My area of research is comets and asteroids, which I have been studying
since 1994. I am interested in the composition and physical properties
of these objects, and how they compare to each other and other objects
in the Solar System. Why? Because Small Bodies can tell us about
the origin of the Solar System -- what were the
compositional and thermophysical conditions in the solar
nebula and in the protoplanetary disk? Understanding
the origin of the Solar System is the
overall "big picture"
question that I (and many planetary scientists) work toward.
However to understand what the small bodies tell us about
our origins, we need to know what has happened to these objects
in the intervening 4.6 billion years sinice they formed.
In other words, I study cometary and asteroidal evolution.
This is not an easy task, but it is an interesting question
in its own right, and it is critical if we are to make sense
of what the Solar System is like today.
The experimental methods that I use almost always involve
telescopes, both on the ground and in space. I primarily
work at visible and infrared wavelengths, but have also
done radio experiments as well.
Here're some projects I'm working on either
as PI or as coI, in no particular order:
Behavior and nucleus properties
of comet 2P/Encke.
- Comet Encke is an unusual comet with a very short
orbital period. We have been working to understand the
nucleus's size (with infrared observations) and spin state
(with visible observations). We would also like to figure out
its shape since its diurnal light curve is very odd, having
only one bright peak (instead of two). Furthermore, we are studying
the comet's activity behavior when it is near aphelion. The
comet seems to be intrinsically brighter at 4 AU than it is
at say 2 or 3 AU, even though one would expect a comet to become
less active the farther it is from the Sun.
comet 29P/Schwassmann-Wachmann 1.
- Comet S-W 1 is in a near-circular orbit just
outside Jupiter's orbit. It has a long history of having
outburst behavior, where its normal constant level of
activity will be punctuated with brightenings of a few
magnitudes. We are studying the evolution and devlopment
of the comet's coma before, during, and after these outbursts.
By watching how the morphology of the coma changes over time,
we hope to determine the comet's spin state and active areas.
We also are interested in understanding the nucleus's size
and shape. Most of the data we have are images
at visible wavelengths.
Size and albedo distributions of cometary nuclei.
- We have obtained a great deal of infrared
and visible imaging of cometary nuclei so that we can
determine the ensemble of nuclear properties. So far
our survey is focussed on the Jupiter-family comets.
Knowing the size distribution is important for understanding
how the JFCs evolve over time; the shape of the distribution
will tell us something about what physical processes dominate.
The albedo distribution is important to know since no-one
has yet done a strong test of the very widespread assumption
that nuclei have geometric albedos near 4%. This seems to be
a reasonable assumption but it is based on very few data points.
Thermal properties of cometary nuclei.
- With infrared photometry, we can use thermal
models to infer the thermal properties of the nuclei. For the
most part, nuclei are expected to be porous, poorly-conducting
bodies, so that when sunlight heats a patch of surface, it
is very hard for that energy to be conducted to nearby areas.
Our results so far indicate that most nuclei have low
thermal inertia, and that there is not much infrared
beaming (which might be expected from a surface that
had say many craters or similar-shaped topography).
There are a few nuclei that intriguingly seem to have
different thermal properties however. This science question
is important for understanding how the evolutionary processes
that comets suffer actually affect their bulk properties.
Long-term studies of comet C/1995 O1 Hale-Bopp.
- Comet Hale-Bopp excited the world in 1997, and
it is now receding from the Sun. It is nearly 30 AU away yet
it still seems to have some coma. We are working on understanding
how and why this comet still shows dust, and what the properties
of the dust are. We also want to compare this dust to what was
emitted back in the 1990s -- is the dust different? Does the
driver of activity in a comet change the properties of the dust
that comes out? How is energy transported through the nucleus?
Do outbursts still occur even at such large heliocentric distances?
Behavior of comet 9P/Tempel 1 after Deep Impact.
- We participated in observing this comet before, during,
and after the very successful Deep Impact event. One of many
interesting things that came out of that was the realization
that the impact did not create a new active area. We are using
large-scale imaging of the coma to analyze
behavior of the ejecta and thereby understand the physical properties
of the liberated dust grains. We are also interested in comparing
that dust to the pre-impact dust. Furthermore, we are interested
in the photometric properties of the ejecta; we have infrared
data that indicate the ejecta became bluer over time (in terms
of its near-IR color). We are investigating what the
physical reason for this phenomenon is.
Activity of distant comets and Centaurs.
- Comet S-W 1 counts as an active Centaur,
in addition to being a short-period comet. Other Centaurs
are also active, and we're interested in understanding
the general phenomenon. Why are some Centaurs active
and others not? What does activity do to their surfaces?
Is the nature of the activity different compared to
normal inner-Solar System cometary activity? These questions
are important for understanding the evolution of
icy bodies as they dynamically migrate from the scattered
disk, through the giant planet region, and into the
inner Solar System.
Physical, thermal, and reflectance propreties of Jovian Trojans.
- We've been using infrared and visible photometry
to understand the properties of Jovian Trojans. In particular
we're interested in understanding why the albedos of the
small Trojans vary so widely, when the
large Trojans are so uniformly reflective.
Trojans probably formed with a lot of ice, so we would
like to now how much ice is left in the big ones and in
the little ones, and whether or not the Trojans could be
another source of the current comet population. Since
Trojans may have formed much farther out in the Solar
System, according to the Nice model, there may be
some similarities/differences between Trojans and TNOs
that could shed light on the question of the Trojans origin.
and reflectance properties of outer Solar System objects.
- As many other people are, we are interested in
the properties of TNOs. This involves imaging and
spectroscopy at various wavelengths. Space-based studies
are very useful for addressing this topic, since there
are not that many TNOs that one can really study in
the first place. Many are too faint.
Surface properties of asteroids in short-period and
long-period cometary orbits (a.k.a. extinct-comet candidates).
- What happens to comets when they die? Some
die catastrophically when they break up into many
fragments that eventually dissipate. However some probably
stick around long enough to lose all their volatiles
and cease activity. At that point, a comet will
look a lot like an asteroid. Is there a way to tell
if a given "asteroid" in near-Earth space is actually
a dead comet? We are investigating ways in which
the surfaces of dead comets and asteroids might differ
(thermal behavior, reflectance, etc.).
We are also very interested in the proto-typical
dead comet -- or really, dormant comet -- 107P/Wilson-Harrington.
This comet was active in 1949 but has not been active since.
Why is this? Are there any unusual surface properties that
could tell us about the recent history of the comet? Alternately,
perhaps there has been some very low-level activity going
on that is just not easy to see without good spatial
resolution and good sensitivity.
Surface properties of near-Earth asteroids.
- We're studying not only NEAs that might
be dead comets but NEAs in general. For example, what
happens to NEAs that are in orbits that take them very
close to the Sun? If they are heated to high temperatures
(~600K to 1400 K) many times, what chemistry happens on
their surfaces? Do they evolve differently than NEAs
that stay cooler?
Currently my group here at UCF is relatively small:
three graduate students (Jenna Jones, Charles Schambeau,
Mary Hinkle) and one undergraduate student (Seamus Anderson).
However I collaborate with others here at UCF
and with many folks around the U.S. and Europe.
If you are an undergraduate or graduate student looking
to go into astronomy, and if comets and asteroids interest you, feel
free to contact me about coming to UCF.
updated august 2016