The Occultation of a 7.3 magnitude star by (3200) Phaethon

July 29, 2019 - Monday morning 4am

Phaethon is the parent of the Geminids meteor shower. An odd object that is small, about 4-6 km, but apparently rather round, from radar images during a close approach. But the diameter is uncertain, and the orbit needs refinement for a Japanese spacecraft mission to this object. So, this is a high-importance event. David Dunham plans to fly out and deploy multiple stations, and we'd like at least 30 stations to get good odds of getting the occultation from at least 3 chords and accomplish the mission. It's on Monday morning at 4:20am. Altitude is 27 degrees in the northeast, inside the pentagon of Auriga.

Preston Predictions page

The expected nerve center for the Central Valley contingent is in Wasco. Here's the preliminary plan

http://iota.jhuapl.edu/20190729Phaethon.htm

Derek Breit's set of maps

Kirk and I have been assigned tracks A22 and A31 respectively. Before deciding our precise observing spots, I carefully did the elevation correction to the track lines so that we were indeed at the expected tracks. I moved the sea level track perpendicular north (i.e. NW) by (0.48x elevation). Very nice that Track A22, elevation corrected, crosses an intersection (Arrowbear Trail and Arcadia Rd - both roads are just rough two-tire dirt trails) on a nice overlook over Carrizo Plain; the precise spot that I have camped at before, for a graze maybe 15 years ago - my second most prolilfic events graze ever with 22 events. And also on another occasion for another asteroid event. The tentative plan is for us to arrive there around sunset or so, eat, debug the "Unit 2" scope issue from the prior night, get some sleep, and then leave Kirk there to observe at track A22, while I drive a few miles back northwest to an entrance to Topaz Solar Farm and observe from Track A31. Then I'll return to camp, and we will either sleep, head to a hoped-for breakfast meeting with other Central Valley people, or head home depending on latest updates. Temperatures will be hot. Low temp at occultation time will be 66F, and high on Sunday in Carrizo may be 101F, 98F on Monday.

In case of scope trouble. One can aim using the chart here at the shown UT (subtract 7 hrs for PDT) and then leave scope alone and start integrating and the target will drift through during the right time. Kirk should print this chart and bring it.

Results:

David Dunham's page on the results and the AAS DPS paper press release and International Academy of Astronautics 2021 "Planetary Defense Conference" paper: D. Dunham's Abstract

Kirk and I ended up staying in Carrizo Plain exclusively, with no travel over the Temblor Range to the other observers (who were very busy with multiple station deployments anyway). I'd hoped to meet up with David D and others in the Central Valley, but we needed a spot to camp and also be able to sleep, and the predicted temperatures at night were still in the 80's in the Central Valley, with nothing but orchards to illegally camp on. Second possibility was the oil town of Reward, but it was just going to be too hard to camp and also transport ourselves to our tracks if we instead went to the Reward/McKittrick area as Dunham eventually did. Our respective tracks at that spot were not as easy to travel between as they were at Carrizo Plain. In fact, I'm a little surprised that the Central Valley group didn't all decide to use Carrizo Plain, as the (empty) "residential" dirt roads crossing north of Soda Lake would be ideal for unmolested unmanned stations and perfect viewing (and 6 F cooler drier temperatures too).

Our Procedure, Detailed...

Kirk met me at my place in Santa Cruz Sunday afternoon July 28 at 4:30pm. We transferred the Cabrillo College "Unit 2" occultation rig into my RAV4, along with my "Unit 1" gear and some camping gear, filling up just about every cubic foot of space, but still with room for rear view mirror sight. I'd originally planned to bring a Coleman stove and cook dinner and breakfast, but we didn't have room for the stove, and instead stopped at Subway in King City for foot-long sub's for dinner on the road. We took Hwy 46 to Shandon, then down Shell Creek Rd to Hwy 58, to Carrizo Plain, to "7 mile Rd" at the far east end of the Plain, and then down to the empty dirt "residential" roads maze that still exists all these years after the economic dust-bowl end to developers' plans for making this a real town. ArrowBear Trail took us up a short hill and a great overlook where elevation-corrected track A22 crossed. This would be our camping location and Kirk's observing site. We arrived about 10:15pm. It typically takes me an hour from arrival at a sight to comfortably be recording, but we of course wanted more for this important event. Also, our plan was to see if Kirk's "Unit 2" Celestron 8SE scope would behave better than it did for the Hopmann occultation the night before, for Kirk. We also really needed to insure a bit of sleep before tackling the occultation. Kirk lost much sleep Sunday morning attempting the occultation by asteroid Hopmann from Bonny Doon north of Santa Cruz at 2:30am. I was even more sleep-deprived, having done an "S24O (short 24hr overnighter) bikepacking trip with visitor Kent who was in town briefly this past week. We mtn biked into Nicene Marks State Park for a delightful guerrilla camp on the upper Aptos Creek Friday night/Saturday morning. Then Sunday morning I competed in the "Wharf to Wharf 10K" race, and re-ran the course backwards to get back home. 14 miles total. After returning from the race, I showered and packed up for the Phaethon trip and waited for Kirk's arrival. We set up Kirk's Celestron 8SE, (on long-term loan from the Cabrillo Astronomy Department) and he used Polaris and Altair as auto-2 star alignment stars. The alignment came back successful, and the scope then went accurately to various objects (the target hadn't risen yet), and we were reassured all was working well. We don't know why the scope was failing to then "GoTo" for the Hopmann event the night before. I believe that there are combinations of 2-stars which the scope can't do the mathematical "solution" for, for some reason. It may have to do with the scope's assumption you are powering up with the scope aimed true north on the horizon before doing any alignment. However, just to be sure things were OK, I advised we keep the scope powered up and tracking even though the event was still over 4 hours away. We had internal AA batteries in the scope and tested that the scope would stay on if you unplugged the external 12VDC barrel plug and then re-plugged it in. We did so, successfully, with now the Orion Dynamo lithium ion power source, and it stayed on and powered right through the occultation 5 hours later. We also verified his Watec 910hx worked as required. One glitch was that somehow his ZR45mc camcorder battery was dead, though Kirk was confident he turned the unit off after the Hopmann event the previous night. No worries - we had plenty of spare camcorder Li-Ion batteries. Satisfied all was in working order, we took a few pictures of the dark skies Milky Way over Carrizo Plain with our camp set up in the dark. Then Kirk snoozed in his lounge chair, while I set up my new Big Agnes Copper Spur UL1 "fast-pack" trail runner's tent, which got its maiden use on the bike-packing trip 2 nights before. I love this new tent! It has a mesh upper so I could watch the stars and keep out the bugs. We set our iPhone alarms to 2:30am, got up, re-verified Kirk's equipment was all "go", then I drove back down the hill to 7 Mile Rd, up to Hwy 58 and to my track A31 a few hundred yards from the entrance to the California Valley Solar Farms. I had an hour after arrival, but was set up and on target with all systems go with still 43 minutes to spare. I took some documentary pix with my Nikon D7000, and waited. During that hour, not a single car drove by on Hwy 58. I love the solitude of Carrizo Plain.

I told Kirk to begin taping 2 minutes before the event, as it was at least possible there may be some occultations from possible debris from Phaethon, being as this is not just another asteroid, but likely a very old comet that is encrusted with its own dirt crust - it's the source of the strongest meteor shower of the year, the Geminids of December. Clearly it's been shedding material. I started my recording 2 minutes ahead also, but with 1 minute to go, a very light touch on the video RCA cable caused the LCD screen to go blank. I pushed together quickly and the video signal returned. As I saw nothing remarkable during that first minute from 11:18 to 11:19, I began the footage for reductions when the signal returned. Very strange. there was not an apparent poor connection. In fact, I tested after the event and I had to pull the RCA jacks fully apart where it connect to the stub RCA from the Watec, before losing signal. Neither was the connection RCA to the Watec body poor, so far as I could tell. I'm mystified why that loss of video happened. I'll have to test on that later. There was a similar loss briefly during the pre-recording, also solved by pushing the RCA jacks firmly together (although there was no more room to push them actually into firmer contact). I've never seen that before. After the video signal returned, still with 1 minute before the event, I stood back and didn't touch anything, and got a successful recording. The star did a complete wink out at the predicted time, which I guessed and reported later as 0.3 sec. But on playback it was actually more like 0.49 sec which would be a near central occultation. The D and R both looked to have taken a bit more than the 1/30 sec between recorded frames. I recorded at the fastest setting, which was 2x fields = 1 frame.

Note: I advised Kirk to defocus the target star slightly to avoid saturation of the pixels, which he did. However, in my own record, I forgot to do this. While watching the video, one can see all of the fainter stars undergoing significant twinkling, while the target star remained remarkably constant, indicating saturation was probably an issue.

LiMovie photometry of RN's tape: I used photometry circles of 5/6/25 for the target, for tracking, and for comparison. Unfortunate that the best available comparison star is so much dimmer than the target.. I used aperture photometry, not PSF photometry. I used 'drift' setting on all 3 stars until just 2 seconds before the event, when I switched to 'linked' tracking so the target circle wouldn't get lost during the occultation. Then right afterward, changed it back to 'drift' for all 3 stars to complete the photometry record. The linked tracking looked very solid during the few seconds it was engaged.

Pyote Analysis

Choices: smoothed secondary 101 pts min/max pts to search = (10,20)

Pyote log file

D: 11:20:13.8904 UT (.0021, .0053, .0096) 1-sigma, 2-sigma, 3-sigma error limits
R: 11:20: 14.3772 UT (.0021, .0053, .0096) 1-sigma, 2-sigma, 3-sigma error limits

Duration = 0.4868 sec. S/N = 8.84

Track A31 Nolthenius IOTA report csv

Track A22 Kirk Bender IOTA report csv

Reductions for the team are proceeding. Here's some of the results. It turns out we have 5 positive chords from the IOTA team, and 2 more from the Southwest Research Institute team. The two closest observers to the observed centerline were Ye and Nolthenius. The actual centerline turned out to be quite close to the predicted line from SwRI (but there were several other differing predictions within about 20 miles of each other. After the close fly-by of Phaethon to Earth a few years ago, with radar observations, there was uncertainty about the diameter ranging from 4km to 6km. We can see that the observed profile is well fit by an ellipse, so that the projected diameter will indeed depend on aspect ratio.

Photos

Kirks Flickr page

David Dunham lead the work to assemble a post paper for the Tucson conference Asteroid Science in the Age of Hayabusa 2 and OSIRIS-REx  on the 2019 Phaethon occultations, highlighted by this one, with the paper titled "(3200) Phaethon - First Successful Occultations by a Small Near-Earth Object"

My full original .avi file (734 Meg)

My 23 sec .mp4 compressed clip centered on the occultation and immediate comments (trim and compression done in VLC. Different computers seem to see either the entire clip, or miss the first 10 seconds of the clip. Not sure why.)

My PowerPoint Presention designed for the American Astronomical Society - Div Planetary Sciences meeting Oct 2020 and PDF version

David Dunham's PowerPoint for the Press at the AAS DPS meeting Oct 2020