AFTER THE 9P IMPACT
Mark Kidger
(Based on a talk given at Annual Meeting of “The Astronomer” held on
Saturday, 2005 October 22)
Abstract
2005 will be remembered
for two astounding feats in solar system exploration following on the success
of the missions Cassini-Huygens, Stardust and the Mars Exploration Rovers in
2004. The Hayabusa landing on the asteroid Itokawa in November and, in July,
the impact of the Deep Impact probe on the surface of comet 9P/Tempel 1. This
article explores the Deep Impact mission and its results, some of which have
been rather unexpected.
Introduction
At 05:44:36.07UT
(spacecraft time), 05:52 (Earth time) on July 4th 2005, despite dark
predictions from some scientists who were concerned that the nucleus would
prove to be a difficult target, Deep Impact’s impacter probe impacted on
9P/Tempel 1. The famous image on the TV screen at the JPL control centre
showing the bright flare of impact was received at 05:54:39.46UT and was the
first confirmation of a successful impact. A recently released series of images
from the camera-spectrograph shows the impact sequence in 12 images taken over
a period of 0.7s. The first pinpoint of light is seen in frame 64 from the
camera. By frame 68, taken 0.281s later, at 05:44:36.30 the impact point was
heavily saturated. The impact was estimated to have occurred at an angle of
around 30º and this can be seen from the bright cloud of expelled material that
moves down in the frame at high speed, fading rapidly; the cloud appears first
in frame 68 at 05:44:36.36 and last seen faintly in frame 70, just 0.121s later
when it was moving off the limb of the nucleus. The brightness of the impact
flare amazed watching astronomers. No one had expected the impact to be so
spectacular in the images from the mother ship. In the control room an
impromptu party broke out as scientists and engineers celebrated the success of
the mission, while astronomers waiting in telescopes all round the world licked
their lips in anticipation of a spectacular show from the comet.
It rapidly became obvious though
that the impact would be visually disappointing from Earth. Despite what was
some occasionally blatant image massaging to make the difference between “pre”
and post-impact more obvious in some images posted in the Internet, nothing
could hide the fact that the comet had brightened much less than expected, even
by the most pessimistic predictions. The series of images (left) taken by
Fabiola Martín-Luis with the 82-cm IAC-80 Telescope at Teide Observatory some 8
hours before impact and then 16h and 40h after impact shows just how small and
temporary the brightening was.
So, what was it that “went wrong”
with the impact and how is it that the project took place in the first place?
Mission
history and background
Deep Impact was included in the NASA
programme of “Discovery Missions”. These are missions intended to give faster,
cheaper science than the old, classical missions on a grand scale such as
Voyager. To qualify as a Discovery Mission a project must cost under $500
million and take 18 months or less from go-ahead to launch. This necessarily
means that the missions must be much smaller in concept and simpler than a big
set-piece mission. Deep Impact was originally proposed in 1996 as a
high-velocity (38km/s) impact with the asteroid (3200) Phaethon, the parent of
the Geminid meteor shower and thus strongly suspected of being an extinct
comet. The idea was to observe the impact and to (briefly) reactivate the
asteroid as a comet. However, the review panel that scrutinised the proposal
rejected it on two grounds: they were unconvinced that Phaethon was a dead
comet and, what is more, sceptical that a probe could be targeted to impact.
The project was revised and resubmitted in 1998 with two key changes: the
target was changed from an asteroid to a low-activity comet and a targeting
sensor was added to improve the probability of a successful encounter. The
mission was composed of two parts: the mother ship and the impacter probe. Instrumentation
was, of necessity, limited by the budgetary and mass constraints. The mother
ship carried a 30-cm telescope, proudly billed as the largest telescope ever
carried into deep space with a high-resolution camera-spectrograph able to take
images in any one of 8 different filters, with a near-IR spectrograph to
analyse the impact plume. Additionally there was a 12.5-cm medium-resolution
telescope for wide-field imaging. The mother probe also had the high-gain
communications antenna and the computer for control and data storage. The
impacter probe, in contrast, was very basic. With a mass of 373kg the majority
of the probe was constructed from copper rather than the more standard
aluminium; this was done because aluminium has an extremely complex spectrum
when incandescent, similar to high-pressure sodium lighting, whereas copper has
just a few, narrow lines, more like low-pressure sodium. The more complex the
emission spectrum of the probe when it exploded, the more difficult it would be
to separate the spectrum of the impact plume from the contamination due to the
probe itself. The impacter carried a 12.5-cm telescope identical to that of the
mother ship. However, contrary to popular belief, it carried no explosives. The
impacter was a kinetic energy weapon only; at 10.2km/s the kinetic energy of
impact was equivalent to 14 times the probe’s mass in TNT; only the carrying of
a nuclear device on board would have increased the energy of impact.
After a delay of two weeks a perfect
launch took place on January 12th. The delay had no impact on the
encounter date, which was driven by a desire to encounter the comet at
perihelion (in fact, as is well known, as the comet’s perihelion passage was on
July 5th, the mission team decided to advance the encounter by one
day to take advantage of the coincidence in date. Unlike the Giotto fly-by of
1P/Halley, Deep Impact was a low-velocity encounter. Comet Halley has a
retrograde orbit obliging the approach to he head-on; in contrast, as 9P/Tempel
1’s orbit is prograde and almost in the plane of the ecliptic causing the
encounter to be a tail-chase.
9P/Tempel
Comet 9P/Tempel was chosen because
it is a highly evolved object, having been in the inner solar system with
perihelion distance less than 10AU for 300 000 years. Prior to 1648 the comet
had a much larger perihelion distance than now, making it much fainter. A close
encounter with Jupiter reduced the perihelion distance sharply and later
encounters have reduced the perihelion distance still further. It is no coincidence
that Ernst Tempel discovered the comet from Marseille on April 3rd
1867 shortly after a further downward drop in perihelion distance. The 1867
apparition was almost the best possible with the comet magnitude 9 at discovery
and the comet observable for almost 5 months until August 27th. The
comet was recovered from Marseille in 1873 by Stefan (of "Quintet"
fame) and then by Tempel himself in 1879 after which a new encounter with
Jupiter led to the comet being lost until Elizabeth Roemer recovered it in a single
image in 1967. Since 1967 the comet has been seen at every apparition. With a
5.5-year period, alternate apparitions are very good and very bad. However, a
new encounter with Jupiter in 2024 will increase the perihelion distance once
more and make the comet much fainter again. An extensive ground-based observing
campaign has been carried out to support the Deep Impact mission. Karen Meech
has led this. This campaign started in 1999 with a programme to measure the
rotation period of the nucleus. This suffered from enormous difficulties but
eventually a period of 1.71 days (40 hours) was measured. The light curve
amplitude of 0.6 magnitudes suggested a highly elongated nucleus with a mean
diameter of 5.2±0.4km, when combined with
an unknown rotation mode, this made the nucleus potentially an extremely
difficult target to hit; this was one of the biggest surprises of the mission
as the nucleus photographed by the probe was far less elongated than expected.
In parallel with the ground based science programme there was also a targeting
astrometry programme coordinated by Steve Chesley at JPL. This involved almost
exclusively amateurs taking high-precision astrometry to improve probe
targeting. This was successful in that the pre-release aim was within a few hundred
metres of the nucleus and considerably more accurate than after the first
post-release targeting manoeuvre.
The mainly Spanish
“Observadores_cometas” team contributed over 5000 photometric measures in
apertures from 10-60” (observations are still continuing), more than 2100
astrometric measures, 99 estimates of the total visual magnitude and some 150
images by 32 observers or groups of observers from 4 countries. The total
visual magnitude estimates are fitted by a relation of m1 = 6.39 + 5
log D + 24.68 log r. This is
similar to the light curve fit derived in 1999-2000. It has been suggested that
9P has been 20% less active in 2004-05 than in its previous return but, as it
was a particularly poor apparition with almost no data taken within 6 months
either side of perihelion we are talking about activity at high heliocentric
distance (looking at the light curve from 1999-2000 the dispersion is so large
that measuring a 0.2 magnitude difference in level with the current apparition
looks to be difficult).
The activity of the comet has been
strongly asymmetric about perihelion in 2004-05 with the highest value of Afr and thus dust production about 100
days before perihelion. However, interpretation is complicated by the fact that
much larger values of Afr are obtained with
smaller apertures (the graph, below, was obtained with 10”, corresponding to a
physical diameter of 5400km in mid-April) than with large ones. In fact, of the
some 200 comets that we have studied, none has 
had an activity curve
that was similar to this, although it is interesting that 150 days after
perihelion the amount of dust production is now close to that registered
pre-perihelion at the same “r” and the trend is now similar too. There is no
big rise in dust emission at impact (at T-1d); in fact, it is less than 30% in
a 10” aperture and smaller than the amplitude of the variations prior to
impact. An interesting feature of the activity of the comet pre-impact were the
well-publicised outbursts, for example, those seen by the HST on June 14th
and by Charles Morris on June 23rd. Water vapour production from the
SWAS instrument on SWIFT shows variations of over a factor of 2 prior to impact
superimposed on an apparent downward trend, with the SWAS data showing a
possible period of 8-9 days, similar to the separation between the HST and
Morris outbursts and consistent with the period of the “rippling” in the data for Afr shown above. An interesting
question is how much of the observed post-impact brightening was due to impact,
as both the SWAS and the Spanish data show the impact coming slightly after the
bottom of the cycle, leading the SWIFT team to comment prior to impact that the
comet was due to start to outburst from this intrinsic cycle approximately 1d
before impact. Such a cycle would be due to a small degree of precession in the
nucleus’s spin, although it is felt that the nucleus spin must be close to its
minimum energy state (i.e. it has only a small amplitude of precession): even a
precession by a few degrees could though lead to an active area becoming
critically illuminated for a few hours every precession cycle during part of
its inbound path and sparking an increase in activity in a similar way to the
nucleus of C/1995 O1 (Hale-Bopp) in Autumn 1995.
Impact
science
High-resolution imaging
is now available of four cometary nuclei: 1P/Halley (top left – Giotto);
9P/Tempel 1 (top right – Deep Impact); 19P/Borrelly (bottom right – Deep Space
1); and 81P/Wild 2 (bottom left – Stardust). Looking at the four nuclei it is hard
to credit that they all represent the same kind of object – evolved periodic
comets – as the differences from one to another are so strong. Certainly, none
of them look like the old Whipple model of the dirty snowball (centre). One of
the principal aims of the impact was to study the internal structure of the
nucleus. The classic model had a core, possibly even of solid hydrogen, wrapped
in layers of ices and covered by a porous crust. More recently though three
models have been favoured: a monolithic model in which the nucleus is built of
a single block of homo-geneous material; a differentiated nucleus, in which the
core may have different structure and composition; and a multicomponent model
(that exists in different varieties and which is the favoured model), in which
the different components of the nucleus may be only very weakly held together
like the evident rubble pile that is the asteroid Itokawa, either by mutual
gravitation, or by some binding icy “glue”. A mantle of dust then covers the
nucleus, possibly with a thin outer covering of ice. Different models exist
that suggest that the mantle covering the underlying pristine material may be
anything from 1-10 metres in thickness; thus it was believed that Deep Impact
would pass right through the dust mantle into pristine ice. By studying the
formation of the crater, its size and depth, it was expected that the structure
and tensile strength of the outer layers of the nucleus could be calculated.
Unfortunately, the large dust plume
that was produced in the impact completely obscured the formation of the
crater. Initially, contradictory information was released but, when all the
images had been returned to Earth, it became evident that no information on the
crater formation process could be extracted from them. The estimated size of
the crater (100-m) is based on the increase in the x-ray luminosity of the
comet after impact, given that the x-ray luminosity of a comet is known to be
correlated with its dust activity hence, from the increase in x-ray luminosity,
we can estimate the amount of dust expelled when the crater formed and thus the
volume of the crater. The characteristics of the dust suggest that the impact
occurred in a layer of extremely fine, highly dissecated dust like extremely
dry talcum powder and that the impact itself was similar to falling into a deep
drift of fresh, powdery snow. Observations from Earth have shown that there was
an enhancement by a factor of 2 in gas production on impact, but that this
lasted only about 15 minutes, while the central coma increased in brightness by
a factor of 5. This, and the lack of reaction in the light curve of the comet
suggests that the impact did not penetrate through to pristine material,
suggesting that either the dust layer is thicker than expected, or that the
impacter was stopped at a smaller depth than anticipated.
At 10.2km/s the approach to the
nucleus was rapid and high-resolution images of the surface are only available
for the last 15 minutes or so. The last image that was received was taken 3.7s
before impact from 38km altitude and has a resolution of 1-m. At the time of
this talk no processed images from the impacter have been made available. The
last image (right) that shows the entire nucleus, taken from 3000km, 300s
before impact, is particularly interesting. We can see at least four distinct
types of crater, including formations that look like the mouths of geysers and
bright, fresh impact craters. The impact point is marked with a small circle.
What is particularly interesting is to compare the plateau-like structure
marked by an ellipse, with a “meseta” structure on Mars observed by Mars
Express that is thought to be due to sublimation of permafrost; the
similarities between the two structures are evident.
Despite the rather
disappointing visual response of the nucleus on impact, some activity was seen.
HST observations show an obvious dust hood and high-resolution imaging from La
Palma with the NOT shows an apparent increase in the brightness of dust jets
that were visible on the evening before impact (but, once again, care must be
taken with interpreting these images as the exposure time was greater in the
post-impact images and the filtering used to enhance the jets does not conserve
the intensity on processing, in particular, it is not obvious that all the jets
should brighten so obviously post-impact rather than just the one from the
impact site. The Swift, ultraviolet light curve (left) shows how rapid the post
impact rise in brightness was and how quickly it started to decline. A similar
pattern is seen in visible and near-IR monitoring, with the largest effects on
impact logically seen in the smallest apertures.
Some of the most eagerly awaited
results have been those from the mother probe’s infrared spectrometer. At the
time of the talk the first results had just been released and analysis was at
an extremely early stage, with less than 2% of the spectra received from the
probe processed to date. The results though are interesting and cast some light
on the nature of the surface. 
The spectra cover a range
from 2-4.5 microns that contains a number of spectral bands of interest. The
slit of the spectrograph was centred slightly south of the impact point. The
aim was to measure the spectrum of the debris cloud that was expelled.
Two spectra are shown: the lower
line is the spectrum 0.1s before impact – this spectrum is essentially
featureless. The upper curve is the spectrum taken 0.6s after impact; on it is
superimposed a model spectrum (the grey curve). The model supposes that there
is a cloud of carbon dioxide and water vapour at 1400K that is seen
superimposed on a hot spot on the nucleus that is heated to 800K by the impact.
The model gives a good fit to the observed spectrum except between 3.1 and 3.7
microns where a strong emission is seen. This band corresponds to the
wavelength of the carbon-hydrogen bond found in organic compounds. The
implication of this spectrum is that a significant quantity of water and carbon
dioxide was released on impact however, the enormous strength of the band from
the C-H bond indicates that huge quantities of organic compounds were present
in the surface layers and volatilised by the impact. This has confirmed the
suggestions that the material of the dust mantle on the surface of cometary
nuclei is rich in organic compounds; it also confirms the Giotto observation
that a significant amount of relatively high molecular mass polymers were
present in the dust in the coma of 1P/Halley.
Conclusions
The results of the impact
were initially disconcerting but, despite the failure to fulfil the prime
objective of the mission: to image the formation of the crater and to measure
its size, the mission has been a resounding success. The fact that the initial
impact was so spectacular made the lack of a strong, sustained reaction from
the nucleus disappointing. Based on the best available knowledge of the
structure of the nucleus it was expected that the impact would release about as
much gas and dust as in as much as a week of normal activity, hence the
expectation that the comet would become an easy naked-eye object. It was also
expected that the crater formed would expose bare, virgin ice that could make
the comet brighter and more active than usual for several returns. What
actually happened is that rather than hitting a relatively solid surface the
impacter fell into deep, extremely fine and dissecated dust, penetrating a
considerable distance but evidently not exposing fresh ice from the sub-surface
layer. The two moderate-sized craters close to the impact site are somewhat
ill-defined in the images suggesting that the dust drifts at the impact point
may have been especially deep compared to other areas of the nucleus and were
certainly extremely depleted in volatiles. Interesting though there are some
bright albedo features very close to the impact site that are suggestive of ice
or frost deposits; these are also seen around the edges of the meseta north of
the impact point. This suggests that not all areas of the nucleus are similar
to each other in structure. Probably the results are not going to be sufficient
to allow major conclusions about the internal structure of the nucleus, which
is disappointment, although as analysis proceeds we are going to learn an
enormous amount about the nucleus of 9P/Tempel 1 in general.