View Full Version : How many asteroids are brighter than magnitude 13.5 or so?

2004-May-13, 05:56 AM
Anyone know exactly or approximately? Assuming most favorable viewing geometry at some point during the year.

Brady Yoon
2004-May-13, 06:01 AM
All I know is that one asteroid is brighter than 6th magnitude, Vesta.

Kullat Nunu
2004-May-13, 09:34 AM
Hard to say.

Absolute magnitude of an asteroid is its visual magnitude when asteroid is placed 1 AU away from the Sun and viewed at zero phase angle from a distance of 1 AU (i.e. viewed from the center of the Sun).

Typical main belt asteroid lies something like 1-2 AUs from the Earth so they would be 1-4 times dimmer (0.8-1.5 mag), if distance from the Sun is exluded; real brightness is less than that: they orbit 2-3 AUs from the Sun thus dimming additional 4-9 times (1.5-2.4 mag). So visual magnitude for a main belt asteroid would be about 2-4 magnitudes more than its absolute magnitude. Note that is for opposition, usually they are dimmer. On the other hand, asteroid move on elliptical orbits so sometimes they are closer and thus brighter.

Distant minor planets (about thousand known) almost all have absolute magnitudes < 13.5. Because they all lie so far away, their visual magnitudes are over 18 even for the very largest ones.

There is 17558 asteroids listed (ftp://ftp.lowell.edu/pub/elgb/astorb.html) whose absolute magnitudes are less than 13.5 mag.

If we chose only asteroids with absolute magnitudes < 11 and exclude all the asteroids whose semimajor axis is over 4 AUs only about 3000 asteroids remain.

On the other hand, several large near-Earth asteroids come much closer than 1 AU so they too can have visual magnitudes under 13.5 mag. Most NEAs are too small to reach that brighness even during close approaches.

Hopefully I got it right. :lol:

2004-May-13, 03:24 PM
Anyone know exactly or approximately? Assuming most favorable viewing geometry at some point during the year.

As a quick sample, Guide8 tabulates slightly less than 200 asteroids brighter than magnitude 13.5 right now. That's without checking to see how many are in some observable direction. As one might guess, lots of the low-numbered asteroids stay permanently brighter than that, but the list includes 5 without permanent numbers but bearing 1999-2001 discovery designations (well, unless I've neglected ephemeris updates even more than usual). Four of them (Ceres, Pallas, Vesta, and 20 Massalia) come in brighter than 10.0, and another 13 are between 10 and 11.

2004-May-14, 01:10 PM
Less than 200 sounds about what I was expecting to hear. I guess I should've clarified that I was talking about apparent visual magnitude from here and not absolute magnitude. 13.5 or thereabouts is the resolution limit of my telescope so I'm just wondering about how many would be within my grasp.

Heavens Above has a nice page for the brightest asteroids at any given time.