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the comets could have a bulk density approaching 1
,
700 kg
/
m
3
,iftheyare
collision fragments from the core of a once molten parent body.
4. Methods for Estimating Cometary Bulk Densities
4.1.
Non-gravitational force modeling
As a comet sublimates, a so-called non-gravitational force is exerted on the
nucleus due to conservation of linear momentum. Since this force mod-
ifies the cometary orbit with respect to the purely gravitational orbit,
measured non-gravitational changes of the orbit may be used in combi-
nation with thermophysical model calculations in order to estimate the
mass and bulk density of the comet. In a pioneering study, Rickman
40
-
42
estimated the bulk density of Comet 1P/Halley, and found 500
∼
ρ
bulk
∼
1
,
200 kg
/
m
3
, with a preference for the lower value. Sagdeev
et al.
,
43
esti-
mated
ρ
bulk
= 600
+900
−
600
kg
/
m
3
, while Peale
44
obtained a preferred value
ρ
bulk
= 700 kg
/
m
3
(although he could not exclude any value in the range
30
4
,
900 kg
/
m
3
). In spite of the large error bars associated with
each individual estimate, the combined results seem to indicate that the
bulk density of 1P/Halley is below
≤
ρ
bulk
≤
1
,
000 kg
/
m
3
.
Rickman
et al.
45
estimated the bulk densities of 17 “old” short-period
comets (objects having made at least 30 revolutions since capture), and
although the results were uncertain for individual objects, the sample as a
whole indicated a typical density of
ρ
bulk
∼
500 kg
/
m
3
.
Comet 19P/Borrelly was studied by Farnham and Cochran,
46
∼
who
830 kg
/
m
3
, with a preferred value of
found a density 290
∼
ρ
bulk
<
∼
490 kg
/
m
3
. Davidsson and Gutierrez
47
studied the same object,
obtaining 180
∼
ρ
bulk
∼
300 kg
/
m
3
. The reason for the discrepancy may be
that Farnham and Cochran assumed a momentum transfer coecient that
is substantially larger than obtained through direct simulation Monte Carlo
modeling of the gas in the near-nucleus coma, as performed by Davidsson
and Gutierrez.
Davidsson and Gutierrez
48
also studied Comet 67P/Churyumov-
Gerasimenko, obtaining 100
∼
ρ
bulk
∼
370 kg
/
m
3
for the best possible
reproduction of observational data (including the rotational lightcurve
amplitude, water production rate versus time, and non-gravitational
changes in the orbital period, longitude of perihelion and longitude of
the ascending node). An upper limit
ρ
bulk
∼
600 kg
/
m
3
was suggested if
uncertainties in the empirical data were considered. The study of Comet
ρ
bulk
≈
