The Occultation of a 11.5 star by the Asteroid Didymos and its Moon Dimorphos

Nov 12, 2022 at 2:09:16 am from the Mojave Desert

Nature paper showing changed shape of Dimorphos after impact

This is high value event. Didymos's moon Dimorphos was the target for the NASA DART mission, and it is critical to determine with high precision the new orbit of Dimorphos and Didymos after the DART collision. The DART Mission is to test out an idea for deflecting an Earth-crossing asteroid by using passive mass impacting. It might at first seem like a simple freshman Newtonian calculation, but there are variables. How much of the energy of impact goes into heat and not momentum change? How does the surface mechanical properties change how the energy is absorbed? Didymos/Dimorphos is not a threat to Earth, but testing this strategy is vital for successful planning of a deflection of a dangerous asteroid in the future. It's also important for any new orbital targeting of commercially valuable asteroids which some would like to move to more Earth-convenient orbits.

We've already gotten 6 successful past Didymos occultations. But nearly all have been just a single chord. This Nov 12 event is favorable - a bright star and promises to draw some observers from Arizona, Nevada, and California to help get multiple chords. The asteroid is only about 1 km across, but the orbit is now well enough known so that the odds of getting a "hit" are high, unlike for Phaethon in 2019.

This is the best opportunity of the year for a Big Science event to excite my Astro 8A students. Not only is this the most accessible high-value science event of this year, it's also on a weekend, favoring being able to ask for volunteer students for this optional Astro 8A Observation Astronomy field trip. But, it's a 1000+ mile round trip. Exhausting even by Cabrillo Field Trip standards. It's possible that even my most eager-beaver astro fan students may not be able to take that much time off for an optional event. So, my team (me, and former student Kirk Bender) will be taking lots of photographs and documenting the details of the process, and the planetary science that's also to be learned in this geology-rich area.

I'm requesting a Cabrillo van to use for transporting the equipment I've been focusing for the past 3 years for asteroid occultation precision timings equipment grants. We have 3 good rigs - currently used by myself, Kirk Bender, and teaching assistant Karl von Ahnen. I am going to try to interest one of my bright and motivated future astronomer students in Astro 8A to join Kirk and I as observers and data-recorders for this event. It'll be tough; It's a long 8 hr drive to the desert area east of Amboy, CA on Friday evening Nov 11, to arrive for the asteroid event that happens at 2:09am Nov 12 Saturday.

Update: Just Kirk and will be the observers. I reserved Cabrillo College van N, to be picked up Thur by 2pm. A trail running accident 2 weeks ago caused a cracked rib and still hurting, and the cloud forecast shows clouds only clearing very shortly before the event. So it's still iffy as of Wed afternoon as I write this. Our Astro 8A students are not able to take this much time off for the event, even Cade whom I'd hoped could do our 3rd track station. So... we'll document and bring it back as learning experience in the classroom for Astro 3, and for 8A.

Target star details.
The target star is in western Cancer, halfway between Procyon and the Beehive Cluster. It's at a very good 51 degrees altitude, and the 85% waning gibbous moon will be 30 degrees above the target, in Gemini just a degree above M35. The target star is fairly bright at v=11.5 magnitude and so a 0.31 second event should be do-able at 2x setting, if skies cooperate. Clear skies and 30 degrees from the moon shouldn't present a problem. There's a slightly dimmer star above the target by about 40 arc seconds. Should be marginally able to be no problem with interference, depending on seeing conditions.

I've chosen the most convenient and most promising observing site along the narrow 1km wide path. It is along Essex Rd. between I-40 and Route 66, east of Amboy. That'll be our destination. If clouds dictate, we can also try further north at Castle Mountain, or south, along Route 66. Current prediction early in the day, is for clouds to be with us on the entire drive, but be clearing past the eastern sky by the time of the event, as a storm system exits.

As of Nov 9, IOTA's Steve Preston is recommending the JPL orbit #197 from Horizons, and John Irwin sent the .kmz file which has the elevation corrected path and tracks for observers. I've used it to make the 4th chart below and choose sites. Norm Carlson and I agreed to switch tracks, given his slightly larger telescope he took track 2W and I am now on 1E. Kirk is not yet assigned, but likely I'll place him on the centerline, which is unoccupied.

Weather: Current prediction as of 30 hours before the event, is for clear skies. A bank of cirrus that originally was expected over the site, is, with later and later weather updates, now moving faster than earlier predictions, east. So the bank of cirrus is now expected to not interfere at all. All of the CA Mojave looks clear. Low temperature at Pisgah Crater that night is 33F. It might be a big warmer at Essex Road, but our elevation is similar. Light winds at the site and all around the desert, is ~6mph variable direction. 6 mph slight breeze shouldn't be a problem.

Event time is 2:09:16am at Essex Road.

Our chosen sites: on the centerline and 1 track north, along Essex Rd about halfway between I-40 and Rt 66. Elev =1800 ft.

SDSS image, with the neighbor star to the right, in the north-up chart, magnified.

Camping:
Priority plan is camp at my occultation site on track 1E: along the dirt road which runs SW from an intersection with Essex Road at track ~1.7E. The Google Earth image shows this dirt road is on flat ground, and my 1E station is just ~200m from Essex Road, which itself is unlikely to have more than ~1 car per hour on it, at that time. There's good flat sand to set up tents and be comfortable, setting up a table / campstove as well.

Post Occultation Planned Activities:
After the occultation: I'm not sure what other observers will be close enough to us to meet before or after the event, at this time. As of Nov 10, it looks that there will be no other observers close to our site. Paul Maley and Norm Carlson will try it from the path where it crosses I-10, near Chiriaco Summit. So, our communications with the other team members from Arizon and SoCal will likely only be by phone and text.

In any case, it would be valuable as additional education to study the limestone caverns of Mitchell Caverns in the Providence Mountains not far from the path. It's part of a national recreation area, and the Providence Mountains are among the highest in the Mojave, with pine trees growing at the elevation of Mitchel Cavern (or, at least there were in the 1970's when I last visited Mitchell Caverns).

Cultural History: We'll show images on this page and a later PowerPoint, of the deep past geological history. And, also I will link images from the 1950's and 60's when Route 66 was an iconic road from Southern California to the rest of the country. Relics of this past can still be found, and provide a setting for the early Space Age (especially Amboy, and Amboy Crater - a cinder cone from millions of years ago). We'll also stop at Ludlow, which has some old buildings showing the evolution of the area from the 1950's to the present.

Pisgah Crater: West of the Providence Mountains, and along Route 66 (a very quiet road after I-40 was built) is Pisgah Crater (Pisgah Crater map - geology), which is a significant volcanic feature from an earlier geologic epoch when California had a subduction zone along its coast. There are lava tubes underground which have been targets for cavers for many years. And a panoramic view of the desert geology from the summit area. I've taken photo workshop groups to this spot many times, ~10-20 years ago. It's a popular spot at cool times of the year, says this source. "This volcano is a very popular geology research site, not uncommon to find university classes and professional seminar groups around the mountain. Geological survey activity also occurs at the mountain every so often. Moreover, the volcano erupted at least three times, producing porphyritic basalt, and rocks from the first eruptive phase are almost aphanitic." During the Route 66 Photo workshops which I helped lead some years ago, Pisgah Crater usually had a school group or another car or two visiting at the times we were there in the mid-morning. Our photo groups were older and not into caving, but I've been into a couple of the lava tubes and they're not dangerous to go into. Bring head lamps!

These destinations would fill Saturday pretty well. There's supplies available in Amboy and Ludlow, but regular large markets we won't find till close to Barstow. drive back towards Santa Cruz and then at midnight, set up for the Pippa Occultation, which is a decent event at 22 deg altitude at 12:31am Sunday. It's 5hr 13 minutes drive from Pisgah Crater to the sites I've chosen along I-5 not far from Mercey Hot Springs road. If we left Pisgah after photographing sunset, we'd have plenty of time to stop for gas and a Subway dinner and get there before 11pm. We could then drive home afterwards, arrive after a long night of driving, but at least have all Sunday to recover. Or, we could camp at Rainbow Basin if we're too tired. Can decide later.

Results and Post Event Analysis

Event Observers:
Robert Jones - (from Running Springs, CA) with the largest scope at 14" and thus best scope to go after the DART target moon, Dimorphos, which was predicted to be on a track offset from the Dimorphos path. Not possible to get both moon and primary asteroid from the same site. Fortune favors the brave; The Dimorphos event duration was predicted to be only 0.07 seconds long (!). But the S/N for this 11.5 mag star was expected to allow his using 1/60 sec frame rate and get this moon. If successful, this would be the FIRST confirmed (as it turned out Roger Venable got the first, later confirmed) precision astrometry of Dimorphos after the DART impact deflected its orbit. And, such an observation is not possible for the brighter occultation event of Nov 14 which will have more observers; Dimorphos will be behind Didymos so no occultation possibility. Robert did his job, and got lucky too - his site was in the Dimorphos shadow as hoped but certainly not assured, based on preliminary data from the DART post-impact astrometry.

Paul Maley - drove out from Tucson, Arizona and took track 1W, the closest-to-centerline track on the west side. He set up near I-10, ant got a successful 0.25s occultation

Kirk Bender - I gave Kirk the centerline, as he has the better camera, and it would best make use of his equipment. More on his efforts below.

Richard Nolthenius - I took track 1E, the only observer on the east side of the predicted path. More on my results below.

Norm Carlson - took track 2W and had a good event about 0.30 seconds in duration

Robert Jones - took the track recommended by JPL orbit #197 to attempt to get Dimorphos

Narrative

I got final sky charts and key information pages printed and stored in a folder, finished packing the last of the occultation gear and camping gear on Friday afternoon. Kirk and I left my place in the van at 2:20pm Friday. Kirk was co-pilot, manning the iPad and computerized access to the rest of the event observers. Paul Maley decided to stick with the Interstate 10 site as weather looked finalized and clear. I promised Paul we'd not put Kirk at 1W or too close to it. Route: To get to our site, I drove Kirk and myself in a Cabrillo College van to Hwy 101 south to Hwy 46, across to I-5 and then to Wasco, where we stopped at a Subway to buy some dinner.

The Didymos Occultation

We arrived at my site on the dirt road at midnight, time enough to set up the campstove table, my tent, and the 8SE telescope and gear table, before then driving Kirk to the centerline, where we decided it was best for him to observe. I feared our 8" scopes were unlikely to be able to reliably detect an event that was much shorter than 0.1 seconds. With Norm at 2W, Paul at 1W, me at 1E, then Kirk would take the centerline.

We set up his site an hour ahead of the 2:09 am event time. I drove back to our camp and got my equipment set up. With the temperature near 40F, it was important to keep the camcorder warm before and during the even. Kirk and I have both had camcorder failure-to-record at low temperature (one of these was on the vital Hayabusa I re-entry mission in Australia, for me in 2010). For this event, I took an Orion insulated Dynamo Li-Ion battery box and configured it to hold my camcorder. Kirk used some clothing to minimize heat loss . As back up, Kirk and I would video record the external LCD monitor screen, as the video signal itself does not seem subject to cold temperatures. It was the camcorder recording heads device that were worrisome.

Kirk's Station:
The telescope and video gear set up OK, and Kirk got onto the target area after a 2-star alignment using Polaris and Procyon, which were the same two stars I chose. These are far apart on the sky, and Procyon was not far from the target star, so they seemed the ideal choice. The problem came later, as the barrell connector to the scope from the battery turned out to be fragile at these cold temperatures, and even a moment's lack of power means the 8SE scope has to be re-initialized again from scratch with 2-star alignment, which typically takes up to 12 minutes to complete if all goes well. After locating the target in the Q70 eyepiece, and while then taking out the eyepiece and diagonal to insert the Watec 910hx videocamera, the scope lost power more than once and had to be re-started. The last power-loss was too close to event time, and in the end, Kirk could not get on-target and try video recording until after the event. We learned a lesson - have a back-up power connector, and unfortunately, the barrell connectors come in more than one size yet look identical from the outside. Another source of intermittant power trouble. Fortunately, the other 4 observers all got good data, so the loss of Kirk's data was not crucially fatal.

Nolthenius' Station
I had no trouble getting the scope 2-star aligned and the sent it GoTo to the target. But I did not identify the star field in the eyepiece. There's a bit of backlash and drift in the gears of most telescopes, including the 8SE scopes. The best solution I've found is to tell the scope to GoTo a nearby easily identified Messier deep sky object and then note the offset from the pointing to the valid pointing. I sent the scope to M44 and could see that the scope was pointing close but not perfectly, and with the offset, could then ID the star patterns near the occultation target star. I make our finder charts so that the eyepiece circle chart is scaled and oriented just as one will see it in the eyepiece a half hour ahead of the event. Our event was 51 degrees in altitude, in western Cancer, and this was low enough that field rotation was not a problem. It's best to realize that the star field rotation may be slightly different than the printed chart. The convention is to begin taping at least 10 diameters before the predicted event time. In our case, that's only 2.5 seconds! Much better to do a minute before and a minute after, for such short events, just to get enough data points so that a decent statistical base can be there for the "false positive test". 10 diameters is a ball-park figure for where one should find any undiscovered moons, but there are exceptions in the literature of asteroid moons more than 10 diameters away.

I had brought an Orion Dynamo Li-Ion powered battery box, with foam cutout, as an ideal place to put the camcorder and keep it from cooling off by radiating to the night sky, and also brought two "blue ice" packs. One for Kirk, which I kept warm up until I left him at his station by keeping inside my pants. And one for me. I found as I assembled the equipment that I could not get the video input cord to reach the camcorder from the OccBox, and so instead I placed the warm blue ice pack underneath the camcorder and left it in its usual place, on top of the top shelf of my custom made "OccBox". I started recording at 2:08:25am, and had charged up the triple size large Li-Ion camcorder battery, and also a second regular sized battery completely, just the night before. Nevertheless, the cold seemed to zap the power from the battery and the power reading dropped rapidly during the brief recording. I made sure to get my little red PowerShot camera on its small tripod and aimed at the external LCD screen monitor during the event. During the event, I could see the target star quite bright and easy at 2x, which was gratifying as I worried it would be scintillating too often near 0 level and make the false positive test difficult. In actual fact, the seeing was good, the stars were steady, the target bright, and the brief 1/4 second occultation was very easy to see live onscreen. I could have gotten an even shorter event without trouble, at that magnitude. The recording was left going till 1.7 minutes after the event before shutting down. The camcorder battery, again, started at full and dropped rapidly to half, and then down to just 10% while still recording. The next morning, I waited till the sun had warmed the camcorder to a decent temperature in the 60's, and then played it back. But, it showed no apparent recording, the playback screen looked black. At this point I was very glad I'd done the backup, which did record successfully. Later, at the end of the long day and while re-charging inside the van as we drove back, Kirk checked the recording again and said that it did indeed record. But, later after full recharge and room temperature operation, the play back only contained the audio record and the VTI time stamps, and no star field... or so it seemed.

Lessons:
* Make sure the battery is fully charged and healthy enough for operation at cold temperatures. (actually I'd done that).
* Always do a back up recording of the external monitor
* It's sometimes a good idea for recording faint stars to adjust Gamma to be less than 1, says David Herald. Play with it to bring out best S/N. I've always left Gamma=1 to make for more accurate photometry, but it might not be optimum for low noise on faint stars. Kirk adjusted gamma on his scope after I picked him up and before we took his equipment down, and it did make a positive difference on his gear. Too high or too low, and the visibility of faint stars is compromised. Don't assume gamma=1 is best if getting magnitudes is not essential. For my own tests, I have not yet seen lowering gamma as helping visibility of faint stars. However, for bright stars, lowering gamma may be a good way to avoid saturated pixels and yet avoid the photometry measurement troubles of having an out of focus star.
* Also, consider analyzing by NOT subtracting the background (use 'appsum' in PyOTE). For short and/or faint events, this can help. In tonight's event case, the dip in brightness is to zero so it's not so important. However, high sky noise can then swamp signal, so it's only a possible option to try.

Analyzing the back up recording...

The first issue is that the Canon point/shoot PowerShot, and my Nikon D7000 as well, and nearly all cameras in fact, use 25 frames/second recording rate. However, the miniDV camcorder records at 30 frames per second. This means that the back-up recording is taking images at offset times and at a different rate than the original frames it is recording. This then requires that PyOTE operate with manual time stamps. I had to go in and look at the first and last frames and give the best timestamp. There's going to be some uncertainty there since there will be a blend of two frames on each backup recording frame. I estimated the proper first field time (left number of the two) by obsering if it was a blend of two frames and how much of which frame dominated by the intensity of the numbers, and interpolating. It seemed to work well. For 3802 frames of recording, the 25 frames/sec time stamps exactly filled the interval. No dropped frames warning.

David Herald noted that while my procedure was well-motivated, it would actually be best to find two PowerShot frames which showed very strong time numerals and then weak superposed numerals of the next NTSC time stamp. This would best guarantee that the beginning of the NTSC frame matched the beginning of the PAL PowerShot frame. In otherwords, take the bounding frames for PyOTE's calculation to be an an integer number of beat frequency periods. This was not yet done in the table of images of my reductions shown in the table immediately below. Why? Because as it turned out, I didn't have to use the Powershot frames at all. See farther below, as the original camcorder screen seems was so dark that I thought initially it was altogether not recorded. Yet could easily be reduced by just adjusting the background/range in PyMovie).

Below Table: The Images and photometry from the PowerShot video off the external monitor. This is a good educational in case this back-up method is needed in the future...

Nov 16. Update! I tried again after recharging again the big camcorder battery at home and replaying, I could see, darkly, the video signal on the original taping. Great! No need now to use the PowerShot video for the analysis. So, I was then able to download the tape and reduce the original, using the display background/range option in PyMovie, and get a better reduction. I could also, in PyMovie, use "reduce in field mode" as I'd recorded at 2x setting in the Watec. This doubled the number of display points, and the quality of the fields was easily good enough to give a solid reduction. Here's the new reductions. New PyOTE Log file

The new reductions including my updated analysis, are shown on the IOTA website. Here's the sky plane reduction. Exciting to see now, for the first time post-DART impact , we have solid relative positions between Didymos and Dimorphos. Congratulations, now, too to Roger Venable who first got a detection in occultation simultaneousl for Didymos and Dimorphos.

Reporting:

* IOTA report files Nov 16.
* ACROSS was sent the original video via WeTransfer, and the IOTA reductions as well. on Nov 17 evening.