More Information on Occultations by Near-Earth Asteroids - New 2024 January 3
The Asteroids, Comets, Meteors Conference (ACM 2023) in Flagstaff, Arizona brought major asteroid scientists and occultation workers from around the world together in June 2023.
One paper there, about the 20 most dangerous km-sized asteroids during the next 1000 years, gives us important new targets for occultations
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 2024, as described in the .pdf document linked to at the top of this page. Another important asteroid is (98943) 2001 CC21, see below. _ _ _ 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, 2023 JUNE 18-23 This conference, abbreviated ACM 2023, was held in Flagstaff and online in June 2023. 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. 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 _ _ _ 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 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 ______ David and Joan Dunham, 2024 January 3 e-mail: dunham@starpower.net cell phone: 301-526-5590