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
NEO Q&A session #0
Q0: When was 2003-CR20 discovered
and who discovered it?
A0: The asteroid was discovered by
team using the Haleakala-AMOS facility (NEA
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
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
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,
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
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
NEO Q&A session #0 ,
and in particular the
While looking forward 100 or more years
is important, looking backward in time is
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
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:
- Uncertainty about the position/momentum of the asteroid.
Additional observations help a lot here.
- Uncertainty about the position/momentum of the rest of the solar system.
- Roundoff and other numerical errors in the N-body solver used to predict future position/momentum of Earth and asteroid.
- 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
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,
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 /
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
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
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)
telescope to visually see 2003-CR20
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
First of all, the odds of 2003-CR20
impacting the Earth is low, so talking
about impact locations is very
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
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
about 1112.43 days (3.045667 years).
It currently comes within 0.5734 AU of the
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.