Plume non-detection and NASA's pre-impact estimates
Chris Anderson
I guess what I am struggling with is this: How did NASA get their estimates of visibility so wrong?
I base this question on the following, admittedly very crude, estimate. I have tried, wherever possible, to err on the side of being conservative.
The official NASA word was that the impact of the Centaur booster might be visible in a 10” to 12” backyard telescope. Now, if I read that correctly, they meant “visible to the human eye with a 10” to 12” scope,” not “capable of being imaged with a CCD attached to a 10” to 12” scope”. Assuming that’s what they meant, let’s give them the benefit of the doubt and assume a 12” (0.3 meter) scope for purposes of comparison.
So far, the largest instrument I have seen that has reported a definitive non-detection of an optical plume is the 6.5 m MMT in Arizona (as per Sky & Telescope’s web site). The MMT has a light gathering power that is (6.5/0.3)^2 = 455x more than a 12” scope, however they also used a beam splitter to feed light to a total of three instruments, only one of which was an optical camera. So, if we assume that the beam splitter divided the incoming light evenly three ways, the camera still got 150x more light than a 12” scope.
According to the label on MMT’s video stream, the camera used for optical imaging was a StellaCam. The video is smooth, not jerky, suggesting that they were not integrating (stacking frames), but it is impossible to tell where they had the gain set.
In my experience a StellaCam II, when attached to our 5” spotting scope and operated in full gain mode but not integrating frames, produces images that are roughly similar to what the eye sees through our 24” scope. Given the difference in light gathering power between these two instruments (24/5)^2, that equates to the StellaCam being approximately 20 times more sensitive than the human eye. On Friday morning, our StellaCam was set around 60% gain to avoid saturating the image, so I’d guess that the MMT’s was set lower and/or they used filters to avoid saturating and/or the beam splitter was giving the camera a less-than-one-third share.
All things considered, depending on the exact gain setting on their StellaCam, NASA’s estimates for the brightness of the plume were off by roughly 1000 (and maybe more, since no ground-based optical instrument did, to my knowledge, succeed in doing so).
So, my question remains: How did NASA miss the mark by 3+ orders of magnitude?
Perhaps reframing the question in terms of geometry (was the plume simply too low to clear the crater wall?) rather than brightness allows for the estimated visibility to have been less egregiously overestimated. But, in my opinion, NASA’s published estimates were coming from the wrong side of the bell curve.
I understand that imaging science is typically less scientifically productive than spectroscopy. But grossly overestimating the visibility of something like this does have an negative impact (no pun intended) on NASA’s image, especially given the big PR push that NASA made (“have an LCROSS viewing party,” “participate in ‘citizen science’”, etc.) and leaves people like myself who are on the front lines of informal astronomy education with egg on our faces.
If I sound a little shrill, it’s because I worked for weeks to organize a public viewing opportunity for this event with a live video feed from our 24” telescope to our planetarium theater, and filled the theater to capacity, only to find myself having to try to explain to a packed house why they got up early on a Friday morning and paid $8 to see bupkis. (That being said, we did provide the NASA coverage and a breakfast, but still I wonder how many of our patrons who showed up and paid Friday morning could ever be induced to do so again for a similar event in the future. My guess is: Not many.
And I’m already seeing the conspiracy wing nuts in the blogosphere spouting about how NASA duped us into looking at the Moon so they could be sure telescopes wouldn’t be looking somewhere else where they had some nefarious plans unfolding. Or the only-slightly-less-crazy who, because they don’t understand the nature of imaging vs. non-imaging science, are calling for NASA to lose all its funding because of the “failure” of LCROSS and the “waste” of $79M of taxpayers’ money.
My advice to NASA is: Next time, be very conservative in your estimates of visibility. Bad PR usually trumps good science in the court of public opinion.
Chris Anderson
Production Specialist, Faulkner Planetarium
Coordinator, Centennial Observatory
Herrett Center for Arts & Science
College of Southern Idaho
Twin Falls, Idaho, USA
Tim Swanson
Too Deep of hole in the wrong spot. Conspiracy be damned. I seriously doubt its about what we could not see taking place.
I think they should have shot for the surface not a crater. Like I previously stated, why the crater. Everything from the crater was previously blown out by what made the crater.
We dropped an expensive rock into the crater and had greater hopes than what was delivered. We did not shoot a missile with explosives at the moon.
Bottom line in my opinion.... Over estimated - Over anticipated plume. So many wanted so much more from this and it did somewhat fizzle. I am sure there will be years of data from this event from what went wrong and right.
Again, I am a novice - with a big camera that was eagerly awaiting more than what happened. I will say, I have learned more than I could have ever imagined with this group and thank you all for your contributions.
Tim
From: cand...@csi.edu
To: lcross_ob...@googlegroups.com
Date: Tue, 13 Oct 2009 13:34:28 -0600
Subject: [LCROSS_OBS: 1269] Plume non-detection and NASA's pre-impact estimates
canopus56
IMHO is the short answer will be found in the LRO +90 sec Diviner
image which shows an off-axis ray. This indicates a low angle oblique
impact similar to (if memory serves correct) the 4 degree impact angle
of Apollo 14's Saturn IVB on the moon. Since the angle of the
incoming spacecraft was about 65 degrees, my speculative guess is that
it probably impacted by an near tangential graze on the side of a
small crater wall, effectively creating an impact angle relative to
the local surface of close to 90 degrees.
This scenario was a known and unavoidable experimental risk.
> My advice to NASA is: Next time, be very conservative in your estimates of visibility.
> Bad PR usually trumps good science in the court of public opinion.
governmental air of invincibility or "hype" either up or down, what
their scientific predictive judgments are. A scientific experiment
is not a "trial" in the public media. The LCROSS Team did emphasize
that this was an experiment and did not have a guarenteed result.
Science education for young people can best be served by giving them
an inside look at how science is done - which includes the risk of
experimental failure or large departures from the expected results.
The public should insist that NASA not curb their expectations or
their experiments to just doing "safe science" where the results are
largely known in response to criticism by a miniscule tin-foil hat
crowd. Whether in the science sphere or the political sphere, in the
new media environment, the public is becoming more savy at critical
thinking skills when consuming information. The public does not need
to be "protected" from good science.
That may be incompatible with and require some public communications
policy changes to accomodate public outreach venues such as your
planetarium.
Clear Skies - Kurt