[meteorite-list] Sentry: A Monitoring System From NASA To The Web

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:02:32 2004
Message-ID: <200203272339.PAA02249_at_zagami.jpl.nasa.gov>

http://spaceguard.ias.rm.cnr.it/tumblingstone/issues/current/eng/2002cu11.htm#Sentry

Sentry: A Monitoring System From NASA To The Web
by Livia Giacomini
Tumbling Stone, No. 12
March 12, 2002

It took two years of hard work, but finally, on March the 12th, NASA
announced that Sentry, its new automatic asteroid impact monitoring system,
was beginning to be operated out of Jet Propulsion Laboratory. Sentry was
built largely by Drs. Steve Chesley and Alan Chamberlin with technical help
from Paul Chodas.

To be more precise, Sentry is a highly automated system, designed to help
scientists better communicate about the discoveries of new, potentially
threatening Near Earth asteroids (NEAs) and their follow-up observations.
While completely independent from other scienitific teams, it is in constant
communication with the NEODyS CLOMON impact monitoring system, operated in
Pisa, and researchers from the two systems are cooperating to check and
improve their results.

But how does this new tool work? First of all, it is important to say that,
as it usually is today in the scientific world, Sentry is a technological
tool which implements web based technology. In fact, all the data calculated
is posted daily on the web, in the public Sentry risk page
(http://neo.jpl.nasa.gov/risk/) which constitutes a very important tool for
research teams around the world as well as for amateurs.

The procedure to determine these parameters is quite clear: data about NEAs
is drawn each day from the Minor Planet Center in Cambridge, and it is used
to update the orbits, Earth approaches, and last but not least, Earth impact
probabilities. Based on these calculations, several asteroids are added
monthly to the web page which can be considered as an updated list of
"dangerous" objects which must be followed up. These objects are in fact
characterized by two aspects: first, they have orbits that can bring them
close to Earth, and second, only a limited number of observations are
available for each of them, so that their trajectories are not well-enough
defined.

Normally, these NEAs are listed on the Sentry Impact Risk page only to be
removed to a second no-risk page soon afterward. But pay attention, this
procedure doesn't mean that there was an error while calculating the risk.
In fact, as new observations become available, the knowledge of the object's
orbit is improved (its region of uncertainty becomes smaller) and its risk
promptly recalculated. The most likely outcome of the all procedure is that
the object becomes harmless and is therefore removed from the risk page.

But how does the public outreach of the Sentry system work? A first
characteristic of the Sentry risk page is that the risk is presented using
both the usual Torino Scale and the new, more technical, Palermo Technical
Scale (click here to go to T.S. issue number 11, to know more about the two
scales: http://spaceguard.ias.rm.cnr.it/tumblingstone/issues/num11/eng/palermo.htm).

Thanks to the two scales, the risk of a single object is compared to the
so-called background level (which is the average risk from the entire NEO
population). A Palermo Technical Scale value less than zero and, in most
cases, a Torino Scale value of zero, indicates that this risk is below this
background level, and that the event can be considered only of academic
interest, and not deserving public concern.

On the other hand, on the very rare cases when events have a Palermo
Technical Scale value greater than zero, a Technical Review is requested to
verify the calculations before the prediction is placed on the Risk Page.
But Sentry is also a scientific instrument, meant to give to scientists all
data and information about NEAs. For this reason, independently from the
associated risk, for each object of the Risk Page there is a separate page
providing more detailed technical information.

But let's come down to mathematics. The real question is: how are the Earth
impact probabilities for near-Earth objects calculated? Every day,
observations and orbit solutions are received from the Minor Planet Center
and once an object has been classified as a NEA, and as soon as enough
observations have been collected, an orbit determination process is used to
find the orbit which best fits all the observations. But how is this done?
The main idea is that an object's orbit follows some equations that take
into account the gravitational attraction of the Sun, the planets, the Moon,
and the three largest asteroids, Ceres, Pallas, and Vesta. Given an
observed, initial position of an asteroid, its further positions can be
computed solving these equations of motion. The difference between these
computed values and the actually measured ones are called observation
residuals. The overall orbit of the object (and therefore the six orbital
parameters that characterize it) is determined iteratively adjusting the
calculated and the observed positions until the sum of squares of all the
observations residuals reaches a minimum value (this mathematical procedure
is called minimum squares fitting, see this issue of T.S. to know more).

The final result of the orbit determination process is called the nominal
solution. Of course, slightly different orbits may still fit the
observations and this set of orbits lies within what is called the
uncertainty region, as all the points inside the region are called virtual
asteroids. Whenever new optical or radar observations become available,
automatic updates of this orbit are calculated, giving obviously priority to
the objects that seem more dangerous. It is therefore clear that, as new
observations of the object are made, the nominal orbit can change and its
region of uncertainity can become smaller and smaller.

Orbits and regions of uncertainty are fundamental to make the evaluation of
the real risk associated with the asteroid, or in other words, the impact
probabilites. In fact, once the nominal orbit and its associated uncertainty
region have been estimated, Sentry can simulate the object's motion in time
for up to 100 years. This is done to determine its close approaches to the
Earth, published in the Earth Close Approach Tables together with the
relative impact probability. This projection in the future of the
uncertainty region is not a simple task from a mathematical point of view
and it is not always possible. A first mean to achieve this goal is to
compute these parameters by projecting the uncertainty region to the close
approach time via so-called linearized techniques. Since these techniques
lose accuracy when the uncertainties become large, close approaches may be
calculated up to decades into the future for objects with well-known orbits,
but only a few months or years for objects with poorly known orbits.
On the other hand, Sentry also wants to estimate long-term possibilities of
impact for objects with poorly known orbits. To estimate this risk it uses
more sophisticated non-linear methods, which are integrated to linear means
whenever the uncertainties in a close approach prediction are large.

Sentry, together with other scientific services such as NEODyS, is just one
example of the great efforts that every day is been made and must be made to
improve our knowledge of asteroids and NEOs. In this optic, by the year
2008, NASA has a congressionally mandated goal to find 90 percent of all
Near Earth Objects larger than 1 kilometer. Of them, only about 500 have
been found. An estimated 500 or so, still remain undiscovered.
Received on Wed 27 Mar 2002 06:39:30 PM PST