maximum fly-by velocity that a spacecraft might encounter a comet with and is a
consideration in the design of ESA’s forthcoming Comet Interceptor mission. For
comparison, Giotto completed a fly-by of 1P/Halley at a relative velocity of
68.373 km s
À1 .
Currently, comets are classified according to their orbital characteristics.
Figure 1.4 illustrates Levison’s classification from 1996 with some minor
modifications.
The orbital period is used to separate comets into two main categories. Shortperiod comets (SPCs) are those with P < 200 years whereas long-period comets
(LPCs) have P > 200 years. The naming convention for comets is related to this
division.
1 Comets that are defined to be SPCs or when there are confirmed observations at more than one perihelion passage are referred to as periodic and their names
are prefixed with a number and the designation P/ (e.g. comet 19P/Borrelly). Some
periodic, numbered, comets no longer exist or are deemed to have disappeared. The
prefix D/ is used in this case (e.g. 20D/Westphal). There are currently eight comets in
this category. Reappearance or re-discovery cannot be excluded and has indeed
Fig. 1.4 The Levison classification of comets
1 http://www.minorplanetcenter.net/iau/lists/CometResolution.html
8
1 Light Curves, Orbits, and Reservoirs
j j ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
2GM ⨀
r h
r
:
ð1:16Þ
This can be used to calculate the heliocentric speed of a comet on a parabolic orbit
at the Earth’s distance from the Sun. This is around 42 km s
À1 which, when
combined with the Earth’s orbital speed (~30 km s
À1 ), gives the worst case impact
velocity at the Earth for a comet entering the Solar System on a parabolic, retrograde,
orbit and illustrates the potential danger from such an object. In order to imagine the
consequences, a 67P-sized comet hitting the Earth at this velocity would be equivalent to around 6 Â 10
12 tons of TNT. For comparison, the “Fat Man” atomic bomb
that was exploded over the Japanese city of Nagasaki at the end of the Second World
War was equivalent to 2 Â 10
4 tons of TNT. This also gives an estimate of the
maximum fly-by velocity that a spacecraft might encounter a comet with and is a
consideration in the design of ESA’s forthcoming Comet Interceptor mission. For
comparison, Giotto completed a fly-by of 1P/Halley at a relative velocity of
68.373 km s
À1 .
Currently, comets are classified according to their orbital characteristics.
Figure 1.4 illustrates Levison’s classification from 1996 with some minor
modifications.
The orbital period is used to separate comets into two main categories. Shortperiod comets (SPCs) are those with P < 200 years whereas long-period comets
(LPCs) have P > 200 years. The naming convention for comets is related to this
division.
1 Comets that are defined to be SPCs or when there are confirmed observations at more than one perihelion passage are referred to as periodic and their names
are prefixed with a number and the designation P/ (e.g. comet 19P/Borrelly). Some
periodic, numbered, comets no longer exist or are deemed to have disappeared. The
prefix D/ is used in this case (e.g. 20D/Westphal). There are currently eight comets in
this category. Reappearance or re-discovery cannot be excluded and has indeed
Fig. 1.4 The Levison classification of comets
1 http://www.minorplanetcenter.net/iau/lists/CometResolution.html
8
1 Light Curves, Orbits, and Reservoirs