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 The
Near Earth Space Surveillance
(NESS) mission concept has been supported by
UTIAS/SFL and a team of asteroid scientists, through
the Canadian Space Agency (CSA) and Defence Research and Development
Canada (DRDC Ottawa). NESS uses a single satellite to perform
a dual mission: searching for and tracking Earth-approaching
asteroids, and tracking satellites in Earth orbit. There are
aspects of both of these activities that are best accomplished
using an orbiting observatory. The concept is to implement NESS
using a small imaging telescope mounted on a low-cost microsatellite-class
platform, based on the design developed for the MOST stellar
photometry microsatellite mission.
Introduction
The
capabilities of microsatellites continue to grow more sophisticated
with each passing year, allowing them to be used to carry
out types of missions that once required much larger satellite
platforms. In the early 1990s, miniaturization of digital
electronics enabled microsats to demonstrate orbiting store-and-forward
packet radio communications. Miniaturization of optical sensors
then allowed optical remote sensing to be carried out from
this class of platform.
Up
until recently, imaging space science missions still relied
on larger, more expensive satellites. However, the development
of miniaturized attitude control system components such as
reaction wheels and star trackers, has allowed the development
of the MOST mission for the Canadian Space Agency, in which
a microsatellite-based space telescope employs an attitude
control system similar to that of the Hubble space telescope.
These
new capabilities also enable other types of missions to take
advantage of the benefits of using the microsatellite approach-such
as shorter development times, and much lower costs than for
larger satellites-while achieving high levels of performance
and capability. The Near Earth Space Surveillance mission
is one such mission.
Mission
Rationale
Mission #1: Asteroid Science
NESS
proposes to use a space-based telescope to observe asteroids,
in order to extract scientific knowledge from those observations.
This is, of course, an activity that can be (and is) carried
out using ground-based telescopes. Being located on the ground,
however, imposes some limitations that interfere with the
ability of scientists to make some very important types of
observations; by being in orbit, NESS is able to fill these
observational gaps.
Science
Context
 Asteroids,
the smallest planetary objects in the Solar System, are studied
for several important reasons. Until recently, they could
only be studied from afar by astronomers, who have discovered
them one by one, and measured their orbits, their brightness
and their reflectance spectra. With the advent of planetary
exploration space missions, such as NASA's current Near
Earth Asteroid Rendezvous (NEAR) mission, spacecrafts
are underway to inspect some asteroids directly, collect from
them and analyze samples, and eventually return samples to
Earth for detailed study.
This
science has already resulted in an improved understanding
of the evolution of the solar system, and of the genesis of
Earth and the other major planets, and much more remains to
be discovered; for example, asteroid and comet collisions
may have played an important role in the genesis of life on
Earth (more
info). In addition to their scientific value, some asteroids
represent a threat to the Earth; there are many Near Earth
Asteroids (NEAs) which follow orbits that bring them close
to the Earth, and there is ample evidence that asteroids of
all sizes have collided with the Earth throughout history,
causing damage and climatic disruptions ranging up to the
level of world-wide mass extinctions (e.g., click
here). Recent international efforts have greatly improved
the ability to predict far in advance how closely large asteroids
will approach the Earth, potentially providing enough advance
warning to allow action to be taken to avoid damage from such
collisions, making this area of science potentially of the
greatest possible practical importance to the entire population
of the Earth.
The
international NEA effort is carried out by a community of
researchers who perform three broad tasks. The first is performing
search programs in order to discover new NEAs. The second
is carrying out astrometric and photometric data processing
on these discovery observations, followed by computation of
orbital elements and rapid dissemination of these elements
to many follow-up observers. This enables the third task,
rapid astrometric confirmation and follow-up of the discoveries,
in the absence of which newly-discovered NEAs would promptly
be "lost" again. In addition to these NEA-focused
tasks, some astronomers also perform photometric, spectrographic
and polarimetric observations of asteroids of all types, from
which information on asteroid size, albedo, spin rate and
composition can be deduced.
Potential
NESS Science Opportunities
Aten-class
NEA Search: A space-based observatory has a very great
advantage over observatories on the ground, when used to search
for Atens. This is because most of the time Atens are closer
to the Sun than the Earth is, and so a telescope must point
less than 90 degrees away from the Sun's direction to track
them (that is, the target's "phase angle" is >90
degrees). When this is done from the ground, proximity of
the Sun to the telescope's line of sight causes sky-glow,
which obscures all but the brightest, closest objects. This
may explain, at least partly, why the known population of
Atens comprises a mere 7% of the known population of the near-Earth
asteroids: there may be nearly as many Atens as Apollos, but
because Atens are hard to observe with current ground-based
instruments, most of them may go undetected.
However,
a telescope in Earth orbit does not have to contend with sky
glow; while scattered light is an issue, suitable baffle design
can allow such an instrument to point quite close to the Sun's
direction (perhaps closer than 45 degrees), while maintaining
a very dark image background level. The observed population
of NEAs is highly biased toward the most easily observed objects.
The current sample of NEAs is complete only for objects in
the 8 km diameter range. The level of completeness for asteroids
in the 1 km range is estimated as 12%. Observational methods
are unreliable in determining an accurate estimate of the
number of objects less than 1 km in diameter (sub-critical
impactors). NEA populations may be estimated by extrapolation
from the known sample of objects and combined with calculated
dynamical lifetimes to produce estimates of the numbers of
objects in each size range; de-biasing these results in a
population estimate of 2.6 x 10^5 NEAs in the range of diameter
100-1000 metres. The total number of critical impactors (diameter
> 1 km) is expected to be 1000-3000. If the Aten population
accounts for about 25% of the NEA sample then we would expect
there to be several hundred Atens larger than 1 km in diameter
and perhaps 60,000 in the 100-1000 m range, providing a strong
likelihood of NESS making numerous discoveries of new Aten-class
NEAs.
In
addition, theoretical investigations of the long-term orbital
behavior of NEAs have shown that Aten objects may dynamically
evolve into orbits that are completely interior of the Earth's
orbit (as "Inner Earth Objects" (IEOs), or Arjunas);
NESS could have the capability of detecting these hypothetical
objects.
NEA
Rapid Astrometric Confirmation and Follow-up: While
many newly-discovered NEAs can be followed-up using ground-based
instruments, these are relatively poor at doing follow-up
tracking of NEAs with aphelions close to the Sun and comparable to that of Earth's
orbit. This is due to the short time spent by such objects
at opposition to the Earth before disappearing into the dawn
sky; objects that are discovered during extreme close encounter
(by definition, these are Potentially Hazardous Asteroids)
will traverse the night sky in a matter of days (or in some
cases hours!) and fade into the daylight sky before a fully-reliable
orbital solution can be attained.
An
orbiting telescope such as NESS would be able to track this
type of object effectively for a considerably longer time.
Note that objects of this type tend to suffer large numbers
of close Earth encounters before actually impacting the planet,
and predicting their orbit's evolution with time requires
numerous observations over a long period of time; only a space-based
astrometric telescope could provide the observations necessary
to keep track of these objects.
Ground-based
instruments are also completely incapable of tracking asteroids
whose aphelions are less than that of Earth's orbit. These
classes of objects represent an important segment of the NEA
population for which only NESS would be able to perform astrometric
observations.
Compositional
studies: Analysis of visual and near-infrared spectral
reflectances and spectral albedos provide the primary means
of characterizing the surface mineralogy and petrology of
asteroidal bodies. Ground-based colorimetry has contributed
much toward understanding the mineralogy of the NEAs; to date,
however, only a dozen Aten asteroids have been studied in
this way. Addition of an ECAS (Eight Colour Asteroid Survey)
broad-band filter system would allow NESS to be used to determine
the mineralogical properties of an asteroid simultaneously
with the acquisition of astrometric data. The further addition
of photometric/polarimetric sensing capabilities could allow
NESS to determine asteroid spin rates, sizes, albedos and
compositions, for NEAs that are inaccessible to existing ground-based
observatories.
Mission
#2: Satellite Tracking
Canada
and the United States are partners in NORAD, the North American
Aerospace Defense Command. NORAD historically has used the
satellite-tracking services that are now provided by the U.S.
Space Command, to distinguish between ballistic missiles approaching
North America, and the >23,000 detectable man-made objects
currently orbiting the Earth. This satellite tracking function
is fed by data collected by the radar and optical sensors
of the Space Surveillance Network (SSN), which continually
measure the distance and/or direction from each sensor to
satellites passing overhead.
In
the past, Canada contributed sensors to this data-collection
activity-a set of Baker-Nunn optical film cameras located
in Canada. With the advent of electronic imaging sensors,
the Baker-Nunn instruments have been decommissioned, replaced
by new instruments such as GEODSS. Canada's Department of
National Defence has recently initiated a Surveillance of
Space (SOS) program, in order to contribute new Canadian sensors
to the SSN. Ground-based optical sensors are essentially optical
telescopes, which take images of patches of the sky, which
are analyzed to find the moving star-like images of satellites.
There are several limitations that constrain the operations
of any such sensor:
- They
cannot operate during day-time, or too close to sunrise
or sunset, due to sky-glow caused by the Sun obscuring the
faint images of the satellites being tracked.
- Their
effectiveness can be diminished at times when the Moon is
in the sky, again due to sky-glow.
- Their
effectiveness is diminished by cloudy or misty weather,
frequently to the point of zero effectiveness.
- Less
than half of the sky is visible to the sensor at any one
time, due to obscuration by the Earth.
For
these reasons, the amount of useful operational time from
any one optical sensor can be very low, depending somewhat
on geographic placement. Another significant geographic factor
is that sensors located outside of Canada's borders would
be difficult to maintain and operate, for logistical reasons.
It is notable that several of these limitations are not applicable
to a space-based optical sensor, and the constraints imposed
by the remaining limitations are significantly reduced. An
orbiting optical sensor could be much more productive than
a ground-based one, as well has having a much more reliable
and rapid response time.
One
question that arises naturally relates to telescope aperture
size: can a telescope with sufficient photometric sensitivity
to carry out a useful satellite-tracking mission, be built
and flown within the small size constraints of an affordably-small
satellite? The answer is "yes," as has been demonstrated
by the Space Based Visible (SBV) experimental telescope 4,
flown as a payload aboard the BMDO's Midcourse Space Experiment
(MSX) satellite in 1996. This telescope has a 15-cm aperture-interestingly,
the same aperture size as for MOST's telescope-and is routinely
detecting satellites as faint as M=15. The sensor's performance
is good enough that, after its experimental phase concluded
successfully, the Space Surveillance Network began using it
as a Contributing Sensor.
For
these reasons, in addition to including several new ground-based
optical sensors, the DND's SOS program will also involve Canada
launching a satellite system to be used to track other satellites.
This element of the Canadian SOS system will likely primarily
do routine tracking of communications satellites in geostationary
orbit, and other high-orbit Earth satellites (which in NORAD
parlance are called "Deep Space Objects").
Based on the MOST satellite's precise photometry and imaging capabilities, the DRDC has developed an interest in the use of microsatellites to conduct some of this research in a rapid, low-cost way. The feasibility of doing this has been reinforced by the striking similarities between the MOST and SBV instruments, and the similarity in attitude control performance between MOST and the MSX satellite.
This
has led to the concept of DRDC participating in the NESS mission,
to carry out research in support of the development of SOS
system requirements. (To pick just one of many examples, fundamental
design decisions such as the question of how much image processing
for target detection should be done on the ground versus on
the satellite, depend critically on difficult-to-predict details
of pixel-level imager noise characteristics. Flying an engineering
model imager in the relevant environment, i.e. in orbit, would
be a powerful way to validate analytical models of this type
of effect.) This concept places a schedule requirement on
the NESS mission: that it be developed promptly enough so
that flight test results are available in time to be used
by the designers of the operational SOS system.
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