Fields on the HOP Page

Observatory and Observatory Code

The ephemeris of each asteroid is calculated for a topocentric position. The Minor Planet Center has provided the position of many observatories throughout the world. By selecting one of these observatories, you automatically supply the proper latitude and longitude to HOP.

If you are not observing from one of the listed observatories, please select an observatory near your location. Generally, the ephemeris error introduced by this approximation is not observable.

Limiting Magnitude

This is the faintest object that you can currently observe. Limiting magnitude can vary, depending on local conditions and integration time, However, any asteroid predicted to be fainter than the value you select will not be considered. Remember that the absolute magnitudes of asteroids are not always well known, so the calculated values can be in error.

Maximum Zenith Angle

This is the maximum angle from the zenith that you choose to observe. Any asteroid at a greater zenith angle will not be selected.

Minimum Solar Elongation

Any asteroid closer to the Sun than this value will not be selected.

Minimum Lunar Elongation

Any asteroid closer to the Moon than this value will not be selected. Users should change this value as the lunar month progresses. At new Moon, set the value to zero. Near full Moon, the value must be larger. When the Moon is near the horizon, you can observe closer. General advice is to be aggressive with this parameter (especially when the Moon is not full) and make it small. Then see if you can observe the object.

Minimum Galactic Latitude

Any asteroid closer to the galactic plane than this angle will not be selected. Be aggressive with this parameter and make it as small as you dare. Remember that the width of the galactic plane is larger in northern-hemisphere summer. Also remember that there are dark regions near the plane where asteroids can easily be observed.

Field of View

The field of view is used to calculate an instrument's probability of imaging an asteroid.

Probability of Imaging

Any asteroid that has a probability of imaging less that this value will be ignored. The probability of imaging is calculated by taking into account the positional uncertainty of the asteroid and the field of view of the observer's instrument.

Your Name and Email Address

Your name and email address are useful to others if they wish to communicate with you about your observing plans. If you mark an asteroid with your intent to observer, your name and email address will be attached to the asteroid. Then other observers may talk to you if they have issues of common interest. You may not schedule an asteroid for observation (see below) if you have not provided your name and email address.

Asteroid Selection Criteria

Here the user can select asteroids based on various criteria. Each criterion allows the user to assign a relative importance rating from 0 to 9, with 9 as the most important. There are two major classes of criteria: The first concerns asteroid orbital type and the second concerns reasons to observe. At least one of the asteroid type criteria must be non-zero and at least one of the reasons to observe must produce a non-zero score.

The selected asteroids are given a score based on the product of your ratings and an internal rating in each category you select. The final list of asteroids is ordered by the total score, normalized to 100.


This is the set of asteroids whose perihelia are greater than 4.95 AU and that are not Trojans.


This is the set of asteroids that are neither Transjovians nor Mars-crossers nor inner planet-crossers. It consists mostly of main-belt objects but also contains objects that don't fit into the other categories.

Mars Crossers

This is the set of asteroids whose perihelia are between 1.3 and 1.666 AU, and whose aphelia exceed 1.381 AU.

Inner Planet Crossers

This is the set of asteroids whose perihelia are less that 1.3 AU. It comprises Atens, Apollos and Amors.

Danger of Loss

We considered asteroids whose positional uncertainty is greater that 1 deg in the next 10 years to be in danger of loss. The internal rating is calculated as the ratio of the current uncertainty to 1 deg. If the asteroid has an uncertainty greater than 1 deg for the entire 10 year period, it is considered lost and is given the lowest score.

Maximize Orbit Improvement

The rating for these asteroids is calculated to be the ratio of the current ephemeris uncertainty to the next peak ephemeris uncertainty. If the current uncertainty is greater than the next peak, the asteroid is given maximum score.

It is possible for an asteroid with a large ephemeris uncertainty to receive a low score because the peak ephemeris uncertainty is much larger than the current uncertainty. If the ephemeris uncertainty is large enough that the asteroid will soon be difficult for you to find, then it is a good target to observe now.

Numbered Asteroids with Substandard Orbits

If the ephemeris uncertainty of a numbered asteroid exceeds 2 arcsec at anytime during the next 10 years, the orbit is considered substandard. The rating is calculated by the ratio of the current uncertainty to the maximum uncertainty.

Numbering Imminent

This service cannot be implemented until we receive observations from the Minor Planet Center on a daily basis.

Asteroids Needing Confirmation

Asteroids in need of immediate follow-up observations are given high ratings here.

Mass Determination

Asteroids that have or will have a close encounter with other massive objects such that observations will aid in determining their masses will be selected.

Spacecraft or Occultation Targets

Asteroids that are space mission or occultation targets are rated at maximum value and all others are rated at zero.


The user can select options that control the way the selected asteroids are displayed.


The user can select the parameter on which the output is sorted. "Relative Score" is the default and it orders the asteroids by their need for observation. The other sort options are obvious. If "Orbital Elements" output style is chosen, RA, Decl, RA motion, and Decl motion become meaningless as sort parameters because they are not included in the output list. If the user selects one of the meaningless sort parameters, the program will default to "Relative Score" as the sort field.

Sort Direction

The user can select the direction of the sort, i.e., ascending or descending.

Maximum Number of Asteroids

The user can select the maximum number of asteroids that will appear in the output list. The absolute maximum is 100.

Scheduled Asteroids

The user can include or omit asteroids scheduled by other users. If the user omits scheduled asteroids, the scheduled asteroids do not count in the total number of asteroids selected. However, they do count if they are included. For example, if you select "Include" and get 100 asteroids, all of which are scheduled by other observers, you could select "Omit" and get a different set of asteroids, none of which will be scheduled by other users.

Find Asteroids

Clicking this button invokes the routine that produces a list asteroids that meet the specified selection criteria.

Observation Scheduling

You may indicate your intent to observe an asteroid by clicking the button next to the asteroid name. You must also click the 'Schedule' button at the bottom of the page after you have made all your asteroid selections. This last button will write your choices into the scheduling database.

The asteroid will remain scheduled to you for 48 hours or until we know that the object has been observed, whichever occurs sooner. (On some occasions, we may not be aware that your observations have been submitted to and accepted by the Minor Planet Center.) You must provide your name and email address before you will be permitted to schedule an observation.

General etiquette:

When you schedule an asteroid, your name, email address and observatory code will appear in place of the scheduling button. If you are unwilling to display your name and email address, do not schedule the object.

Selection Scores

There are eight selection scores. Each score can range from zero to nine. The scores are formed from the product of the user rating and the internal score for that parameter. The result is normalized to 9. The eight scores are listed below. (For convenience, we repeat some information given above):

Relative Score

All the scores for each asteroid are summed and sorted. The scores are normalized to 100 and ranked with the highest score first.

Schedule for Observation

The user may schedule a particular asteroid for observation by clicking the button immediately to the right of the asteroid name. Several asteroids may be chosen at the same time. To enter the choices into the scheduling database, the user must also click the 'Schedule' button at the bottom of the page.

RA and Decl

These fields give the RA and Decl of the named asteroid at 00:00:00UT of the day indicated at the top of the table. The user can infer an approximate ephemeris for the time of observation by using the RA Motion and Decl Motion presented in the next two columns. A better ephemeris can be obtained from asteph.

© Lowell Observatory 2008
Web Curators: Ted Bowell & Bruce Koehn
Last modified: 2008 April 29