2023 Occultations by Near-Earth Asteroids - Updated 2023 June 17
Occultations by Didymos, target of NASA's 1st planetary protection DART Mission, had the highest priority in January, but during the rest of 2023, it is too far away with no viable occultations
In 2022 October, a large expedition in Japan observed a significant shift during an occultation by (3200) Phaethon, so 4 occultations in North America are now of great interest, to better understand the evolution of this important object
We are also interested in (98943) 2001 CC21, a flyby target of Hayabusa2, with favorable occultations during the last quarter of 2023
The Asteroids, Comets, Meteors Conference (ACM 2023) in Flagstaff, Arizona will bring major asteroid scientists and occultation workers from around the world together from June 18th to 23rd.
One paper there, about the 20 most dangerous km-sized asteroids during the next 1000 years, gives us important new targets for occultations
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 at the bottom of this page. Also near the bottom are 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. See farther down this page for maps and other UPDATED information about the good bright occultations by NEAs during the rest of 2023. Results of the Didymos/Dimorphos occultations are given in the .pdf document, and in this Web page of past Didymos events. 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 sections below on NEA occultations during the 2nd half of 2023, especially by ( 3200) Phaethon and by the Hyabusa2 target, (98943) 2001 CC21. _ _ _ As described in this paper given at the Planetary Defense Conference in April 2023, Phaethon appears to be veering from its ephemeris following its 2022 May perihelion passage. We want to confirm and/or quantify that with new observations of Phaethon occultations; we have 4 fairly good chances during the rest of 2023, as described in the .pdf document linked to at the top of this page. Another important asteroid is (98943) 2001 CC21, see below. _ _ _ 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. Six large expeditions have already been mounted in Japan this year; 2 were clouded out and the others had 10 and 12 stations that had no occultation, except for an event on Mar. 5 that had one positive that was used to update the orbit. Unfortunately, the Mar. 5 star had a high Gaia RUWE, and a larger-than-expected error, which caused the new orbit to be wrong, with all observers having no occultation for the subsequent events, including 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 The early 2023 events by this asteroid were better than the ones late this year because the distance will be about 3 times greater, around 0.5 AU, so the event durations are shorter. Page 5 of the .pdf document has a map and table of only the brightest 2001 CC21 events in North America during early 2023. There are many more observable events that we will try to document with another update probably in October. In the meantime, you can find events in your area using the late 2023 Occult input file given at the bottom of this page. For the Google Earth (GE) files that we might generat with Occult (see the 2nd paragraph of the "Google Earth files for NEA occultations" below), we will use 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 600m 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. _ _ _ OTHER NEA OCCULTATIONS - FUTURE EMPHASIS ON MOST THREATENING PHAs Although Didymos was included in the searches, none of its events are observable during late 2023 because the solar elongation is less than 15 deg. The searches were made to mag. 14.0, a useful faint cut-off for these very short events when observed with all but the largest telescopes. Besides Didymos and Phaethon, other events considered were 9 of mainly the lower-numbered objects from a new list of asteroids that are really potentially hazardous during the next 1000 years, according to a presentation for next week's Asteroids, Comets, Meteors (ACM) meeting; the abstract for the talk lists 20 objects 1km across or larger whose mean orbit intersection distance (MOID) with Earth will pass through zero during the next 1000 years, and whose current orbit is indeterminant (error in the mean anomaly becomes greater than 180 deg.) within the next few hundred years due to accumulation of the current uncertainty in the mean motion of the object's orbit. So these objects are most important to observe with high-precision observations (occultations and radar) to reduce the current errors in their orbital elements, to enable more accurate propagation of their orbits to see if there might be any possible collisions when the MOID for them becomes very small. The 9 objects we considered are: (4179) Toutatis, (4183) Cuno, (5011) Ptah, (7092) Cadmus, (7482) 1994 PC1, (66391) Moshup, (143404) 2003 BD44, (143651) 2003 QO104, and (314082) Dryope. Starting with the predictions for 2024, we will consider all 20 objects in the list. _ _ _ TANTALUS AND SISYPHUS SUCCESSES (2102) Tantalus and (1866) Sisyphus are relatively large NEAs in orbits highly inclined to the ecliptic, so they would be especially threatening, if their MOID ever goes to zero. Fortunately, they are not in the list of 20 mentioned above, so that will not happen anytime soon. But keeping a close watch on these will improve their orbits to better assess their long-term risk. An occultation of an 8th-mag. star by Tantalus was successfully observed in New Mexico on May 7th, as described in this article in Stardust, publication of the National Capital Astronomers. Similarly, Steve Messner recorded the first occultation of a star by Sisyphus on 2022 Nov. 26, as given in IOTAs North American Events Report Page for 2022. The orbits of these objects will need to be updated with these occultation observations that will result in an order of magnitude reduction of their uncertainties, but in both cases, the occultations were observed from the predicted central lines, showing already that the true errors of the current orbits are much less than the current formal errors, so the current orbits can be used for good predictions, keeping in mind these results. _ _ _ 14th ASTEROIDS, COMETS, METEORS CONFERENCE, JUNE 18-23 This conference, abbreviated ACM 2023, will be held in Flagstaff and online next week. Clicking on "Program and Presenter Information", including the program with links to abstracts, you can see the interesting agenda, including several papers using results from occultation observations. Some of the NEA occultation-related abstracts are linked to below. The virtual registration fees are low enough ($200) that some amateurs may be interested in attending; some amateurs may qualify for the cheaper student rate. In-person registration is open on site starting on June 18; registration for virtual attendance is available to the last day of the conference, June 23. Some abstracts about Near-Earth Asteroids (NEAs) IOTA Asteroidal Occultation Results Apophis and Didymos occultation results by Damya Souami for ACROSS Potentially Hazardous NEAs important for occultations _ _ _ 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. _ _ _ 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. _ _ _ _ _ _ BACKGROUND INFORMATION ABOUT NEA OCCULTATIONS our NEA occultations presentation for ACM 2023 scheduled for June 23rd; the full conference Web site is: 14th Asteroids, Comets, Meteors (ACM) Convention, Flagstaff, AZ, June 18-23 Planetary Defense Conf. (PDC 2023) NEA Occultations paper PDC 2023 NEA Occultations presentation Narration for the PDC 2023 NEA Occultations presentation PDC 2021 NEA Occultations paper more about early Phaethon and about Apophis The May 7th occultation by (2102) Tantalus published in Stardust of the National Capital Astronomers. The presentations at the NASA Small Bodies Assessment Group meetins during 2022 are mostly superseded by the more recent information in the 2023 links above. 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 _ _ _ _ _ _ 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; for predictions, it is close enough to the current JPL#204 orbit but it should be updated for the most precise work. That can be done for individual occultations using Occult4 by right-clicking on a line for the event. Worldwide 2023 events for our selected NEAs (except Phaethon) to Mag. 14 Worldwide 2023 events for Phaethon to mag. 14, but you must ignore any after October 10. For the 9 NEA events in late 2023 mapped and tabulated in the main .pdf document For all of the NEAs considered for the rest of 2023 worldwide, 1662 occultations, see discussion above. Using this, you MUST ignore all Phaethon events that occur after mid October because the osculting initial epoch is in mid June, and Phaethon passes though perihelion in mid October; after that, the actual path diverges strongly from the path predicted by Occult4, due to the program's limitations. ______ OTHER IOTA ASTEROIDAL OCCULTATION PREDICTION WEB SITES Bright Main-Belt Asteroids, has links to the others Trojan Asteroids (Lucy targets are emphasized) Selected Special Main-Belt Asteroids IOTA main asteroidal occultations predictions ______ David and Joan Dunham, 2023 January 5; updated Jan. 6, 6pm EST, Jan. 16, 19, 28, Feb. 23, and June 17 e-mail: firstname.lastname@example.org cell phone: 301-526-5590