SENTRY: AN AUTOMATIC NEAR-EARTH ASTEROID MONITORING SYSTEM
NASA's Near-Earth Object Program Office (http://neo.jpl.nasa.gov) announces the arrival of the Sentry automatic impact monitoring system. In development for nearly two years, Sentry is a highly automated, accurate, and robust system for continually updating the orbits, future close Earth approaches, and Earth impact probabilities for all Near-Earth Asteroids (NEAs).
When interpreting the Sentry Impact Risks Page (http://neo.jpl.nasa.gov/risks/), where information on known potential NEA impacts is posted, one must bear in mind that an Earth collision by a sizable NEA is a very low probability event. Objects normally appear on the Risks Page because their orbits can bring them close to the Earth's orbit and the limited number of available observations do not yet allow their trajectories to be well-enough defined. In such cases, there may be a wide range of possible future paths that can be fit to the existing observations, sometimes including a few that can intersect the Earth.
Whenever a newly discovered NEA is posted on the Sentry Impact Risks Page, by far the most likely outcome is that the object will eventually be removed as new observations become available, the object's orbit is improved, and its future motion is more tightly constrained. As a result, several new NEAs each month may be listed on the Sentry Impact Risks page, only to be removed shortly afterwards. This is a normal process, completely expected. The removal of an object from the Impact Risks page does not indicate that the object's risk was evaluated mistakenly: the risk was real until additional observations showed that it was not.
While completely independent, the Sentry system is meant to be complementary to the NEODyS CLOMON impact monitoring system operated in Pisa, Italy. Personnel from both the Sentry and NEODyS systems are in constant communication, cross checking each other's results and providing constructive feedback to continuously improve the efficiency, accuracy, and robustness of both systems.
The Sentry system was developed largely by Drs. Steve Chesley and Alan Chamberlin with significant technical help from Dr. Paul Chodas. Ron Baalke provided our web site updates.
Donald K. Yeomans Manager, NASA Near-Earth Object Program Office
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.
NASA'S NEW ASTEROID SENTRY STANDS WATCH
From Space.com, 14 March 2002 By Robert Roy Britt Senior Science Writer
NASA announced this week a new Web-based asteroid monitoring system, called Sentry, to monitor and assess the threat of space rocks that have been discovered have a chance of hitting Earth. The setup is designed to help scientists better communicate with each other about the discoveries of new, potentially threatening asteroids and the follow-up observations that typically show those asteroids to be, in fact, no threat.
While no large asteroid is currently known to be on a collision course with our planet, experts say an eventual impact is inevitable and the consequences could be grave, up to and including global devastation that might destroy civilization as we know it. The odds of such an impact in any given decade are extremely low, and most experts agree that there would likely be at least 10 years of warning if such an object were ever spotted.
Smaller asteroids, however, are more likely to hit Earth in any given year and could cause significant local or regional damage. The odds are low in any given year. But over the course of a generation, the chances of such an event become significant. The odds of a locally or regionally destructive asteroid hitting an inhabited area in a given 50-year period are about 1-in-160, according to experts.
In recent years, asteroid experts around the globe have struggled to develop a system to catalogue and track newly spotted Near Earth Asteroids - those that are close enough to Earth's orbit to warrant scrutiny -- and to properly communicate any possible threats to the public.
However, asteroids move so slowly against the background of stars that when one is first discovered, astronomers cannot pin down its exact path. Therefore, a wide range of possibilities are generated for the rock's possible orbit around the Sun, and often Earth becomes a possible target in those projected paths.
A handful of false alarms, in which scientists said there was a remote threat that a particular asteroid would hit Earth in a certain year, have made headlines and frightened the public. The first and most notable was an asteroid called 1997 XF11, which briefly loomed as a frightening nemesis until four years ago this week, when new observations revealed it would miss the planet.
A similar but less publicized "threat" emerged last August with an asteroid called 2001 PM10. Data on the rock was available on a public website and was hyped by uninformed web users before the fresh observations removed the risk.
Since the 1997 XF11 situation, researchers have argued, sometimes vehemently, over how to better manage their data and make more informative public announcements.
The Sentry system
The new Sentry system, developed over the past two years, is partly a response to this perceived need. It is operated out of NASA's Jet Propulsion Laboratory. The system's online "Risks Page" included 37 asteroids as of Thursday morning.
"Objects normally appear on the Risks Page because their orbits can bring them close to the Earth's orbit and the limited number of available observations do not yet allow their trajectories to be well-enough defined," said JPL's Donald Yeomans, manager of NASA's Near-Earth Object Program Office, which oversees Sentry.
"By far the most likely outcome is that the object will eventually be removed as new observations become available, the object's orbit is improved, and its future motion is more tightly constrained," Yeomans said in a statement. He added that several asteroids will be added to the list each month, only to be removed to another "no-risk" page soon afterward.
Sentry follows other attempts to deal with the publication of asteroid risk data. A color-coded disaster yardstick called the Torino Scale, developed in 1999 and designed in part to inform the media and the public, has gone largely unused. On the Torino Scale, a zero or one represent remote risk, and a 10 means it's time to sell the farm.
All but one of the asteroids currently on the Sentry list are zeros on the Torino Scale. Topping the list, though, is a space rock named 2002 CU11, discovered Feb. 7. It presently has a 1-in-100,000 chance of hitting Earth on Aug. 31, 2049. But as its orbit is refined, it is quite possible that this asteroid, like many before it, will be categorized harmless.
Copyright 2002, Space.com