Amateur Observations of
Asteroid Didymos/Dimorphos, B. Gary, 16", Hereford Arizona
B. Gary, Latest Update: 2022.10.24, 05 UT
This web page is no longer being updated due
to lack of interest on my part. Plenty of others, especially on
DART's photometry team, are conducting observations so there's
no need for mine.
I neglected DART's impact of the asteroid Didymos's orbital
companion (Dimorphos) until Richard Hoagland asked if I was
observing it to detect changes after impact. He suggested that
Dimorphos might have been completely destroyed by the impact,
and wondered if I could do some observations to confirm this. So
I made my first observations Oct 01 (UT) and was able to observe
between clouds for ~ 2 hrs. Here's what I found:
Figure 1. Light curve for a 2-hour observing session.
The "?" symbol indicates onset of a fade.
Three features are of
interest in this light curve: 1) a possible fade at 8.85 UT,
2) a possible brightening at 10.12 UT, and 3) a possible fade
starting after the brightening. (A
wild speculation: Features 2 and 3 are due to an orbiting dust
cloud producing forward scattering brightening before it
After more image sets are available I will try to derive a
Didymos rotation curve. The Didymos rotation variation can be
obtained by phase-folding with a 2.26-hr period, and deriving
a smoothed brightest-value curve. After subtracting that curve
from a LC like the one above the residuals would contain the
eclipse and transit features, which in-turn would allow for
eventual determination of the new orbital period for Dimorphos
(the smaller asteroid, that one that was impacted).
Update: October 2 (2nd observing session)
Figure 2. Light curve for 3 hours on October
Maybe this 1/2-hr, 3 % fade (centered on 9.11 UT) is a transit
or eclipse, that can eventually be used to determine if the
DART mission objective was achieved. All of my graphs have
been submitted to the DART team, which must be busily working
away with all their LC observations to solve this complicated
A trailing tail is present; maybe sideways and leading ones
are also present.
Figure 3. 3.5-minute of exposure. North up, east
left. FOV = 4.4 x 3.3 'arc. Asteroid motion is to left and
slightly up (e.g., ENE).
Update: October 3 (3rd
Figure 4. Light curve for 3 hours on
There's a fade feature from 9.3 to 10.4 UT (centered on
9.85 UT) with depth ~ 9 %. It's different in length and
depth compared with the fade feature on the previous day so
they are probably not associated with the same eclipse (or
transit). I have no idea how to explain them..
Figure 5. Still 3 tails, the longest one is 4.5
'arc long. North up, east left. FOV = 9.4x5.3 'arc.
This image can be compared with an image taken with a
4.1-m telescope in Chile (SOAR observatory), link1 and
is rotating slightly faster than the "spin barrier" for
rubble pile asteroids. This is shown in the next graph:
Figure 5. Scatter diagram for many asteroids
showing rotation rate vs. diameter. The spin rate of ~ 10
rotations per day (P_rot = 2.40 hours) is referred to as a
"spin barrier." Faster spinning rubble-pile asteroids have
insufficient gravity to overcome centrifugal forces,
meaning that they disintegrate, spewing rubble in all
directions (in its equatorial plane), and eventually disappear. The
red circle is the location of Didymos (pre-impact).The
blue line shows the track Didymos would take toward
disintegration if it's spin rate were to increase slightly
(from 10.6 rotations/day to whatever destabilizes it).
Didymos is already sitting on the
"spin barrier" (red circle in above figure), and a small
increase in its spin rate would force it to be on a
"disintegration runaway" track (leftward, along the blue
line in figure). While on this track Didymos would have
insufficient gravity to hold itself together as it spun
itself apart while shedding debris into a disk that could
extend to beyond its Hill sphere (e.g., producing long tails
in several azimuthal directions). This debris field could
endure until it disappeared by dispersal into interplanetary
space (contributing to the Gegenschein). During this mass
loss process the orbit of Dimorphos would grow larger, with
longer periods, until it too would fling itself free of
Didymos. All of these behaviors would be observable from
Earth, so it's just a matter of time before we'll know if
the entire Didymos/Dimorphos system is destined for
disintegration and disappearance.
Pravec, P., C. A. Thomas and 8 others, 2022, "Photometric
Observations of the Binary Near-Earth Asteroid (65803) Didymos
in 2015-2021 Prior to DART," The Planetary Science J.,
3, 175 link
B.Gary home page
This site opened: 2022.10.02.