About Tom's Asteroid Flybys Service
This service is intended as an aid to visually observing close-approaching
asteroids. It is based on data available at the
Minor Planet Center (MPC).
For purposes of clarity, I work at the same institution, but am not
directly affiliated with the MPC. This is a personal
How to read the data
The main page shows a list of all predicted approaches for the next year.
By default this list is sorted by time of closest approach. By clicking
on the column headers for "Object Name", "Maximum Brightness", and
"Maximum Angular Speed", you can sort the list by those fields.
Some table cells are marked with colors. Generally
red cells are
items which are the most interesting (for instance, brighter, faster,
or newly added).
Orange cells are
also interesting, but not as much, and
yellow cells are
Here are the descriptions of what each field means:
MicroPlots If turned on, this is the first (unlabelled) column
in the table, and shows a tiny plot of where the object will be in the sky.
The plot is layed out the same as the fullsize plots on the detail page.
This can quickly tell you that some objects are not in your hemisphere,
and if you know which parts of the sky in the full plot are up at night,
it can also hint whether this object is up during the day or night.
Object Name This is the name of the object as found on
the MPC website.
When This is the predicted time when the
asteroid will be closest to the Earth. This information is accurate
to within about 15 minutes.
Dist This is the predicted minimum distance to the
Earth, in Astronomical Units. 1 AU is the distance from the Earth to the
Sun, about 93 million miles. The moon is about 0.0025 AU from the Earth,
and almost all of the "close" approaches listed will be farther away than
this. This field shows red if an asteroid comes as close as the moon.
V This is the minimum visual magnitude (maximum brightness)
the asteroid is expected to reach. Smaller values here are brighter.
Many asteroids may be brighter than this outside of the five-day bracket
around the closest approach.
Unfortunately, most close approaches will be too dim to see without
fabulous equipment and conditions.
|Magnitude||What it means|
|4 or less||Naked-eye viewable, even in most urban locations
(assuming no clouds of course). EXTREMELY RARE!|
|4-6||Naked-eye viewable, in ideal dark-sky conditions away from
cities. EXTREMELY RARE!|
|6-10||Visible in binoculars and small telescopes, with
adequate sky conditions. VERY RARE!|
|10-14||Visible in medium-sized (6") telescopes under good
|14-18||Visible in large telescopes (12") with ideal
|18 and up||Huge telescope required (25" or more)|
When V When will the object be at it's
brightest? This is not the same as it's time of closest approach. Due
to observing geometry, it'll generally be brighter when it is more directly
away from the sun. If it maintains the same brightness for several hours,
the first hour is listed. Often maximum brightness occurs at
the very beginning of the 5-day bracket
that I use. In these cases, the object is likely to be even brighter before
my 5-day window. This is because objects reflect the most light
when they are at opposition (on the opposite side of the earth from the
sun). Of course, closer objects are also brighter, but the more distant
"close approaches" are still likely to be brighter at opposition than at
closest approach. Perhaps a future feature will be to calculate and
plot the opposition for these asteroids.
Speed What is it's fasted apparent motion in the
sky? This is measured in arcseconds per minute. If you are looking
through a telescope with a ten arcminute field of view (fairly high
magnification for a medium-sized telescope) then an asteroid moving
at 600 arceconds per minute would cross the field of view in one
minute (the motion would be readily obvious). An asteroid moving at 10
arcseconds per minute would cross the
same field of view in an hour (it wouldn't appear to be moving without
watching it for a while).
When Speed When will it appear to move the fastest?
If maximum speed is maintained for more than an hour, first hour
of maximum speed is listed.
H (Absolute Magnitude) This is a measure of how bright
is, if you were at some fixed distance from it. This can be correlated
somewhat to it's size, although loosely, because asteroids can be bright
because they're big, or because they are very white. Still, a magnitude of
18 is fairly big, at least a half a kilometer, and up to one and half
kilometers. The NASA Near Earth Orbit web pages has a
table of magnitudes
and estimated diameters .
Size This is an estimate of the diameter of the asteroid,
based on absolute magnitude. It's a range of likely minimum and maximum
sizes, based on high bright asteroids typically are. See above.
Opps (Oppositions) This tells us how many times this object has
passed the earth since we've discovered it.
Observation This tells us how long this object has
been observed. If the entry is a range of years, the orbit is going to
be very well understood. If the entry is only in days, then the orbit
probably needs refining. If you are interested in providing measurement
data, you might be able to help out. Read more at the
Minor Planet Center.
PHA (Potentially Hazardous Asteroid)
This does NOT mean
that this is going to hit the Earth. It means that someday, many decades
from now, beyond our ability to accurately plot it's orbit, it is very thinly
possible that it could. Basically, PHAs are asteroids that we need to keep
an eye on in the very long term.
Added/Updated The two dates here show when the object was
added or updated. Together with color highlight, this makes it easy to
find newly discovered asteroids.
How I obtain the data
The data available here is derived from two web pages at the MPC.
The first is
Close Approaches To The Earth.
This web page lists predicted
close approaches with Earth. It changes daily, because existing asteroids
are constantly having their orbits updated, especially if they are fairly
new, and because new asteroids are constantly being added shortly before
or after a close approach. My data updates daily from this source.
If you are interested in visually observing
a close approach, you should check my web page regularly.
That web page does not estimate
visual brightness, angular speed, or positions in the sky. To obtain
this information, I use the
Planet & Comet Ephemeris Service.
I request hourly data, starting the day before the close approach, for
three days, and scan for the maximum speed and brightness and when they
occur. At this point, I don't go any finer than hourly. If a close
approacher has the same speed or brightness for several hours, I will
list the first hour in which it is predicted to be at it's
maximum speed or brightness.
I use Boston as the location for all these requests. For very
close approachers, this can introduce errors relative to other positions
on the planet
but it should be close enought to get a general idea.
I also request the oribital elements from this service by selecting
"MPC 8-line" in the formats for orbital elements section. From this,
I obtain the absolute magnitude. I also determine whether or not this
is a PHA based on output on this web page.
Note that if the information for a particular approach on MPCs
Forthcoming Close Approaches
web page has not changed, I do not check the Ephemeris Service for