PLEASE NOTE:
*
CCNet SPECIAL 3 March 1999
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INVESTIGATING NEAR-EARTH ASTEROIDS
==================================
Alan Harris and John Davies
From ASTRONOMY & GEOPHYSICS: The Journal of the Royal
Astronomical
Society, 40 (1) Feb. 1999, pp. 10-13*
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*The permission of the editor of A&G to circulate this
article on the
CCNet is gratefully acknowledged.
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Alan Harris and John Davies report on current astronomical
investigations of near-Earth asteroids, biased somewhat by a
thermal
infrared perspective and recent results obtained with the UK
Infrared
Telescope in Hawaii.
ABSTRACT
Recent publicity, including the Hollywood blockbusters "Deep
Impact"
and "Armageddon", has raised the level of public
awareness that
comets and asteroids frequently fly by the Earth at distances
close
to zero in astronomical terms. In fact, the Earth moves among a
swarm
of such objects, mainly asteroids, and more are being discovered
almost daily. Knowledge of the size distribution, surface
properties
and compositions of near-Earth asteroids is essential for
investigations of their origins, their relation to meteorites
and comets, the role they have played in the evolution and
development of the terrestrial planets and life on Earth, and the
current threat they pose to civilization as potential impactors
on
the Earth. Techniques applied to the study of near-Earth
asteroids
and some recent results are described.
INTRODUCTION
Asteroids are remnant material from the processes of formation
and
initial development of planets, and therefore a source of
information
on conditions in the early solar system. Most asteroids orbit in
the
main belt which lies between the orbits of Mars and Jupiter.
Collisions between main-belt asteroids give rise to fragments,
the
orbits of which can evolve under the gravitational influence of
Jupiter to become highly elliptical and eventually cross the
orbits
of Mars and the Earth. The asteroid population known as
near-Earth
asteroids (NEAs) consists mainly of such objects, but possibly
includes some nuclei of evolved or extinct comets.
New NEAs are being discovered almost daily by automated search
programmes such as the University of Arizona's
"Spacewatch" project,
NEAT (Near-Earth Asteroid Tracking) which is run from the Jet
Propulsion Laboratory in Pasadena, and LINEAR (Lincoln
Near-Earth
Asteroid Research) run by MIT's Lincoln Laboratory. At present
the
only European NEA search programme is ODAS, operated jointly by
the
Observatoire de la Cote D'Azur in France and the German Aerospace
Center (DLR). At the time of writing the number of known NEAs is
around 600, of which over 200 are larger than 1 km (large enough
to
cause a global catastrophe if they impacted on Earth). The
current
rate of discovery of NEAs with diameters of 1 km or more is
around 40
per year and it is estimated that some 2000 such objects remain
to be
discovered.
While close approaches of NEAs to the Earth may not bode well for
the
long-term future of our civilization (see, for example, Ostro and
Sagan 1998) they provide astronomers with a unique opportunity to
study a large population of planetary bodies in detail with
groundbased and orbiting facilities. A few NEAs each year are the
subjects of international observing campaigns which utilize
optical,
infared and radar instrumentation.
OBSERVING NEAR-EARTH ASTEROIDS
A major problem for NEA observers is that a small asteroid's
apparent
brightness typically passes rapidly through a peak and then
diminishes as it flies by the Earth. This means that useful
observing
windows, while accurately predictable, are often limited to a few
weeks or less. An observer may have to wait many years before a
favourite NEA comes round again.
Rotation rate is a basic parameter of NEAs that can be routinely
measured using modest groundbased optical telescopes equipped
with
modern CCD cameras and data-reduction software. In general, as a
non-spherical object rotates, the amount of light reflected in
the
direction of the observer varies. Observers compile lightcurves
from
their photometry and find the rotation period that best fits the
data, taking into account the changing observing geometry and
light
travel time as the object moves along its trajectory in
near-Earth
space. Measured NEA rotation periods are mostly in the range of a
few
hours to a few days. Lightcurve observations spanning sufficient
time can also provide important information on an object's shape
and
the direction of its rotation axis. Since most small asteroids
are
probably fragments of larger bodies, knowledge of their dynamical
states and shapes can throw light on the collisional evolution of
asteroids in the main belt.
Spectroscopy in the optical and near infrared regions provides
crucial information on mineralogy. Furthermore, the spectral
reflectivity of asteroid surfaces provides a basis for
classifying
them into one of some 14 types with similar spectral
characteristics.
The taxonomic system ranges from very dark objects with albedos
of
0.05 - 0.1 (e.g. classes D and C), which have features indicating
the
presence of organic molecules and silicates similar to those
found in
carbonaceous chondrite meteorites, to objects with albedos of 0.5
or
higher (class E) having the spectral signature of iron-free
silicate
minerals. In the case of NEAs the most populous class appears to
be
S, characterised by the presence of pyroxene, olivine and metals,
and
albedos in the range 0.1 - 0.3. There appears to be an
unexplained
lack of C-type NEAs compared to the main belt population. The
study
of NEA taxonomy provides important clues as to the nature of the
parent bodies.
One of the most powerful techniques for probing NEAs is radar.
Objects which approach the Earth to within about 0.04 AU (about
15
Earth-Moon distances) can even be "imaged" using a
technique, first
applied to asteroids by Steve Ostro and colleagues of the Jet
Propulsion Laboratory, that combines time delay or range
information
with the Doppler frequency spread (resulting from the target's
rotation) in the radar echo. In combination with results from
optical
and infrared observations, radar data can provide a wealth of
information on NEAs, including extremely accurate orbital data,
size,
shape, surface roughness and composition, rotation rate and axis
orientation. Facilities used frequently for NEA radar
observations
are the 70-m and 34-m antennae of the NASA Goldstone Deep Space
Communications Complex in California and the 305-m dish at
Arecibo in
Puerto Rico operated by Cornell University. Unfortunately, the
signal/noise ratio of radar observations decreases with the
fourth
power of distance, so high precision work is limited to
relatively
few objects which make very close approaches.
ABSOLUTE BRIGHTNESS AND MAGNITUDE
To facilitate comparison of the absolute brightnesses of
asteroids,
observed optical magnitudes can be converted to
"reduced" magnitudes,
V(alpha), by normalising to unit heliocentric (r) and geocentric
(Delta) distances measured in astronomical units. That is,
V(alpha) =
Vobs(alpha) - 5log(r.Delta), where Vobs(alpha) is the observed
optical magnitude at solar phase angle alpha. V(alpha) can be
further
reduced by correcting to alpha = 0 (opposition). This is more
complicated, requiring knowledge of the dependence of the
object's
brightness on solar phase angle (the "phase curve"),
but a standard
procedure is commonly used (Bowell et al, 1989). The absolute
magnitude of an asteroid is taken as V(0) averaged over the
rotation
period. The absolute magnitude is thus independent of observing
geometry; it is a physical property of the object that depends on
its
size and albedo. The albedo is an important indicator of surface
composition and taxonomy. However, a spherical object with a
diameter
of 2 km and an albedo of 10% will have the same absolute
magnitude as
one with a diameter of 1 km and an albedo of 40%. Thus, in
general,
size and albedo cannot be determined independently on the basis
of
optical photometry alone.
Fortunately, thermal infrared measurements provide a second
relationship linking size and albedo. A large, dark object may
have
the same absolute optical magnitude as a small light object but
its
emission in the thermal infrared will be greater because it is
bigger
and hotter. Given optical photometry and spectrophotometric
observations in the 10 - 20 micron region made with a suitable
infrared telescope, such as the UK Infrared Telescope (UKIRT) on
Mauna Kea, both the size and albedo of a NEA can be accurately
established.
SIZE AND ALBEDO DETERMINATIONS
While various methods, such as occultation measurements and radar
observations, can provide accurate size information in certain
cases,
most published asteroid diameters and albedos are based on
thermal
infrared data. The accuracy of asteroid size and albedo
determinations from optical and infrared measurements depends on
the
validity of the various assumptions embodied in the thermal model
used. The most commonly applied simple asteroid thermal model is
the
so-called "Standard Thermal Model" (STM) which assumes
an idealized
non-rotating (or slowly rotating with negligible thermal inertia)
spherical object in thermal equilibrium with incident solar
radiation; that is, the temperature of each surface element is
determined by the amount of incident solar energy and the
bolometric
albedo and emissivity. The temperature decreases from a maximum
at
the subsolar point to zero at the terminator and there is no
emission
from the night side. Given that there is no such thing as a
spherical, non-rotating asteroid it is perhaps surprising that
the
STM performs so well, at least when applied to main belt
asteroids.
The STM, with parameters as given by Lebofsky et al (1986), was
used
in the derivation of albedos and diameters for some 2000 (mostly
main-belt) asteroids in the IRAS Minor Planet Survey (Tedesco
1992).
An alternative model is sometimes used, which differs from the
STM in
the form of the temperature distribution over the object's
surface. A
rotating asteroid will emit significantly on the night side if
its
rate of rotation and/or thermal inertia are high enough. To
account
for this, the "Fast-Rotating Model" (FRM) assumes a
surface
temperature distribution that depends only on latitude, that is,
it
decreases from a maximum at the equator to zero at the poles. It
is
assumed that the object's spin axis is perpendicular to the plane
containing the asteroid, observer, and Sun.
While the STM works well with main-belt asteroids, neither of
these
simple models produces consistently good results with NEAs. This
is
hardly unexpected given the irregular shapes of NEAs and the fact
that, unlike main-belt asteroids, they are often observed at high
solar phase angles. Furthermore, small objects with low
gravities,
which arise as fragments from a collision of larger bodies, may
be
expected to have surfaces consisting mainly of bare rock, rather
than
the mature, dusty surfaces characteristic of the Moon and large
asteroids that have retained the ejecta of numerous impacts.
Since
rock has a higher thermal inertia than dust, such objects may
have
different thermal characteristics compared to larger main-belt
asteroids. Most NEAs probably have thermal characteristics
somewhere
between the extreme cases described by these simple models.
Unfortunately, the lack of information on physical parameters
such as
shape, spin axis orientation, sense of rotation and surface
roughness
seriously reduces the usefulness of more sophisticated
thermophysical
models in the case of NEAs.
A Near-Earth Asteroid Thermal Model (NEATM), which is a modified
version of the STM described by Lebofsky et al (1986), has been
developed at the DLR Institute of Planetary Exploration in Berlin
to
maximise the accuracy of NEA size and albedo determinations from
thermal infrared spectrophotometry (Harris 1998).
The main difference between the NEATM and the STM of Lebofsky et
al
is that a key model normalization parameter, eta, namely an
adjustment to the surface temperature distribution to account for
the
effects of surface roughness and thermal inertia, is derived
individually for each object via spectral fitting to the
observational data, rather than taking a set value derived from
observations of the large main-belt asteroids Ceres and Pallas.
Given
its rotation period, information on an asteroid's surface thermal
inertia can be obtained from the best-fit value of eta. By
collaborating with optical observers it is possible to ensure
that
the thermal infrared and optical measurements used in the
analysis
refer to the same rotational phase, for example lightcurve
maximum,
thus reducing errors caused by an object's non-spherical shape.
In
most cases diameters derived using the NEATM lie between those
from
the STM and FRM and are probably accurate to within 15%.
A SAMPLE OF OBSERVATIONAL RESULTS
Recent (provisional) results from UKIRT observations illustrate
the
usefulness of NEATM. The UKIRT CGS3 spectrometer was used to
obtain
10 and 20 micron measurements of the S-class asteroid 433 Eros in
June 1998 near lightcurve maximum. Eros is a large, well-studied
NEA
with a relatively accurately known size derived from earlier
radar
and thermal infrared observations (see Harris 1998, and
references
therein). Of the three thermal models mentioned above, the NEATM
fits
the data best. The corresponding NEATM diameter is 21 km,
compared to
18.5 km from the STM and 30 km from the FRM. For comparison, the
effective diameter (i.e. that of a sphere of equivalent projected
area), derived from radar and other observations is about 23 km
(Zellner, 1976; Mitchell et al 1998), which is probably accurate
to
10%.
Accurate correction for lightcurve variations leads to even
better
agreement between the NEATM and earlier results (Harris and
Davies,
in preparation). The albedo values resulting from the model fits
to
the data are 0.32, 0.13 and 0.26 for the STM, FRM and NEATM,
respectively. Only the FRM and NEATM albedos lie within the range
expected for an S-class asteroid. It will be interesting to see
if
the NEAR (Near-Earth Asteroid Rendezvous) spacecraft, scheduled
to
reach Eros in January 1999 (now 2000), confirms the NEATM and
radar
results.
Other NEAs that have been observed by the authors and their
colleagues include 2100 Ra-Shalom, 3200 Phaethon, 3671 Dionysus
and
6489 Golevka.
In the cases of 2100 Ra-Shalom and 3200 Phaethon the thermal
spectral
peak lies well to the long-wavelength side of the model
prediction
from the STM, requiring relatively high values of eta to fit the
spectrum using the NEATM and indicating smoother, cooler
temperature
distributions characteristic of rocky, high thermal inertia
surfaces.
Knowledge of NEA surface temperature distributions is important
for
the design of NEA lander missions, of which at least one is
currently
planned. Scientific instruments and mechanical systems may
operate
effectively only in a specified temperature range.
The diameter and albedo of Ra-Shalom, implied by the NEATM best
fit
thermal spectrum, are 2.5 km and 0.13, respectively. This
diameter is
in agreement with radar observations which constrain the
effective
diameter to be larger than about 2.4 km. The albedo, however, is
larger than expected for a C-class asteroid and, together with
observed near infrared colours, indicates that Ra-Shalom may have
characteristics more typical of S-class asteroids.
Among known NEAs Phaethon is one of the few objects considered to
be
candidate extinct cometary nuclei: its orbit is close to the mean
orbit of the Geminid meteors, which suggests it may be their
parent
body. However, a cometary nucleus would be expected to have a
dark,
dusty surface with a relatively low thermal inertia. In the case
of
Phaethon, the UKIRT results based on the NEATM best fit thermal
spectrum imply a surface thermal inertia (taking into account the
known rotation rate) at least six times that of the Moon, i.e.
characteristic of solid rock, which argues against a possible
cometary origin (for further details see Green et al 1985, Harris
et
al 1998). In fact, the search for a convincing "dead
comet" among the
NEAs, in terms of physical rather than dynamical characteristics,
so
far remains unsuccessful.
A joint ESO/UKIRT optical/infrared observing campaign in 1997 on
3671
Dionysus produced an unexpected result. Some optical lightcurve
measurements, made with the 60-cm Bochum telescope at ESO in
Chile by
Stefano Mottola and Gerhard Hahn of the DLR Institute of
Planetary
Exploration in Berlin, showed unusual dips that re-appeared at
regular intervals. The most likely cause was eclipses by an
orbiting
companion or moon, which periodically reduced the total amount of
reflected sunlight observed. Thermal infrared observations made
with
UKIRT and ESA's orbiting Infrared Space Observatory indicate that
the
effective diameter of Dionysus (uneclipsed) is only 1 km or less,
which would certainly make this an exceptionally small binary
asteroid and may have important theoretical implications for the
formation and nature of NEAs. Significant numbers of binary
asteroids
are in fact expected to exist as an explanation for the
occurrence of
many doublet craters on the Earth, Moon and other solar system
bodies.
A good example of the interplay between radar, optical and
thermal
infrared observations of NEAs is the 6489 Golevka campaign of
1995.
This object, discovered in May 1991 by Eleanor Helin with the
46-cm
Schmidt telescope on Mt. Palomar, was initially labelled 1991 JX.
It
was renamed 6489 Golevka in 1996 in honour of the first
intercontinental radar astronomy experiment, during which signals
transmitted by the 70-m Goldstone (USA) antenna were reflected
off
the object and received at stations in Evpatoria (Ukraine),
Kashima
(Japan), and at the 34-m antenna at Goldstone. Optical
observations
from 11 observatories around the world were used to compile
lightcurves, while UKIRT provided a measurement of the 10-micron
thermal emission (unfortunately the object was too faint for
thermal
infrared spectrophotometry). The optical lightcurves showed the
period of rotation to be 6.03 hr and indicated a rather irregular
shape.
As a result of the five-month extensive optical campaign it was
possible to determine the direction of the spin axis. Since the
object passed through opposition (zero solar phase angle) during
the
observing period, it was also possible in this special case to
derive
an estimate of the albedo from the optical data alone using a
suitable photometric model. The result of 0.61 is unusually high
for
an asteroid. The analysis of the UKIRT observations benefited
from
the knowledge of the spin axis orientation, which was taken
account
of in the thermal modelling and enabled more accurate estimates
of
diameter and albedo to be derived from the single 10-micron
observation. The resulting values of 0.30 km and 0.57, for the
effective diameter and albedo, confirm that Golevka is a very
small
and highly reflective object (note how well the independent
albedo
values agree). The results described here (see Mottola et al
1997,
for more details), in particular the period and spin axis
orientation, constitute valuable input for the radar observers in
their modelling of the geometry and physical properties of the
object's surface. Preliminary results reveal a highly irregular
and
angular object with largest dimension around 0.5 km, and are in
broad
agreement with the results from the optical and infrared data
described here (Ostro et al 1995a, Zaitsev et al 1997).
SUMMARY
Near-Earth asteroids are now enjoying greatly increased
attention,
due mainly to the realisation that they have played a major role
in
the Earth's development and could dramatically affect the
long-term
future of our civilization upon it. Astronomical observations
employing a variety of techniques on different types of telescope
are
enabling a catalogue of physical parameters of NEAs to be
compiled.
This is starting to reveal fascinating details about their sizes,
shapes, structure and composition, and their relationship to main
belt asteroids and other solar system bodies. However, the
present
high rate of discovery of new NEAs, which is likely to increase
further as new search programmes become established, leads to a
growing need for follow-up observations using optical, infrared
and
radar telescopes. On the basis of current estimates, many
hundreds of
objects need to be observed in detail in order to adequately
sample
the physical properties of the NEA population and develop an
understanding of their nature, the role they have played in
planetary
development, and the risk they pose to life on Earth.
[A. W. Harris is at the German Aerospace Center's (DLR) Institute
of
Planetary Exploration, Berlin (alan.harris@dlr.de). J. K.
Davies is a
UKIRT support astronomer at the Joint Astronomy Centre, Hilo,
Hawaii
(j.davies@jach.hawaii.edu).]
References
Bowell E et al. 1989 Asteroids II,Univ. Arizona Press,Tucson,
p.524.
Green S F et al. 1985 MNRAS 214 29p.
Harris A W 1998 Icarus 131 291.
Harris A W et al. 1998 Icarus, 135 441.
Lebofsky et. al. 1986 Icarus 68 239.
Mitchell D L et. al. 1998 Icarus 131 4.
Mottola S et al. 1997 AJ 114 1234.
Ostro S J and Sagan C 1998 A&G 39 4.22.
Ostro S J et al. 1995a BAAS 27 1063.
Tedesco E F (Ed) 1992 Tech. Rep. PL-TR-92-2049
Phillips Lab., Hanscom AF Base, MA.
Zaitsev A L et al. 1997 Planet. Space Sci 45 771.
Zellner B 1976 Icarus 28 149.
Copyright 1999, Astronomy & Geophysics, The Journal of the
Royal
Astronomical Society.
*
MY GRANDFATHER PREDICTED IMPACT HAZARD 70 YEARS AGO:
'INVEST TO AVERT ARMAGEDDON', LEMBIT OPIK (MP) URGES GOVERNMENT
From The BBC Online News
http://news.bbc.co.uk/hi/english/uk_politics/newsid_289000/289733.stm
Wednesday, March 3, 1999 Published at 12:55 GMT
The earth is due to be struck by a giant asteroid capable of
wiping out
the entire human race, a Lib Dem MP has warned. Lembit Opik is
urging
the government to invest £1m a year in tracking space rocks to
avert
catastrophe.
He told BBC News Online: "I'm calling on the government to
take
seriously the prospect of asteroid or cometary impact with the
earth.
"Now that's got a pretty high giggle factor, it makes me
sound like one
of those millennium soothsayers, a Nostradamus of Parliament, but
actually it's a very serious threat. We know that the dinosaurs
were
extinguished by a very big global killer that hit us about 65
million
years ago and we're due for another one now."
Mr Opik said the cost of tracking asteroids to the international
community as a whole would be about £70m over the next decade.
The UK should invest between £500,000 and £1m annually towards
this, he
said. "It's a good investment really because for that small
investment
not only do we get peace of mind but we can quite literally avert
armageddon."
The MP said he hoped for a serious and positive response when he
raises
the issue in an adjournment debate in the House of Commons on
Wednesday
evening. "Initially the government were quite sceptical but
I am
optimistic John Battle, the minister who must reply to me
tonight, is
taking it more seriously than they have in the past.
"At the end of the day, if we don't act on it, something
could come out
of the sun and wipe us out, quite literally."
Mr Opik said the campaign was not connected to the rash of
apocalyptic
predictions surrounding the year 2000. "I can assure the
public this is
neither my attempt to campaign for the millennium nor my strange
and
obscure way to become leader of the Liberal Democrats. This is my
genuine interest in the science and the astronomy and ultimately
the
future of our species."
His fascination with the issue stems from family tradition.
"My grandfather was an astronomer and he worked in just this
field. In
fact, he predicted the dangers of an asteroid impact 70 years
ago. They
named an asteroid after him - Opik's asteroid. I'm relieved to
say that
we know where Opik's asteroid is, it's tucked up nice and safe in
the
asteroid belt."
But other space rocks are not so far away and statistically the
earth's
population has reason for concern.
"These global killers seem to hit about once every 30
million years.
What's worrying is that the last one impacted 65 million years
ago and
wiped out the dinosaurs. I'm sorry to say that we're next in line
for
extinction.
"The dinosaurs were wiped out by a massive global killer,
but a quarter
of the earth's population could be wiped out every 100,000 years
when a
one kilometre object hits. If we saw an asteroid hurtling towards
us
then I'm sorry to say all we could do is pray. But we would get
20
seconds and that's not even long enough for the Lord's Prayer.
"If we make this investment then we would get anything from
two years
notice of an impending impact and that's long enough to divert
the
object."
This might involve exploding a nuclear device or towing the
asteroid
out of the harm's way, he said.
Copyright 1999, BBC