chongo's Journal: Hazardous NEO Q&A session #1 / 2003-CR20 FAQ

The following is a summary of the questions that I have received about 2003-CR20. In addition to this FAQ, you should review the previous " Hazardous NEO Q&A session #0 ".

Q0: When was 2003-CR20 discovered and who discovered it?

A0: The asteroid was discovered by the JPL/NEAT team using the Haleakala-AMOS facility (NEA observatory #608) on 2003-02-11.

Q1: How far is 2003-CR20 from Earth now?

A1: As of 2003-02-24, it is about 78.726 million km from Earth.

Q2: Where is 2003-CR20 headed right now?

A2: It is on its way in toward the Sun. It currently below Earth's orbit plane heading up thru it. On 2003-04-07, 2003-CR20 will pass up thru Earth's orbit plane at which time it will be 54.180 million km from Earth. Earth is somewhat ahead of 2003-CR20 this year.

Q3: How close will 2003-CR20 come to Earth this year?

A3: On 2003-04-24, it will be only 50.139 million km away from Earth.

Q4: What is being done to improve the orbit model for 2003-CR20?

A4: Observatories continue to track the asteroid and feed position sightings back to orbit modelers. Because of 2003-CR20's unusual orbit, positional observations are being frequently made.

For the next few months while the object is somewhat near Earth's orbit, 2003-CR20 will be studied in considerable detail. We hope, in the next few months, to learn how fast it revolves about its axis. We also hope to learn the orientation of the axis of 2003-CR20. Additionally we hope to learn if the asteroid has any surface detail that suggests non-uniform light absorb ion or reflection by making a detailed study of the brightness curves of 2003-CR20. Such information will be very important in attempting to establish a long range orbit model beyond 100 years. (See the FAQ #1 from the Hazardous NEO Q&A session #0 , and in particular the Yarkovsky effect ).

While looking forward 100 or more years is important, looking backward in time is also important. Establishing an accurate model over the previous 100+ years will help archivists look thru old telescope photos in the hopes of spotting an image of 2003-CR20 years ago. Finding a mid-1900's image would allow us to establish a very accurate orbit model with a long baseline. Without such a historic photo, we will have to track it forward for a number of years (and not lose it) to improve the model.

Going backward a few hundred years might provide insight into questions such as:

• Has 2003-CR20 been in this high risk orbit for a long time?
• Did it slowly shift into this high risk orbit, or did something else nudge/pull it into its present orbit?
• How long has 2003-CR20 been a hazard? Does recent history suggest that the orbit is continuing to evolve? If so, it is evolving into a higher risk object, or a lower risk orbit? Will be get worse before it gets better?

Q5: How the original error-bar ("Center line miss of 29147km +/- 154080 km") is calculated?

What I'm actually trying to understand are the (unpublished) error bars around the odds estimate. They're obviously huge, that's why the odds change so much with each additional week of observation. Here are the things I can think of that contribute to that error:

1. Uncertainty about the position/momentum of the asteroid. Additional observations help a lot here.
2. Uncertainty about the position/momentum of the rest of the solar system.
3. Roundoff and other numerical errors in the N-body solver used to predict future position/momentum of Earth and asteroid.
4. Modeling error in the N-body solver. For example, it might ignore relativistic effects. And it is a good bet it incorrectly models the Yarkovsky effect, if it even tries.

The first three points should be straightforward to deal with in the error analysis. But the fourth one is a lot harder, since modeling error is usually thought of as bias, not a source of variance. However the magnitude of the modeling error increases with time so one might try to fold it into the error bars.

I'm trying to get a feel for how accurate the odds are and what can be done to make them more accurate.

A5: You have listed a number of the more important factors that contribute to orbit model uncertainty. Although factor #3 (numerical errors) is fairly minor with respect the other factors.

The "odds" estimate for a given close approach might very well be off by a factor of 2x or 3x in the short term (the next dozen or two years, or before the first few close approaches) and 5x to 10x in the longer term (up to 100 years, or beyond several close approaches). An approach nearly 100 years from now with "1 in a 1,000,000" odds could actually be anywhere from "1 in 100,000" to "1 in 10,000,000".

The overall odds in the next 100 years depends on how the close approaches are distributed throughout the time-span. The "error bar for the odds numbers" is typically of by a factor of 2x to 5x.

To improve model accuracy, we seek more observations, better observations, more accurate physical information.

With additional time and observations, we then work the model backward in the hopes of being able to find the object on several old photographs / photographic plates / digitized images. If a good set of historic images can be found, then a much longer baseline can be used to improve the model. By a good set we mean several quality images, with reasonable information such as accurate exposure times. We also hope to find several images spread over a number of days to years.

In a quite a few cases, useful historic data cannot be found. Without historic images, the only thing we can do is track the asteroid over an extended period of time ... assuming that you can continue to track it (you don't lose it).

Q6: How difficult is it to see the asteroid? How big of a telescope do I need to be able to view it?

A6: As of 2003-02-24, 2003-CR20 is at an apparent visual magnitude of +19.16. A +19 magnitude object is about 300,000 times fainter than the typical human eye can see. It is more than 150 times fainter than Pluto.

A typical 35 year old person with reasonable sky conditions in a very dark location with the Moon set would need a 2-meter (79-inch) reflecting telescope to visually see 2003-CR20 right now. Telescopes of that size or larger are only found at major observatories.

With a CCD or astro camera, a moderate to large amateur telescope can record an image of the asteroid.

Q7: If 2003-CR20 hits the Earth, where on Earth it is likely to hit?

A7: At the present time, we do not have enough information to predict impact locations. First of all, the odds of 2003-CR20 impacting the Earth is low, so talking about impact locations is very pre-mature.

Second, the error in time is frequently as much as +/- 12 hours. So in many cases, we don't even know which side of the Earth is facing the asteroid path.

Third, even if we know what side of the Earth is facing the in-bound asteroid, the margin of error off of the model's center line is frequently as large or larger than the Earth itself! Q8: How long does it take 2003-CR20 to complete one orbit around the sun? How close and how far away from the Sun does it get.

A8: The orbit period of 2003-CR20 is currently about 1112.43 days (3.045667 years).

It currently comes within 0.5734 AU of the Sun at Perihelion (closest approach to the Sun), and 3.6289 AU at Aphelion (farthest distance from the Sun). An AU is the average Earth-Sun Distance.

We say "currently" because close encounters with the Earth will be altering the orbit over time.

Questions, comments and corrections welcome.