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 project.

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 slightly interesting.

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 conditions. RARE!
14-18	Visible in large telescopes (12") with ideal conditions.
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 an asteroid 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 Forthcoming 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 Minor 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 changes.

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