chongo's Journal: 2002-XY38 asteroid update for 2002-Dec-31 19:45 UTC

We have a new entry near the top of the current impact risks . It is an asteroid called 2002-XY38.

This asteroid appears be about ~90 m in diameter. Its impact velocity, IF it were to hit the Earth is in the low to medium range: ~12.9 km/sec. If 2002-XY38 were to hit the earth, the collision would result in significant local devastation. The devastation area would be somewhat larger than the Tunguska event in 1908.

The 2002-XY38 asteroid has a Torino impact hazard scale value of 1. It also has a cumulative Palermo Scale value of -2.53, mostly because of its small size and moderate impact speed.

The chance of impacting the Earth in the next 100 years is moderate: about 1 in ~1,300. Even so, at those odds there is a 99.923% chance that it will NOT hit the Earth in the next 100 years.

The 1st close approach is not until 24 Feb 2084. The model shows over the next 100 years the closest approach is about 625 km. The other 7 approaches in the next 100 years range from 1300 km to 5500 km above the Earth's surface.

It is very likely that over the next few days or weeks, additional observations will allow the model for 2002-XY38 to be refined. It would not surprise us to see a refined model reducing the impact risk to the point where this object is removed from the potential Earth impactor table.

It should be noted that 2002-XY38's listing is preliminary with only 33 observations spanning only about 24 days ... not much on which to form an super accurate model.

P.S. There is no change is status for 1997-XR2. That asteroid remains as the only other non-zero Torino impact hazard scale object. The only reason why 2002-XY38 is listed higher than 1997-XR2 is because its Palermo scale is only 0.09 higher.

Further updates on XY38 will be posted to chongo's journal .

Thanks for keeping up the updates

[Shanes]

Thanks for the update. It's the first torinoscale>0 since NT 7 as far as I have noticed.

Do you know if the virtual trajectories are calculated backwords in time aswell? I mean some trajectories should coincide with earth (or some other object) sometime long ago and should therefore be exluded. Or?

My second question is related to what is usually said about impact risks like this:
"It would not surprise us to see a refined model reducing the impact risk to the point where this object is removed from the potential Earth impactor table."

Is the probability of a reduction of risk greater than the actual risk of an impact?

Re:Thanks for keeping up the updates

[Shanes]

uhm, of course I meant: Is the reduction of risk greater than the risk of not having an impact.

You know what I mean: If a risk of impact is very small, then of course a future reduction of risc is probable. I just wanted to know if there was something more to it based on the obsevations.

reply to questions about 2002-XU38

[chongo]

``Thanks for the update.''

You are most welcome. It is fun to provide commentary on NEA's.

``It's the first torinoscale>0 since NT 7 as far as I have noticed.''

NT 7 was, for a period time, a >0 Torino impact hazard scale [nasa.gov] object. The press did run with that story for a while but interest declined as the model developed into something non-hazardous in the next 100 years. (To be fair we lost most, but not all of our interest in NT7 as well. :-))

One should point out that 1997 XR2 [nasa.gov] has remained a >0 Torino scale [nasa.gov] object for a while now. In fact, it has the record for both holding on to the top hazard spot as well as holding on to a >0 Torino scale [nasa.gov] value the longest of any known asteroid.

Some people tend to discount the risk from 1997 XR2 because the 1st close encounter is way out in 2101. But an impact of a 0.23 km object is no trivial matter. Toss that object into the middle of the ocean and will have one heck of a Tsunami on your hands!!! (Think 36 times the energy of the Tunguska [psi.edu] ) Such a Tsunami would be MUCH worse than anything that people have ever experienced in recorded history.

The 1997-XR2 object is VERY DARK. From what we heard, the most recent attempt to find it using a large telescope failed to turn up an image. :-(

Deep scope time is hard to get as there are many well deserving research programs of more pressing nature (or with a less patient observer) than a year 2101 concern. But we are hopeful that somebody someday will pin down 1997 XR2 again and allow for a much improved model to be established.

``Do you know if the virtual trajectories are calculated backwords in time aswell? I mean some trajectories should coincide with earth (or some other object) sometime long ago and should therefore be exluded.''

Excellent question!

One can (and does) run the model backward. Any model can be run backward in time. If you limit yourself to the modern Astronomical period, then you know the position and velocity of a good deal (but not all) of the Solar system's mass to a reasonable degree of precision. That ``20-21'' almost perfect hindsight can allow one to run the object's model backward up to a point. But like going forward in time, errors and imperfections cause problems.

One runs a model backward in the hopes of predicting where the object might have been in the past, for several reasons:

1. To see if somebody happened to take a photograph of the object sometime in the past.
   
   The model quality can be quickly and significantly improved if one is able to find the object in an old image/photograph. Instead of waiting for new observations to be made over time, one can sometimes obtain a baseline of months or even years in a single moment by discovering an old image.
   
   Finding an old image is sometimes not easy. A poor quality model may be too inaccurate to pin down a good location in the past.
   
   You may have a potential image taken many years ago. But the margin of error that far back may be so large as to make the odds the object appearing in the a given image to be small.
   
   And just because you find what might be an asteroid in the image does NOT mean that the asteroid is the one you want to find. Images, particularly wide field images, sometimes have multiple asteroids in them.
   
   The exposure time may be too short for the faint image to show. And a short exposure time may make it hard to determine if the direction of the candidate object matches your model.
   
   And just because you cannot find a candidate object in an image does not mean it is not there. Images sometimes have flaws that can hide an object. Images sometimes have flaws that look like a potential object.
   
   The sky conditions may have been too poor to record the object on the image.
   
   Your asteroid may have a shiny/bright side and a dull/dark side. If the wrong side rotated into view, the object may not show up.
   
   The angle that the Sun is shining may be too poor for a good image to form.
   
   The asteroid may be too far away from Earth to show up on an image. (Hint: Most of the easy, bright, large asteroids have been found. The ones that are left to discover tend to either be small and not so bright (but large enough to cause a problem on impact), or large and very dark.)
   
   
2. To look for events in the past that might have altered the course of the object.
   
   You may find that a past close encounter altered the path of the object in a significant way and quickly moved it into a new (and current) orbit. Moving the model even farther back in time (before the close encounter) becomes difficult has close encounters tend to introduce greater uncertainty into the model.
   
   For objects that might hit the Earth, it is not unusual to find that there were one or more close encounters with the Earth in the past that went unnoticed. We have discovered too many significant objects only to find that they have been close by numerous times in the historic past and nobody noticed them before.
   
   You may find that some other object caused the orbit of the object in question to drift. Perhaps your object was semi-locked into an orbit, but a resonance with some other object unlocked it.
   
   One very difficult case is that the object has a moon that is large enough to complicate the model, but small enough to be hard to detect.
   
   
3. To see if the object in question something that was previously discovered.
   
   Sometimes a new object turns out to be an previously discovered object. Perhaps the model for the previous discovery was poor. Your discovery is really just a surprise adjustment in the old object's model.
   
   Sometimes a close encounter with another object (such as the Earth) was not well documented. The close encounter sent the object off in a somewhat unexpected direction. We have too many asteroids that were only discovered during a close but poorly defined encounter. The models for such asteroids tend to be poor and some of such asteroids are lost.
   
   Yes, we do lose asteroids. Asteroid 1950 DA [nasa.gov], a 1.1 km wide object was lost for over 50 years before it was re-discovered in 2001! Space is really big and most asteroids tend to be rather dark. Sorry! :-) :-(
   
   
4. To attempt to model the Yarkovsky effect [physicsweb.org] .
   
   Usually is hard enough just to find an asteroid. It is even harder to determine the axis rotation and the speed of rotation. To calculate the Yarkovsky effect one needs that data, and more, in order to determine its long range (say 100+ years) effect on the asteroid.
   
   Sometimes a past position will allow one to establish an upper bound on the Yarkovsky effect and allow one to produce a better model with a more bounded long range uncertainly.

One hopes to find a past set of multiple observations that correlate well with the current model in question. Sometimes, one needs to follow the object of a period of time before one can even tie it back to a nice set of multiple observations. If you are really lucky, like in the case of 1950 DA, you can win big and extend the model well beyond 100 years in the future.

Sorry to digress, but it is important to understand that running a model backward and finding observational data to support the model is not easy. In fact it is frequently harder to go backward. At least with a discovered asteroid, one has a chance to follow it for a period of time.

So back to your question ...

We do try (and hope) to find information about an object's past but that is not always easy. And when we do find a close encounter, unless that close encounter was well documented, working back beyond that becomes very very very difficult.

People are looking backward along model paths for any sign of 2002-XU38. So far (to our knowledge), nothing even close to useful has been found.

BTW: The same is being done for 1997-XR2, the other non-zero Torino impact hazard scale [nasa.gov] object. No luck so far.

``Is the reduction of risk greater than the risk of not having an impact?

If a risk of impact is very small, then of course a future reduction of risc is probable. I just wanted to know if there was something more to it based on the obsevations.''

There is a saying that NEA modelers use:

``There are many ways to change a close encounter model into a far encounter model, but there is only one way to change it into model into an impacting model: move it into the Earth!''

OK, that saying is a bit simplistic but it does a valid point. Move an orbit in a random direction. The chance that your new orbit moves closer to the Earth (or impacts the Earth) is less than the chance to move it away from the Earth.

Throw a dart at a dart board that lands near, but not on Bull's eye. Call the miss distance of the first dart M.

If you adjust your aim of the next dart by M (in hopes of hitting the center), but select a random direction of adjustment, you have only a 1/3 chance of getting closer to the center.

If, instead, you adjust your aim randomly between 0 and M in some random direction, you have only a 39.1% chance of getting closer to the center.

Now true, dart throwing is not a perfect analogy for asteroids. Earth's gravity sucks after all. :-) So make your darts metallic and place a strong magnet in the center. Throw the 2nd dart with the same force as the 1st dart, but adjust your aim by up to M in a random direction. You can show that the odds of the 2nd dart landing closer to the center is less than 50%.

In practice, perturbing a model usually moves the object away from the Earth instead of toward it. There are a lot more directions away from a target than toward it.

For all of these reasons, applying more observations to an imperfect model usually improves the accuracy of the model and decreases the chance of an impact.

More data only decreases the impact chance, it does not eliminate the impact chance in the near future. Somewhere there is a significant asteroid that ``has Earth's next number written on it''. We may see the day when the asteroid is first detected ... an imperfect model is created ... it is placed on the hazard list [nasa.gov] ... more data is collected ... the Torino scale [nasa.gov] and Palermo scale [nasa.gov] values increase (or at best stay the same) ...

We just hope we can notice that model far enough in advance to cheat that cosmic dart player our of a ``Bull's eye''!

We hope this helps.``Thanks for the update.''

You are most welcome. It is fun to provide commentary on NEA's.

``It's the first torinoscale>0 since NT 7 as far as I have noticed.''

NT 7 was, for a period time, a >0 Torino impact hazard scale [nasa.gov] object. The press did run with that story for a while but interest declined as the model developed into something non-hazardous in the next 100 years. (To be fair we lost most, but not all of our interest in NT7 as well. :-))

One should point out that 1997 XR2 [nasa.gov] has remained a >0 Torino scale [nasa.gov] object for a while now. In fact, it has the record for both holding on to the top hazard spot as well as holding on to a >0 Torino scale [nasa.gov] value the longest of any known asteroid.

Some people tend to discount the risk from 1997 XR2 because the 1st close encounter is way out in 2101. But an impact of a 0.23 km object is no trivial matter. Toss that object into the middle of the ocean and will have one heck of a Tsunami on your hands!!! (Think 36 times the energy of the Tunguska [psi.edu] ) Such a Tsunami would be MUCH worse than anything that people have ever experienced in recorded history.

The 1997-XR2 object is VERY DARK. From what we heard, the most recent attempt to find it using a large telescope failed to turn up an image. :-(

Deep scope time is hard to get as there are many well deserving research programs of more pressing nature (or with a less patient observer) than a year 2101 concern. But we are hopeful that somebody someday will pin down 1997 XR2 again and allow for a much improved model to be established.

``Do you know if the virtual trajectories are calculated backwords in time aswell? I mean some trajectories should coincide with earth (or some other object) sometime long ago and should therefore be exluded.''

Excellent question!

Any model can be run backward in time. If you limit yourself to the modern Astronomical period, then you know the position and velocity of a good deal (but not all) of the Solar system's mass to a reasonable degree of precision. That ``20-21'' almost perfect hindsight can allow one to run the object's model backward up to a point. But like going forward in time, errors and imperfections cause problems.

One runs a model backward in the hopes of predicting where the object might have been in the past, for several reasons:

1. To see if somebody happened to take a photograph of the object sometime in the past.
   
   The model quality can be quickly and significantly improved if one is able to find the object in an old image/photograph. Instead of waiting for new observations to be made over time, one can sometimes obtain a baseline of months or even years in a single moment by discovering an old image.
   
   Finding an old image is sometimes not easy. A poor quality model may be too inaccurate to pin down a good location in the past.
   
   You may have a potential image taken many years ago. But the margin of error that far back may be so large as to make the odds the object appearing in the a given image to be small.
   
   And just because you find what might be an asteroid in the image does NOT mean that the asteroid is the one you want to find. Images, particularly wide field images, sometimes have multiple asteroids in them.
   
   The exposure time may be too short for the faint image to show. And a short exposure time may make it hard to determine if the direction of the candidate object matches your model.
   
   And just because you cannot find a candidate object in an image does not mean it is not there. Images sometimes have flaws that can hide an object. Images sometimes have flaws that look like a potential object.
   
   The sky conditions may have been too poor to record the object on the image.
   
   Your asteroid may have a shiny/bright side and a dull/dark side. If the wrong side rotated into view, the object may not show up.
   
   The angle that the Sun is shining may be too poor for a good image to form.
   
   The asteroid may be too far away from Earth to show up on an image. (Hint: Most of the easy, bright, large asteroids have been found. The ones that are left to discover tend to either be small and not so bright (but large enough to cause a problem on impact), or large and very dark.)
   
   
2. To look for events in the past that might have altered the course of the object.
   
   You may find that a past close encounter altered the path of the object in a significant way and quickly moved it into a new (and current) orbit. Moving the model even farther back in time (before the close encounter) becomes difficult has close encounters tend to introduce greater uncertainty into the model.
   
   For objects that might hit the Earth, it is not unusual to find that there were one or more close encounters with the Earth in the past that went unnoticed. We have discovered too many significant objects only to find that they have been close by numerous times in the historic past and nobody noticed them before.
   
   You may find that some other object caused the orbit of the object in question to drift. Perhaps your object was semi-locked into an orbit, but a resonance with some other object unlocked it.
   
   One very difficult case is that the object has a moon that is large enough to complicate the model, but small enough to be hard to detect.
   
   
3. To see if the object in question something that was previously discovered.
   
   Sometimes a new object turns out to be an previously discovered object. Perhaps the model for the previous discovery was poor. Your discovery is really just a surprise adjustment in the old object's model.
   
   Sometimes a close encounter with another object (such as the Earth) was not well documented. The close encounter sent the object off in a somewhat unexpected direction. We have too many asteroids that were only discovered during a close but poorly defined encounter. The models for such asteroids tend to be poor and some of such asteroids are lost.
   
   Yes, we do lose asteroids. Asteroid 1950 DA [nasa.gov], a 1.1 km wide object was lost for over 50 years before it was re-discovered in 2001! Space is really big and most asteroids tend to be rather dark. Sorry! :-) :-(
   
   
4. To attempt to model the Yarkovsky effect [physicsweb.org] .
   
   Usually is hard enough just to find an asteroid. It is even harder to determine the axis rotation and the speed of rotation. To calculate the Yarkovsky effect one needs that data, and more, in order to determine its long range (say 100+ years) effect on the asteroid.
   
   Sometimes a past position will allow one to establish an upper bound on the Yarkovsky effect and allow one to produce a better model with a more bounded long range uncertainly.

One hopes to find a set of observations in the past that correlate well with the current model in question. Sometimes, one needs to follow the object for a period of time before one can even tie it back to a nice set of multiple observations. If you are really lucky, like in the case of 1950 DA, you can win big and extend the model well beyond 100 years in the future.

Sorry to digress, but it is important to understand that running a model backward and finding observational data to support the model is not easy. In fact it is frequently harder to go backward. At least with a discovered asteroid, one has a chance to follow it for a period of time.

So back to your question ...

We do try (and hope) to find information about an object's past but that is not always easy. And when we do find a close encounter, unless that close encounter was well documented, working back beyond that becomes very very very difficult.

People are looking backward along model paths for any sign of 2002-XU38. So far (to our knowledge), nothing even close to useful has been found.

BTW: The same is being done for 1997-XR2, the other non-zero Torino impact hazard scale [nasa.gov] object. No luck so far.

``Is the reduction of risk greater than the risk of not having an impact?

If a risk of impact is very small, then of course a future reduction of risc is probable. I just wanted to know if there was something more to it based on the obsevations.''

There is a saying that NEA modelers use:

``There are many ways to change a close encounter model into a far encounter model, but there is only one way to change it into an impacting model: move it into the Earth!''

OK, that saying is a bit simplistic but it does a valid point. Move an orbit in a random direction. The odds are that your new orbit will be farther from Earth.

Throw a dart at a dart board that lands near, but not on Bull's eye. Call the miss distance of the first dart M.

If you adjust your aim of the next dart by M (in hopes of hitting the center), but select a random direction of adjustment, you have only a 1/3 chance of getting closer to the center.

If, instead, you adjust your aim randomly between 0 and M in some random direction, you have only a 39.1% chance of getting closer to the center.

Now true, dart throwing is not a perfect analogy for asteroids. Earth's gravity sucks after all. :-) So make your darts metallic and place a strong magnet in the center. Throw the 2nd dart with the same force as the 1st dart, but adjust your aim by up to M in a random direction. You can show that the odds of the 2nd dart landing closer to the center is less than 50%.

In practice, perturbing a model usually moves the object away from the Earth instead of toward it. There are a lot more directions away from a target than toward it.

For all of these reasons, applying more observations to an imperfect model usually improves the accuracy of the model and decreases the chance of an impact.

More data only decreases the impact chance, it does not eliminate the impact chance in the near future. Somewhere there is a significant asteroid that ``has Earth's next number written on it''. We may see the day when the asteroid is first detected ... an imperfect model is created ... it is placed on the hazard list [nasa.gov] ... more data is collected ... the Torino scale [nasa.gov] and Palermo scale [nasa.gov] values increase (or at best stay the same) ...

We just hope we can notice that model far enough in advance to cheat that cosmic dart player our of a ``Bull's eye''!

We hope this helps.

Re:reply to questions about 2002-XU38

[chongo]

Clarification:

The comment:

``But an impact of a 0.23 km object is no trivial matter. Toss that object into the middle of an ocean and will have one heck of a Tsunami on your hands!!! (Think 36 times the energy of the Tunguska) Such a Tsunami would be MUCH worse than anything that people have ever experienced in recorded history.''

This should have read:

``But an impact of a 0.23 km object is no trivial matter. Toss that object into an ocean and will have one heck of a Tsunami on your hands!!! (Think 36 times the energy of the Tunguska) If the ocean impact were anywhere close to land, than the resulting Tsunami would be MUCH worse than anything that people have ever experienced in recorded history.''