Geoscience Reference
In-Depth Information
4.7.
Direct measurements by spacecraft
To date, no less than six comets have been the targets of spacecraft mis-
sions — 21P/Giacobini-Zinner (
ICE
), 1P/Halley (
Giotto,Vega1and2,
Suisei, Sakigake, ICE
), 26P/Grigg-Skjellerup (
Giotto
), 19P/Borrelly (
Deep
Space 1
), 81P/Wild 2 (
Stardust
), and 9P/Tempel 1 (
Deep Impact
). Of these
mission, only
Deep Impact
has resulted in mass and bulk density estimates,
obtained by studying the dynamics of ejecta created when a 370 kg impactor
collided with 9P/Tempel 1, at a velocity of 10
.
3km
/
s. According to A'Hearn
et al.
,
64
the bulk density is
ρ
bulk
= 620
+470
−
330
kg
/
m
3
. The other missions have
not resulted in mass or bulk density estimates since flyby distances and
comet-spacecraft relative speeds have been too large to yield measurable
gravitational perturbations on the spacecraft trajectories.
The ESA mission
Rosetta
, which includes the lander probe
Philae
, will
hopefully obtain accurate mass and bulk density measurements of Comet
67P/Churyumov-Gerasimenko, upon arrival in 2014.
5. Conclusions
In Sec. 2, it was argued that freshly formed comet-sized planetesimals were
likely to have porosities
ψ
∼
0
.
65, which translates to
ρ
bulk
∼
600 kg
/
m
3
for material with
ρ
comp
≤
1
,
700 kg
/
m
3
. The subsequent evolution of these
bodies into the comets we see today may have included collisional alter-
ation, thermal processing, and possibly other mechanisms not considered
here. Collisions could hardly increase the bulk density above
1
,
000 kg
/
m
3
,
unless the constituent grains were deformed and merged in the process,
which is rather unlikely. Radiogenic core melting and gravitational com-
paction of larger parent bodies (i.e., objects larger
10
∼
100 km) may
partially have removed microcavities and increased the bulk density to
1
,
400
∼
ρ
bulk
∼
1
,
700 kg
/
m
3
, at least for a fraction of the comet popu-
lation. However, if amorphous water ice indeed is an important constituent
in comets, as suggested by frequent outbursts, such dense objects may be
very rare, if existing at all.
The methods for bulk density estimates of comets available at present,
are all indirect and not very accurate. Estimates based on non-gravitational
force modeling may be biased toward fairly porous objects, since comets
with very small and uncertain non-gravitational orbit changes (possibly
caused by a comparably large mass and bulk density) are not selected for
study. Other methods only yield a lower limit (rotational stability), cannot
be applied for a su
ciently large sample (tidal breakup), or only target the
than
∼
