What is the criteria for an object to be called a Near-Earth Asteroid
(NEA)?
Perihelion less than 1.3 au. Roughly put,
this means that all objects which can come within 0.3 au of the Earth
comprise the list of NEA's.
What is an astronomical unit (au)?
The mean distance of the
Earth from the Sun, about 93 million miles or 150 million
kilometers. The lunar distance is 0.0026 au and the radius of the
Earth is 0.000043 au.
How likely is an NEA impact?
Maybe more than you expect. (But not so
high that you should spend your retirement savings early!) The current
best estimate is that impacts large enough to cause a global
catastrophe occur on average every few hundred thousand years. These
impacts are the ones large enough to cause significant global climatic
disruption, widespread crop failures and probable societal
collapse. This translates into a probability of a little less than one
chance in a few thousand of such an impact occurring during a typical
human lifespan. Smaller objects, of course, are more numerous, and
impact more frequently. Impacts capable of causing severe local or
regional disasters occur once every few hundred years, depending on
how you define "local" and "regional." Fortunately most of the Earth
is uninhabited so the likelihood of a impact near a populated area is
far less.
If an impact becomes likely is there anything that can be done about it?
This depends critically on the amount of warning and to a
lesser extent on the composition and structure of the asteroid, but in
principle deflection of impacting objects will be feasible.
What is chi? Why is it equal zero in the observational data for the
object I am interested in?
This refers to the
chi-square statistical quantity that is used to determine our cutoff
for observational outlier rejection and recovery. We currently reject
observations if chi**2 > 8 and we recover previously rejected
observations if chi**2 < 7.5. The chi column in the
observational data describes the quality of the observation,
essentially the weighted circular error in arc-sec.
There are a very few unusual objects for which our automated routine
for rejecting outliers in the observational data fails. In such cases
the chi**2 test is not applied and outlier rejection is done
on a manual basis. Of course, objects without uncertainty information are
not subjected to automatic outlier rejection, so they fall into this
category also. See the next question below.
Why is the orbital uncertainty information missing for the object I am
interested in?
There are a very few unusual objects
for which we are unable to compute a reliable orbit. For these we
provide the orbit computed with at least one of the orbital elements
held constant. In these cases the orbit should be considered highly
unreliable, and for practical purposes the object is irretrievably
lost.
How good are your absolute magnitude estimates?
Our computed
absolute magnitudes are in excellent agreement with the computations
of others in the field, but it is important to note that the source of
our data is solely the photometry published by the Minor Planet Center
with their astrometric observations. For the numbered asteroids
(esp. those below 100), our computation can be quite different from
the "official" IAU value because we have not included any photometric
data not reported to the MPC with astrometry. The link to additional
physical information will typically provide a better value in
these cases. However, for unnumbered asteroids our result typically
represents the best estimate that can be obtained with the available
data.
Why are there too few (or too many) digits in the orbital elements?
Our HTML screen displays are designed for human
readability. If you need machine precision (for machine readability),
then you should download the ASCII files. However, the actual
precision of the files is described by the "1-sigma variation" column
in the HTML orbital element tables, which will generally be different
from either of the element formats.
I don't have an observatory code. What should I enter in the services
forms?
If you are doing only occasional
observing, then you can use the default code of 500, or find an
observatory near you and use their code. If you are intending to
submit observations to the Minor Planet Center then you must contact
them and obtain an observatory code. See their Guide to
Minor Body Astrometry.
How often is your site updated?
The database is updated daily, and hence all orbital information is kept current.
My question isn't here. Who can I ask?
We would like to include your question here!
Contact us
and we will try to answer your question as promptly as possible.