Asteroid Concepts

Introduction

To understand what it is I am doing, it is necessary to understand a few concepts.

Asynchronous Binaries

An Asynchronous Binary is an Asteroid that spins on it's axis at a rate different to the orbital period of it's moon/partner as shown is figure 1.  The difference in rate may be either faster or slower.  Note that this has nothing to do with the rate at which the moon/partner spins on it's own axis.
 
Figure 1: Asynchronous Binary                                   Figure 2:  Synchronous Binary

Synchronous Binaries

An Synchronous Binary is an Asteroid that spins on it's axis at the same rate as the orbital period of it's moon/partner as shown is figure 2.  Note that this has nothing to do with the rate at which the moon/partner spins on it's own axis.

Interpreting Binary Asteroid parameters

So what do all those published parameters for a multiple system actually mean?  Lets take a look at a typical system pictorially.

Figure 3:  Schematic of a multiple system

As you can see from figure 3 the system has a primary object and a secondary object.  Both of these objects have their own rotations and the secondary object orbits the primary.  Lets take a look at a real example - (2577) Litva who's binary nature was uncovered by Brian Warner and other members of the BINAST team:

(2577) Litva
P_1 = 2.81257 ± 0.00003h
P_2 = 5.6816 ± 0.0008h
P_Orb = 35.75h
A_1 = 0.23 ± 0.02m
A_2 = 0.08 ± 0.02m
D2/D1 >= 0.39
U = 3

From this data you can see the the Primary object has s synodic rotation of 2.81257hrs, the secondary object has a synodic rotation of 5.6816hrs and the secondary orbits the primary every 35.75hrs (synodic orbit).  A_1 and A_2 indicate the amplitude of the variations seen in the lightcurve.  D2/D1 provides us with the size ratio between the Primary and Secondary objects.  What is not shown, and is required to derive the value of D2/D1, is the amplitude of the eclipsing events.  Typically there are 2 eclipsing events coinciding with the secondary passing in front of and behind the primary object (as can be seen in figures 1 and 2).  Because we are not always in the same plane as the secondaries orbit around the primary, we see changes in this amplitude.  We look at the smaller of the 2 eclipsing events (smaller in amplitude) and determine it's maximum value over time.  Sometimes this cannot be determined as the plane of the systems orbit may never intersect our line of site.  In this case we can only constrain a lower value for D2/D1. 

In the case of (2577) Litva, the secondary is no smaller than 39% the diameter of the primary.  The value of U is the uncertainty in the data.  U can range from 0 to 3, 3 being certain (and provides reliable predictions) while 0 means the data is a possible indication only.

Most of the time you will not see a value for P_2.  This is because most moons are a great deal smaller than their parents or that the secondaries orbit is synchronised with the primary (as the moons rotation is locked to its orbit around earth). 

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