2023 Occultations by Near-Earth Asteroids - Updated 2023 February 23
Occultations by Didymos, target of NASA's 1st planetary protection DART Mission, had the highest priority in January
As Didymos recedes and its events more difficult, a new NEA target emerges
The focus now is on (98943) 2001 CC21, a flyby target of Hayabusa2, with favorable occultations during the 1st and last quarter of 2023
These occultations are described and illustrated (with maps and tables) in this .pdf document. IOTA had campaigns for some of the early 2023 Didymos events. There are hot links to external Web pages, but links to internal files, especially the .xml input files used by the Occult program, are given below. And see at the end, links to papers and presentations about NEA occultations that give important background information. Especially for those less familiar with occultations, some good links are in the short "Basic Information" section after the Goal paragraph below. See farther down this page for maps and other UPDATED information about the good bright occultations by NEAs during the rest of February and later months. Results of the Didymos/Dimorphos occultations are given in the .pdf document, and in this Web page of past Didymos events. The goals for Didymos and Dimorphos, and other information about their occultations, a Basic Information for new observers, information about Fresnel diffraction effects on light curves of occultations by small NEAs, description of the Google Earth files set up to follow terrain with parallel "fence" lines to coordinate site selection to optimize coverage for NEA occultations (since this currently can not be done with Occult Watcher), and the links for Occult input data for generating your own local predictions, are all given after the section below on occultations by (98943) 2001 CC21. _ _ _ THE GOAL FOR (98943) 2001 CC21 This estimated 600m NEA is a flyby target of the Hayabusa2 extended mission (the flyby will occur in July 2026). The Japanese space agency JAXA is interested in characterizing 2001 CC21 that is currently occulting many stars. The goal now is to obtain a FIRST detection of the asteroid via an occultation, in a situation like we were with Didymos before radar and DART tracking refined its orbit last September. The current 1-sigma uncertainty is over 5 path-widths, with typical event rank of 8 on Occult Watcher. We are trying to cover the sky plane around the current best orbit (Now solution #206 from the JPL Horizons Web site) in a semi-organized fashion, but the changing direction of the object's motion complicates this. Four large expeditions have already been mounted in Japan this year; 2 were clouded out and the others had 10 and 12 stations that all had no occultation. 3 more campaigns are planned in Japan in March, with up to 100 observers for an event on Mar. 26 involving 5.1-mag. HIP 103668 = 6 Persei; see their Web site at http://hal-astro-lab.com/asteroid/2303_2001CC21_index.html (Google translate does a pretty good job translating this to English). Once anyone records an occultation by the object, the orbit will be determined well enough that subsequent efforts will be successful with only a few observers. For now, we need to cover the predicted paths as best we can out to 12 or more path-widths (to 2-sigma or better). _ _ _ 2001 CC21 OPPORTUNITIES IN NORTH AMERICA Currently, 2001 CC21 is only about 0.13 AU from Earth and passing through northern Milky Way fields, producing many occultations. We do not have any very bright events like the 6 Persei event in southern Japan, but we do have many readily-observable events of stars as bright as 9th mag. However, the events are decreasing as 2001 CC21 moves both farther from Earth, away from the Milky Way, and towards the Sun in our sky, so try to take advantage of the opportunities we have during the rest of February, and during March. During the northern summer, 2001 CC21 will be too close to the Sun to observe, but we will have another season of occultations during the last quarter of 2023, but by then, the distance will be about 3 times greater than it is now. Pages 7 and 8 of the .pdf document have a map and table of the better 2001 CC21 events in North America during early 2023. For the Occult Watcher Cloud (OW Cloud) links below, the default now uses JPL#207 and the 2nd line use JPL#205, but the errors are 8.65 and 5.25 p.w., respectively, on Feb. 24. OW Cloud is good for its zoomable Aladin star charts and for basic information, but most of that can be obtained from the header of the Occult map that you see by clicking on the date in the events section below. For the Google Earth (GE) files, gener- ated with Occult (see the 2nd paragraph of the "Google Earth files for NEA occultations" below), I have Used 400m for the sky plane fence spacing (that will project to a larger distance on the ground) to try to get 1 chord, making it very unlikely that the object could slip between them. For some events with many stations, an interval of 300m would be better, to give a high probability of getting 2 positive chords for confirmation. Although the OW Cloud predictions all seem to use JPL#205, the GE files are calculated with JPL#207, the latest orbit on Horizons now. Be sure to use ONLY GE and the provided files, to select observing sites; let me know if you might try an event and I can suggest lines to use, to optimize the coverage; if there is not time to get a line assignment from me, just select a line not far from the center and put that station on OW, so others know where you will be. If you are at a fixed site, let me know which fence line it is closest to, and declare it on OW. We are still figuring out a good way to use the past negative observa- tions, to select the best fence lines to "cover new ground" to avoid duplica- ting past miss observations. _ _ _ FEB. 24 This good 10.2-mag. event occurs around 9:32 UT, crossing New Mex. east of Las Cruces and west of Albuqueque, and crosses western Montana and east of Calgary; the path is also over Acapulco, s. Mexico, but the alt. is only 8 deg. there. OW Cloud link Google Earth file Unfortunately, mostly cloudy skies are forecast over most of the path. We were considering trying for the mainly clear area around Albuquerque, but to get there from our home near Phoenix, we would need to travel on some 2-lane roads with hazardous winter weather conditions, and some of those roads may be closed as a result. It looks like they are open, but we decided to spend our time posting all of this as being more important than a time- consuming personal effort to observe this event. _ _ _ FEB. 25 This 12.2-mag. event is fainter than the 12.0-mag. limit for the main 2001 CC21 map and table in the .pdf document. It will occur around 5:39 UT across southern Arizona, and over the San Francisco Bay area. OW Cloud link Google Earth file Unfortunately, very cloudy skies are forecast over virtually all of the path across the southwestern USA. _ _ _ FEB. 26, 6h UT The path for this 11.8-mag. event passes just east of Dallas, then across Oklahoma, n.e. of Ft. Collins, Colo.; and on to British Columbia. OW Cloud link Google Earth file This far in advance, this cloud cover map is good enough for all 3 of the Feb. 26th occultations that all occur within an hour of 7h UT (= 1am CST, as shown on the GFS cloud cover forecast maps). Colorado south and east of Denver is most likely to have clear skies; unfortunately, the path farther south, and over B.C., looks like it will be very cloudy. THERE IS ANOTHER OCCULTATION ON A SIMILAR PATH 2 HOURS LATER; SEE the "FEB. 26, 8h UT" section below. _ _ _ FEB. 26, 7h UT This is one of the best events, with the star mag. 10.1. The path passes over Boston and near Sherbrook, Quebec. OW Cloud link Google Earth file The GFS cloud forecast is poor, but the ECMWF model shows fewer clouds over southern New England; see the link to it in the 1st Feb. 26 event (6h UT) above. _ _ _ FEB. 26, 8h UT This 12.2-mag. event is fainter than the 12.0-mag. limit for the main 2001 CC21 map and table in the .pdf document. The path passes over s. Texas, then over Lubbock and south of Denver, and east of Calgary. OW Cloud link Google Earth file SEE the FEB. 26, 6h UT section above for the earlier occultation in the same general area. The two paths intersect 30 miles w. of Casper, WY at 6:04:58 UT for the first event and at 8:14:10 UT for this one. Trying to observe in that area would be useful for each station to get two different chances to catch an occultation; now, only a little more cloudiness is expected in the intersection area than south of Denver. See the link to the cloud cover forecast in the 1st Feb. 26 event (6h UT) above. County Rd 201 (Poison Spider Rd) crosses both fences 8 miles north of the intersection point, but it's a dirt road. A better option would be around Alcova, WY, where both paths are crossed by paved County Rd 220, 14 miles south of the intersection point. _ _ _ 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 late 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 may hinder 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. _ _ _ BASIC INFORMATION ON HOW TO RECORD OCCULTATIONS For those less familiar with recording occultations, you can learn about the main techniques, equipment, and software currently used in a good primer by George Viscome. Some more detailed information can be found in the observing tab of IOTA's main Web site. WEATHER FORECASTS Usually, we will provide little, if any, cloud cover forecast information on the event Web pages, like we did for some previous events. Mobile observers are encouraged to get their own cloud cover forecasts for their target areas, using Web sites and advice that I give on this cloud-cover forecast Web page. _ _ _ GOOGLE EARTH FILES FOR NEA 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. In Dec. 2022, Dave Herald 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 Didymos events. However, they are fine for NEAs with no known moons, such as for (98943) 2001 CC21. _ _ _ DIDYMOS OCCULTATIONS DURING THE REST OF 2023 See the map and table of Didymos occultations on page 5 of the .pdf document. The paths of the two occultations in March do not cross the USA, but only parts of s.e. Canada and Mexico with no active mobile occultation observers. The two events in April cross the central and southeastern USA, but they are faint (14th mag.) and have durations less than 0.07s (2 NTSC video frames at most); we can supply Google Earth or other resources for those events upon request, if observations are planned. 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. In late March, there will be two bright Didymos occultations in Europe. On March 20 at 18h UT, Didymos will occult a 10.0-mag. star across France, n. Italy, n.e. Greece, and w. Turkey. On March 26 at 22h UT, Didymos will occult 7.3-mag. ZC 1196 = SAO 79816 = HIP 38832, spectral type K1III, in a path over central Russia and Kazakhstan. _ _ _ LIMITING DISTANCE WHEN FRESNEL DIFFRACTION SMEARING IS SIGNIFICANT Roger Venable gave a good presentation on factors that affect NEA occultation observations at the 2022 IOTA meeting; you can get his presentation by clicking on "The Appearance of Light Curves of NEA Events", the 10th presentation listed on the meeting Web page. In his talk, Roger gives the formula for the Fresnel Length (FL), the characteristic length for the Fresnel pattern of light produced by a knife edge: FL = sqrt(lambda x distance/2); when using it, you must be careful to give all quantities in meters. For lambda, the wavelength of light, use 600 nanometers = 0.0000006 meter and for the distance, multiply the distance in astronomical units (A.U.) by 1 A.U. = 149597870000 m. As Roger explains, when more detailed calculations are performed, the light curves for occultations by small asteroids, assuming they are spherical (or rectangular) can be calculated. A good way to characterize the light curves is with a dimensionless factor "rho" = Rast / FL, where Rast is the asteroid's radius in meters. This figure shows some light curves as a function of rho and asteroid shape. It is evident that when the distance to the asteroid is such that rho = 0.88 or smaller, the Fresnel diffraction effects are severe enough for plausible shapes that we are likely to encounter, to cause a short occultation to be missed in the case of a noisy recording. We call this effect "Fresnel diffraction smearing"; Roger calls it "diffracted out". Roger calculated the rho value for several observed occultations by Apophis in this table. All the rho values are good except for the last two, which are the most distant events. Their observation was inconclusive due to strong scintillation at the low altitude above the horizon, less than 11 deg., for both events. If you record an occultation of a bright star under good conditions with a high signal-to-noise ratio, one could probably detect an occultation with rho values of 0.88 or less. But nature is rarely that cooperative, so 0.88 is a good working value for finding when Fresnel diffraction smearing becomes a problem. Doing some algebra with the above equations shows that the distance when rho = rhoLim (which we will set to 0.88, but another value could be used) for a given asteroid radius (Rast) is given by the formula distance in A.U. = 2 x (Rast/rhoLim)^2/(lambda*AUinMeters) where AU in meters is 149597870000. The distance for some objects of interest are in the short table below: Asteroid Radius, m Distance, AU (when rho = 0.88) Apophis 169 0.82 Didymos 400 4.60 Dimorphos 80 0.18 2001 CC21 300 2.59 Since the aphelion of Didymos is 2.28 AU in a low-inclination orbit, it can never get more than about 3.3 AU from Earth, so its minimum rho would be 1.04, but that would rarely happen; it would be rare for rho for Didymos to be less than 2, so most of the time, diffraction smearing would not be an issue for Didymos. But Dimorphos, on the other hand, is already diffracted out, as it is already more than half an A.U. from Earth. 2001 CC21 is similar to Didymos - OK most of the time, but at its maximum distance from Earth, rho = 0.93. _ _ _ SKY AND TELESCOPE ARTICLE ABOUT DIDYMOS OCCULTATIONS Damya Souami wrote a good article for Sky and Telescope entitled "How Citizen Scientists are Monitoring the DART Impact's Aftereffects" describing the worldwide efforts to observe occultations by Didymos and Dimorphos, and the prospects for future occultation observations. As noted above, the chances for Dimorphos occultations during this year are past, but there will be more opportunities for occultations by both objects in the second half of 2024. _ _ _ OCCULT INPUT .XML FILES FOR 2023 IOTAs Occult4 program is a free comprehensive occultation prediction program for Windows systems. You can use it with the input files below to generate your own predictions for your observatory or region, as described in this .pdf document. It describes a prediction input file for planetary and asteroidal files called All2023.xml. You can use that file to generate local predictions for many mainly main-belt occultations, but you can replace it with the other files listed below to generate predictions for more occultations, mainly of fainter stars than shown on the maps, or for other parts of the world: When you open these files with your Web browser, you should get a display that looks like this. The larger files may take a couple of minutes to show this appearance, showing a messed-up display before then. Once the proper view appears, right-click anywhere on the display, then "Save as" to a directory on your computer; the best directory is the \Asteroid\ subdirectory in your Occult\ directory. For the 23 NEA events mapped and tabulated in the main .pdf document But these all use orbits from August 2022; for Didymos, better orbits are now available. Worldwide (98943) 2001 CC21 events to mag. 14 for the rest of 2023 with JPL#205. The current JPL orbit is #207, which runs 2.0 km northeast of JPL#205 on the sky plane (that is 3.3 path-widths, about 0.4 sigma). Worldwide Didymos events to mag. 14 for 2023 (but using Aug. 2022 orbit). A newer version using a later JPL orbit is available; see the next line. Worldwide Didymos events to mag. 14 for 2023 Jan. 15 - Dec. 31, using the JPL#201 orbit, which is 380m north of the current orbit, JPL#203, which was confirmed to be within about 100m of the actual orbit by observations of the 9th-mag. occultation recorded from several stations in s.w. Europe on 2003 Jan. 21/22 - see past Didymos events for details. North American Didymos events to mag. 14 for 2023 Jan. 15 - Dec. 31, using the JPL#201 orbit. Worldwide 2023 events for our selected NEAs (except Phaethon) to Mag. 14 Worldwide 2023 events for Phaethon to mag. 14 _ _ _ _ _ _ BACKGROUND INFORMATION ABOUT NEA OCCULTATIONS Comprehensive PDC2021 paper on NEA occultations, .pdf Didymos occultations presentation for SBAG mtg., 2022 Jan., .pdf by D. Dunham SBAG27, 2022 June 7-9 Didymos occultations presentation for SBAG mtg., 2022 June, .pdf by D. Dunham ACROSS presentation for SBAG mtg., 2022 June, .pdf by Damya Souami _ _ _ _ _ _ ACROSS (Asteroid Collaborative Research via Occultation Systematic Survey) On 2022 March 1, the ACROSS team announced their project and Web site: It is our pleasure to introduce a new project for occultation campaigns: ACROSS (Asteroid Collaborative Research via Occultation Systematic Survey) https://lagrange.oca.eu/fr/home-across This campaign is led by the Nice Observatory (OCA) and the Aristotle University of Thessaloniki (AUTh), and funded by the European Space Agency (ESA). The goal is to support observations of occultations by NEAs, specifically the asteroid system composed by (65803) Didymos and its satellite Dimorphos. The focus is to obtain positive chords before and right after the impact date of the DART mission satellite, expected to impact Dimorphos on September 26th, the main objective being improving our knowledge on the orbit of Didymos such that we can track the change caused by the impact. A secondary group of objects being campaigned for is a potential set of fly-by targets of the Hera mission that will follow-up DART. Other promising NEAs are in our "training" list. We share through our web site two different sets of events: involving bright stars (V < 10), and a general prediction file (XML by WinOccult) for large telescopes (V<16), that can be checked for specific regions. You may also find there details on our project, news and results of ongoing campaigns and tutorials on how to attempt observations of NEA events. For priority events, updates are provided through the Occult Watcher Cloud (OWC) web site, under the "ACROSS" campaign tag. Specific campaigns will be advertised. It must be pointed out that, due to the nature of NEAs, these are fast events, both because they move fast in the sky, and because a good portion of the NEA database is comprised of small objects. Therefore, only those who can track fast events should consider attempting these events. It is also preferable that you have a mobile station, as these are events with very small shadow paths. The trade-off is that, if positive, they will allow us to greatly improve the orbit of a NEA. Two Didymos events already being worked on are a mag 10 event in Abu Dhabi on September 20th and a mag 13 event in Spain on August 25th, which are addressed in greater detail on our website, and we want to cover as much ground as possible. On behalf of every member part of this mission, we thank you in advance for your interest and your contributions, and we look forward to working with you. Best regards, ACROSS Team Coordinators: Paolo Tanga and Kleomenis Tsiganis Core Team: Damya Souami, Joao Ferreira, Alex Siakas, Lyu Abe, Rodrigo Leiva, Luana Liberto, Pascal Oberti ______ David and Joan Dunham, 2023 January 5; updated Jan. 6, 6pm EST, Jan. 16, 19, and Jan. 28, and Feb. 23 e-mail: dunham@starpower.net cell phone: 301-526-5590