<p><a href="https://astronomy.stackexchange.com/a/50744/7982">@ConnorGarcia's &quot;impactful&quot; answer to <em>Why did they decide to hit Dimorphos in the retrograde direction rather than prograde; was it a &quot;coin-toss&quot; or were there implications for observing?</em></a> quotes McQuaide et al. (2021) <a href="https://ui.adsabs.harvard.edu/abs/2021plde.confE.251M/abstract" rel="nofollow noreferrer">Double Asteroid Redirection Test (DART) Phase D Mission Design &amp; Navigation Analysis</a> near the end of Section II. Trajectory Overview:</p>
<blockquote>
<p>Dimorphos orbits Didymos with a period of 11.92 hours. The planned retrograde  impact will reduce this orbit period. The retrograde impact was chosen over a prograde  impact partly because the resulting increase in the orbit period for a prograde impact  would cause Dimorphos’s orbit around Didymos to be nearly exactly 12 hours, synchronized with Earth’s diurnal period. Earth-based observations would see  Dimorphos in the same configuration each observing opportunity. A retrograde impact  and resultant orbit period reduction allows for more unique observations from night-tonight. <strong>Finally, the retrograde impact also provides for better lighting conditions, as it occurs on the sunlit side of Didymos. Secondary reflection from Didymos may allow  imaging of the night side of Dimorphos.</strong></p>
</blockquote>
<p><strong>Question:</strong> What does &quot;Secondary reflection from Didymos may allow imaging of the night side of Dimorphos&quot; mean? Why would this be useful? From where would such imaging be done?</p>
<p>At only 0.003 arcseconds wide, the &quot;imaging&quot; of the night side of 171 meter Dimorphos from 12 million kilometers away at Earth (i.e. resolving it) seems unlikely to be possible. So I can't figure out what this refers to. Wikipedia's <a href="https://en.wikipedia.org/wiki/Very_Large_Telescope" rel="nofollow noreferrer">Very Large Telescope</a> puts its resolution at 0.002 arcseconds, so they'd only get a few pixels <em>at best</em>.</p>

CC BY-SA 4.0
2022-10-17T09:06:08.953Z
The ejecta scatter light. This light will slightly illuminate the dark side due to being not in solar direction. That is similar to how the unilluminated part of our Moon is illuminated by Earth shine. (as a comment as I'm not sure that this is actually meant... but sounds possible without doing math on the intensities)
planetmaker
CC BY-SA 4.0
2022-10-17T21:10:05.510Z
Could this refer to imaging by LICIACube which occured a few minutes after the impact? https://en.wikipedia.org/wiki/LICIACube#Cruise_phase_and_flyby
Dave Gremlin
CC BY-SA 4.0
2022-10-18T00:06:32.430Z
@DaveGremlin That seems like it would certainly a big part of the "Phase D Mission Design & Navigation Analysis".
uhoh
<observational-astronomy><asteroids><collision>
What does "Secondary reflection from Didymos may allow imaging of the night side of Dimorphos" mean? Why useful? Imaged from where?
uhoh
# What does "Secondary reflection from Didymos may allow imaging of the night side of Dimorphos" mean? Why useful? Imaged from where?

(Asked by: username_0 on 2022-10-17)

[@ConnorGarcia's "impactful" answer to *Why did they decide to hit Dimorphos in the retrograde direction rather than prograde; was it a "coin-toss" or were there implications for observing?*](https://astronomy.stackexchange.com/a/50744/7982) quotes McQuaide et al. (2021) [Double Asteroid Redirection Test (DART) Phase D Mission Design & Navigation Analysis](https://ui.adsabs.harvard.edu/abs/2021plde.confE.251M/abstract) near the end of Section II. Trajectory Overview:

> Dimorphos orbits Didymos with a period of 11.92 hours. The planned retrograde impact will reduce this orbit period. The retrograde impact was chosen over a prograde impact partly because the resulting increase in the orbit period for a prograde impact would cause Dimorphos’s orbit around Didymos to be nearly exactly 12 hours, synchronized with Earth’s diurnal period. Earth-based observations would see Dimorphos in the same configuration each observing opportunity. A retrograde impact and resultant orbit period reduction allows for more unique observations from night-tonight. **Finally, the retrograde impact also provides for better lighting conditions, as it occurs on the sunlit side of Didymos. Secondary reflection from Didymos may allow imaging of the night side of Dimorphos.**

**Question:** What does "Secondary reflection from Didymos may allow imaging of the night side of Dimorphos" mean? Why would this be useful? From where would such imaging be done?

At only 0.003 arcseconds wide, the "imaging" of the night side of 171 meter Dimorphos from 12 million kilometers away at Earth (i.e. resolving it) seems unlikely to be possible. So I can't figure out what this refers to. Wikipedia's [Very Large Telescope](https://en.wikipedia.org/wiki/Very_Large_Telescope) puts its resolution at 0.002 arcseconds, so they'd only get a few pixels *at best*.

**Tags:** observational-astronomy, asteroids, collision

> **Comments:**
> **username_1**: The ejecta scatter light. This light will slightly illuminate the dark side due to being not in solar direction. That is similar to how the unilluminated part of our Moon is illuminated by Earth shine. (as a comment as I'm not sure that this is actually meant... but sounds possible without doing math on the intensities)  
> **username_2**: Could this refer to imaging by LICIACube which occured a few minutes after the impact? https://en.wikipedia.org/wiki/LICIACube#Cruise_phase_and_flyby  
> **username_0**: @DaveGremlin That seems like it would certainly a big part of the "Phase D Mission Design & Navigation Analysis".  

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