relied solely on in situ analysis of the Moon with 1960s technology, our understanding would be a fraction of what it is today.
The two major issues with returning to Earth are connected to evolution of the
sample during the sampling process and the return trip and the effects on the sample
during reentry. The latter in particular is a significant problem for attempts to acquire
undisturbed cores. The forces due to deceleration on re-entry will undoubtedly
compromise the sample’s integrity structurally, leading to mixing between products
from different depths. It seems apposite to conclude that, for the foreseeable future at
least, an ambition to return to Earth, a cored cometary sample is highly challenging.
Rosetta has demonstrated that constraining the spatial distribution of gas and dust
sources uniquely is vastly more challenging than originally thought. One should
caution that some authors have a relatively low threshold for success in this context.
And yet this remains a key question in the efforts to relate surface structure (thermal,
mechanical, chemical) to activity and evolution. The observational requirements for
future missions needed to break the degeneracy and relate coma gas and dust
densities to surface properties are challenging.
The single point density measurements by COPS have shown the importance of
this type of measurement. Local gas density measurements at multiple positions
within the inner coma would have provided far more rigorous constraints on the gas
flow field. In particular, continuous measurements near the sub-solar point (over
periods greater than the rotation period at a cadence of <1 min
À1 ) would have been
particularly valuable. When coupled with a local velocity and temperature measurement (and we note here the additional importance of determining the temperature
anisotropy as a constraint on the energy distributions), this would have provided us
with a far better understanding of outgassing than the rather ambiguous situation we
have today.
Remote sensing observations of the gas distribution were obtained by Rosetta but
their interpretation remains complex. In a future mission concept, the inversion of
density, temperature, and velocity from remote sensing should be traded against
local direct multipoint measurements to establish the optimum strategy. The critical
aspect is whether the gas flow field can be inverted to produce global maps of source
strength at a resolution that is sufficient to relate the production rate to surface
morphologies and properties. Given the variation in surface morphology seen on
the surface of 67P, the required resolution is potentially <100 m and comparable to
the mean free path for modest activity cases.
There are several properties of the dust that affect the dust flow field pattern in
addition to the source. It is well known that particle size and shape are important. The
dust size distribution remains a critical parameter in all studies of the dust flow field
and its brightness at all wavelengths. Future missions will need much more thorough
instrumentation to address this topic.
The comets that we see today come from three reservoirs: The Oort cloud is the
most distant, and the source of long period and dynamically new comets, while short
period Jupiter family comets (JFCs) come from the scattered disc of the Kuiper Belt.
The modern distinction is based on the comet’s orbit (specifically its Tisserand
parameter with respect to Jupiter), and splits comets into “ecliptic” comets from
442
8 Future Investigations of Comets
The two major issues with returning to Earth are connected to evolution of the
sample during the sampling process and the return trip and the effects on the sample
during reentry. The latter in particular is a significant problem for attempts to acquire
undisturbed cores. The forces due to deceleration on re-entry will undoubtedly
compromise the sample’s integrity structurally, leading to mixing between products
from different depths. It seems apposite to conclude that, for the foreseeable future at
least, an ambition to return to Earth, a cored cometary sample is highly challenging.
Rosetta has demonstrated that constraining the spatial distribution of gas and dust
sources uniquely is vastly more challenging than originally thought. One should
caution that some authors have a relatively low threshold for success in this context.
And yet this remains a key question in the efforts to relate surface structure (thermal,
mechanical, chemical) to activity and evolution. The observational requirements for
future missions needed to break the degeneracy and relate coma gas and dust
densities to surface properties are challenging.
The single point density measurements by COPS have shown the importance of
this type of measurement. Local gas density measurements at multiple positions
within the inner coma would have provided far more rigorous constraints on the gas
flow field. In particular, continuous measurements near the sub-solar point (over
periods greater than the rotation period at a cadence of <1 min
À1 ) would have been
particularly valuable. When coupled with a local velocity and temperature measurement (and we note here the additional importance of determining the temperature
anisotropy as a constraint on the energy distributions), this would have provided us
with a far better understanding of outgassing than the rather ambiguous situation we
have today.
Remote sensing observations of the gas distribution were obtained by Rosetta but
their interpretation remains complex. In a future mission concept, the inversion of
density, temperature, and velocity from remote sensing should be traded against
local direct multipoint measurements to establish the optimum strategy. The critical
aspect is whether the gas flow field can be inverted to produce global maps of source
strength at a resolution that is sufficient to relate the production rate to surface
morphologies and properties. Given the variation in surface morphology seen on
the surface of 67P, the required resolution is potentially <100 m and comparable to
the mean free path for modest activity cases.
There are several properties of the dust that affect the dust flow field pattern in
addition to the source. It is well known that particle size and shape are important. The
dust size distribution remains a critical parameter in all studies of the dust flow field
and its brightness at all wavelengths. Future missions will need much more thorough
instrumentation to address this topic.
The comets that we see today come from three reservoirs: The Oort cloud is the
most distant, and the source of long period and dynamically new comets, while short
period Jupiter family comets (JFCs) come from the scattered disc of the Kuiper Belt.
The modern distinction is based on the comet’s orbit (specifically its Tisserand
parameter with respect to Jupiter), and splits comets into “ecliptic” comets from
442
8 Future Investigations of Comets