Past 2023 Occultations by Didymos and its moon, Dimorphos - New 2023 February 20
Occultations by Didymos, target of NASA's 1st planetary protection DART Mission, had the highest priority in January
Some past Didymos and Dimorphos occultations are described and illustrated (with maps and tables) in this .pdf document. IOTA had campaigns for some of the early 2023 Didymos events. THE GOAL FOR DIDYMOS AND DIMORPHOS On 2022 Oct. 15, the first ccultation by Didymos was observed by Roger Venable in Oklahoma, in spite of hitting a deer on his way to his observation sites. IOTA and the ACROSS project are now trying to secure additional observations. The ACROSS and JPL predictions are currently in close agreement, but more occultations by both Didymos and its battered moon, Dimorphos, are needed to better determine the orbits of both objects, to help ESA'a Hera mission that will launch in Oct. 2024 and rendezvous with the Didymos system in Dec. 2026. On 2022 Oct. 19 and Nov. 12, and 2023 Jan. 21, occultations by both Didymos and Dimorphos were recorded in Florida, southern California, and s.w. Europe, respectively. We are trying to get observations of a few more occultations by Dimorphos; please help us with these efforts. For the events of late January, the motion of Didymos is unusually slow, resulting in occultation durations of almost a second, and with the possibility of recording even Dimorphos occultations of relatively faint stars with integrated video observation. There are no Didymos occultations in North America during February, and by March, the apparent motion of Didymos increases, making the Dimorphos events too short to record with common video systems; also, Didymos will recede from Earth so far that diffraction smearing will also likely prevent observation of their occultations. Success before that allowed a more accurate determination of the Dimorphos post-DART-impact orbit than can be obtained by other means; the asteroid is already too far from Earth to be observed by currently operational radar systems. The errors of the orbit of Dimorphos, determined from rotational light-curve (transit) observations, are growing; the few Dimorphos occultation detections in late December 2022 and January 2023 helped reduce those errors. _ _ _ GOOGLE EARTH FILES FOR DIDYMOS OCCULTATIONS Since the paths for these events are all very narrow, one must travel to the paths with mobile equipment to observe them, rather like grazing occultations of stars by the Moon. And like lunar grazes, it is necessary to adjust the location for elevation above sea level. John Irwin in the UK has helped IOTA with this by supplying special Google Earth files that take elevation into account. Consequently, we can not use Occult Watcher (OW) for these events, since OW does not take elevation into account. Instead, we use the Google Earth (GE) files by J. Irwin that he describes in this .pdf document. Recently, Dave Herald has updated his Occult4 program to generate GE files similar to John Irwins, but Dave uses the same color scheme used in other parts of the program, so unlike Irwin, Heralds central line is green, the limits are dark blue, and the 1-sigma limits are red. The parallel fence lines for observers are light yellow and can be set up with a specified spacing on the sky plane and a specified number from either side of center. But unlike Irwins GE file, the Occult one does not include lines for the occultation by Dimorphos, so Irwins file is preferred. For the late Jan. occultations, we do not yet have GE files from Irwin; I have been able to use Occult to generate them instead, and they are posted for the individual events below. More about some of the past Didymos events is below. For some events, Roger Venable, rjvmd@progressivetel.com, made the line assignments, while I was the coordinator for some other occultations. _ _ _ 2023 JANUARY 1 DIDYMOS OCCULTATION 1h UT, Southwestern USA 1h UT, eastern North America 1h UT, southeastern Canada 1h UT, southwestern Europe 1h UT, n. Africa and Middle East Unfortunately, cloudy skies plagued all of the path from which observers planned to try it. The cloud cover was so extensive in most areas that all observers except for Vadim Nikitin did not leave home to try the occultation. Vadim travelled to south-central Colorado, where he was able to acquire the target star and record it through the time of the occultation. But the observation was through rather thick and variable cloud at the expected event, and his recording is still being analyzed, to see if a real event might be detected in the very noisy recording. Under the circumstances, there was no chance to try for the Dimorphos occultation. _ _ _ ACROSSs Damya Souami noted a faint event, an occultation of a 15.1-mag. star by Didymos around 7:05 UT (2:05am EST) of Tues. morning, Jan 17th, visible from the northeastern USA with zoomable Aladin charts and other event details on Occult Watcher Cloud; for the zoomable Google map there, select the 2nd line (using the JPL#201 orbit) or directly, click here. The path crosses North America from coast to coast, but the current NWS cloud forecast shows that it will be very cloudy along most of the path, with the only predominantly clear area being the Hudson Valley and Conn. The path over the Hudson Valley and Conn. is shown in this map. I think only an 18-in. or larger scope might detect the 1/3rd-sec. occultation of this faint star by Didymos, and larger scopes would be needed for Dimorphos. The Occult input file, using the latest JPL201 orbit, for the event is here. An IR satellite image near the event time showed that it was cloudy along the path in the Hudson Valley, but clear over most of Connecticut, with breaks probably large enough to observe the event from south-central New York and eastern lower Michigan. As far as I know, this event was not observed. _ _ _ 2023 JANUARY 18 DIDYMOS OCCULTATION This was a chance to record an occultation by Dimorphos with video but a large telescope, probably 12-in. or larger, was be needed to record the short Dimorphos occultation of this 13.1-mag. star. The path crossed the southernmost part to Arizona and southern Florida; it also crossed Texas and other south-central US areas, but the IR satellite image near the event time showed that it was clear enough only in southern Arizona and Florida. Occult map, Florida to Houston Occult map, Houston to southern Arizona Path across Florida Zoomed in map, South Palm Beach area Roger Venable, rjvmd@progressivetel.com, coordinated the observations for this event using this Google Earth file from John Irwin, who writes about it here. I added the OWC links and some other information to Rogers Jan. 14th message here. Also, see this explanation of Irwins Google Earth file. Sky Plane diagrams of the Didymos-Dimorphos system for this event by John Irwin: Florida Arizona Norm Carlson observed a miss in Arizona, while Roger Venable had three positives and a miss at his northern station, showing the path shifted about 400m south from the JPL#201 orbit. Dave Herald studied the observations extensively; here is the sky plane plot with his preferred solution. That plot shows Venables northernmost station as line !, Carlson as line 2, the JPL#201 prediction as line 3, and two of Venables positives as lines 4 and 7. The third Venable positive, line 5, had large errors and was discordant, so Herald chose not to use it. _ _ _ 2023 JANUARY 21/22 DIDYMOS OCCULTATION This was our last really good chance to record an occultation by Dimorphos with video with relatively small telescopes but it was hopelessly cloudy across Florida, as this IR satellite imagea shows. It was also too foggy to observe in the Delhi area of India, and many areas of southern Europe were too cloudy for observations, but there were large-enough breaks in the cloud cover over the path across southwestern France and in northwestern Italy to allow observations in those areas. ACROSS coordinated the observations there with a special Web page set up using parallel "fence" lines that took into account elevation above sea level, like the Google Earth files by John Irwin. This message about the coordination by me and Paolo Tanga includes the OW cloud link, that was used for Aladin star charts and other event details, but of course, Paolo Tangas link was used for the path, and for each observer to select a line for observation. This screen view shows the selected lines, the general location, observers and equipment for each, and in this view, the basic results. Occultations by both Didymos and Dimorphos were clearly recorded on the lines 300m, 400m, and 500m south of the JPL#201 predicted central line, vindicating the 400m south shift from the Jan. 18th occultation initially claimed by Roger Venable; that fact was distributed to the Planoccult list for European occultation observers about a day before the event. Clear ccultations by both Didymos and Dimorphos are shown in this light curve by Lionel Rousselot 300m south of center. Just 100m to the north, Leroy Arnaul and others used a QHY 174M camera to record the event at fast cadence, but only Didymos occulted the star at their location; here is their light curve fitted with a new Fresnel diffraction pattern available in the latest version of PyOTE. Occult maps Florida, Jan. 22, 0h UT or 7pm EST s. Europe, Jan. 21, 23h UT s. Europe to India, Jan. 21, 22h-23h UT Steve Chesley supplied sky plane diagrams of Didymos and Dimorphos for Florida and for France. These used a recently-updated orbit of Dimorphos but they are essentially no different than the earlier sky plane diagrams. John Irwin supplied this Google Earth file; it is not completely redundant with Tangas interactive path since Irwins includes the predicted path for Dimorphos that Tanga does not have. This document describes Irwins Google Earth file and its use, while this message is what John said about this event. _ _ _ 2023 JANUARY 23 DIDYMOS OCCULTATION This was mostly a southern Canadian event; in the USA, only Washington state was crossed by the path, over the Seattle area, at 5:05 UT (or Sun., Jan. 22, at 9:05 pm PST). Occult map, Labrador to Lake Winnipeg Occult map, Ontario to s.e. B.C. and n.w. Washington The OW Cloud link gives event information, but for the path, use the JPL#201 path and keep in mind the 400m south shift . In her message of Jan. 13, for this event, Damya Souami copied the OW Cloud link from the Jan. 21/22 event, so it is wrong, going to the earlier event instead. It was mostly cloudy along the path across North America, and as far as I know, nobody observed the event, or travelled to attempt observations with the poor weather forecasts before the event. _ _ _ The JPL#201 orbit was used for each of the next events, so keep in mind the 400m south shift from that orbit, as found from the observations of the Jan. 18 and Jan. 21 occultations. Earlier on Jan. 25, Steve Chesley computed a new Didymos orbit and uploaded it to the JPL Horizons system as #203, using Herald's analysis of the Jan. 18th observations, as well as the other past occultations. But since I had already used JPL#201 to generate the Google Earth files for the next three events, I'm not updating yet, will do that for any events after Jan. 29. Herald found that JPL#203 is 370m south of JPL#201 for the Jan. 28th occultation, in good agreement with the 400m south shift noted above, found from the early estimates of the Jan. 18 and 21 shifts. _ _ _ 2023 JANUARY 26 DIDYMOS OCCULTATION I only found this good 10.6-mag. event on Jan. 15 with a new Occult search using JPL#201, now considered the best orbit for Didymos fitted to the 2022 Oct. – Dec. observations. The #201 orbit shifted by less than a km from the previous 200 orbit, so I do not know why this event was not found during my late Dec. search using #200. For this relatively bright star, the central duration is a nice 0.57s, so a central Dimorphos event should be more than 0.10s. The event altitude is >15 in Wyoming & higher westward. Occult map, Nebraska to British Columbia Here is the OW Cloud link; but for the path, be sure to use the JPL#201 map, but just for preliminary planning since it does not take into account elevation above sea level. Here is the Occult input file for the event. Here is Steve Chesley's sky plane plot of the Didymos system at the occultation time, showing that the Dimorphos shadow passed well north of the actual Didymos path. The IR satellite image showed that it was mostly cloudy along the path across North America, and as far as I know, nobody observed the event, or travelled to attempt observations with the poor weather forecasts before the event. Sorry are posting this after this occultation, but we had sent the information above by email a day before to observers who we thought might try to observe it. _ _ _ 2023 JANUARY 28 DIDYMOS OCCULTATION Here is a last eastern US occultation, although rather faint at mag.. 13.4, so it requires a rather large telescope. However, with a central duration of 0.71s, integration can be used, with a central Dimorphos event about 0.14s so integration might be used to detect it with 10-in. scopes, if sky conditions permit. Unfortunately, the event occurs too soon after sunset to give enough dark time to deploy multiple telescopes. Nevertheless, I will let Roger Venable coordinate observation of this event; contact him at rjvmd@progressivetel.com if you might be interested in trying it. The event has a very high RUWE of 10.5, so the accuracy of the star position may be too poor to really predict this event. We are asking the Gaia team to look into this star; since the EDR3 release, hopefully there are enough new observations that a new solution for the star might be computed that would reduce the RUWE. Occult map, Newfoundland and s.e. Nova Scotia Occult map, Delmarva to s. Alabama Here is the OW Cloud link; but for the path, be sure to use the JPL#201 map, but just for preliminary planning since it does not take into account elevation above sea level. Here is the Occult input file for the event. We have requested a sky-plane plot and Google Earth file for this event. _ _ _ 2023 JANUARY 29 DIDYMOS OCCULTATION Like the Jan. 26th occultation, this 12.20mag. event was only found on Jan. 15. The occultation has a whopping 0.76s central duration, so a Dimorphos occultation could last 4 NTSC frames. The path actually passes over Washington, DC, but with an altitude of only 4 deg. In s.e. Michigan, the altitude is 10 deg., and increases farther west along the path. Here is the OW Cloud link; but for the path, be sure to use the JPL#201 map, but just for preliminary planning since it does not take into account elevation above sea level. Occult map, Nebraska to British Columbia Here is the Occult input file for the event. We have requested a sky-plane plot and Google Earth file for this event, in case there are some observers willing to try it; let me know if you might try it, to coordinate observations. _ _ _ 2023 FEBRUARY 8 DIDYMOS OCCULTATION Damya Souami promoted this faint (15th-mag.) event that might have been observed with large portable telescopes. See the bottom of p. 7 of the .pdf document for an Occult map and more about this event; a map like it shows the path over South America. The path happened to pass over a park with old domes of La Plata Observatory in Argentina, but old equipment and high trees around the domes prevented observation there. Observation from south-central Virginia was considered, but the logistics proved too difficult and the last forecasts showed clouds would threaten any effort there, so no attempt was made for this last relatively long- duration (0.7s) Didymos occultation for North America at least until late 2024. John Irwins Google Earth files were emailed to potential observers in the USA and in South America. _ _ _ The later Didymos occultations will e quite difficult to observe, most being too brief to record reliably with video. If anyone is interested in any of them, we can provide more information. ______ David and Joan Dunham, 2023 Feb. 20 e-mail: dunham@starpower.net cell phone: 301-526-5590