Occultations by (99942) Apophis during early 2021 - Updated 2021 March 6
Apophis is small, but important as it will pass very close to the Earth in 2029 and 2036, with a very small chance of impact in 2068
Two occultations of bight stars by Apophis will occur across N. America on February 22 and March 7, UT
Goldstone radar observations were obtained in early March that have now allowed an accurate path update for the March 7th event. It is JPL orbit solution #204, using radar data from Mar. 3, 4, and 5. The weather forecast maps have been updated below, as of Mar. 6, 1pm EST.
The first predictions of these Apophis occultations were by the Unistellar occultation team and Observatoire de Paris. 2021 FEBRUARY 21-22 This was the brighter event, of a 7.0-mag. star in Hydra. No observations of the occultation were obtained, only a few miss observations; you can read about the event, and the attempted observations, here. _ _ _ 2021 MARCH 6-7 The star is 8.4-mag. NY Hydrae = SAO 136779 = HIP 45887, spectal type G5V, at J2000 RA 9h 21m 22.7s, Dec -6° 40' 18". It varies by less than 0.1 mag. with a period of 1.59 days. The current path errors are similar to those for the Feb. 22 occultation described above, but we hope to reduce them to less than 5 path-widths a day or two before the occultation, after planned radar observations are obtained with the Deep Space Network's large Goldstone antenna. The path passes over the Rocky Mountains from Canada to Wyoming, northwest of Ft. Collins and over Greeley, Colo.; over southern Oklahoma; and crosses the Feb. 22nd path southwest of New Orleans, near the Cocodrie Heliport. The time is around 6:00 UT of Mar. 7 UT. Roger Venable's call for an IOTA campaign to observe this challenging event is here. To see the path in detail, you should download this file by John Irwin and use it with Google Earth. The green lines are numbered from south to north, -21 to (+)21, skipping 0 [John's yellow central line is between lines -1 and (+)1]. Ignore his orange lines, that mark the limits and his limits, in case of a 1-sigma path shift. The SwRI Google Earth file, generated by Marc Buie, is very similar; we'll use it for our deployment. The SwRI light gray central line is at -5.5 in John Irwin's network. Dark blue lines mark the predicted limits. The 72 parallel lines are designated A01 to A72, from south to north; Magenta tracks are within the 1-sigma zone (A25 - A50; A37.5 is at center, while John Irwin's center is at A43.5); green tracks are between 1 and 2 sigma (A12 - A24 on the south and A51 A62 for the north); and purple tracks are greater than 2 sigma (A01 to A11 in the south and A63 - A72 in the north). This pattern doesn't quite cover 3 sigma but it's close. We don't have 72 stations; we'll cover as many tracks as far from center as we can, with the still TBD number (about 40) of stations that we believe we can deploy. The track assignments will be in Excel files that I will email to observers. In Oakdale, many of the stations will be at the parish airfield, but others will be either along Hwy 165 or other paved roads at places we are scouting. For John Irwin's path, if you zoom in on it path, black circled dota appear on the central line, positioned at 10s intervals; if you click on them, the event Universal Time, and the star's altitude and azimuth at the time, will appear. At Oakdale, La., the event time is 5:56:00 UT with alt. 47.5, az. 211.5. The pathes interactively takes into account the topography, shifting south with higher elevations above sea level. This is not the case with Josselin Desmar's Google Earth file, which some of you may have, so you should use either SwRI's, preferred, or John Irwin's instead. Since height above sea level affects this narrow path greatly, we can't use Occult Watcher for planning coverage, since it does not take this into account. Each of the parallel lines are spaced 107m apart, and shifted individually to take into account elevation above sea level. This uses the current JPL orbit #204 solution that DOES include the new radar observations. We are unlucky with this event since the long axis of Apophis will be approximately pointed at Earth, presenting a minimum roughly circular cross-section that has half the size of the mean diameter. The cross-section was provided to Marc Buie by Marina Brozovic. Steve Preston's page for the occultation, including his usual finder charts of different scales, is here, and the path is now available via Occult Watcher and the IOTA feed. I created some finder charts for some tests we recently did in Arizona that are below. So they are for Fountain Hills, AZ, not for any point in the predicted path, but they will not be much different for locations in the path: Naked eye chart, to mag. 6.0 Finder scope chart, to mag. 11.0 Detailed chart, to mag. 12.5 More detailed chart, to mag. 13.0 They show the FOV for an 80mm f/6 refractor. There is also a 5-deg. diameter circle, a typical finder scope FOV, centered on the target. The target star is in the center of each view, with the local vertical direction up, as seen from n. New Mexico; the view will be rather similar for other locations along the path across N. America. The target is about 2.5 degrees northwest of Alphard (alpha Hydrae, a bright 2nd-mag. star). I also have several pre-point charts prepared for Oakdale, La. that you can get from this .zip file. For the Guide charts in it: 3 chart - naked eye 4 charts - for finder scopes 5 charts - for midi scopes (80 mm) 6 charts - telescopic (maxi, 120 mm), only for start & end hours; more can be generated on request, if I have time. IOTA is planning a large mobile effort for this event, in western Louisiana. Oakdale has been selected; most of the planned lines we want to cover cross the Allen Parish Airport, which has offered their support. If the weather allows, observation will also be attempted in Colorado. The current NWS cloudcover forecast is good for there, and now Oklahoma, too. Joan and I have reserved a room at the Best Western Oakdale Inn, 2030 Hwy 165 S, Oakdale, Louisiana, phone 318-335-3155 (but to contact me, it's always better to use my cell phone # given at the bottom of this page). Other IOTA observers are here now, also. WE WILL NEED LOCAL HELP TO PROTECT OUR TELESCOPES. For this event, we need to position our telescopes only about 100 yards apart in order to detect the occultation, so we will need to set up in specific places along relatively open rural roads with narrow shoulders where there might not be adequate vegetation to hide our equipment. Local volunteers are sought to protect our equipment from theft and (maybe using dark towels) from passing car lights that can ruin our observations, if they pass at the critical time, which will be around 11:56pm Sat. evening, Mar. 6 (5:56 of Mar. 7, U.T.), in the area. If you plan to try to observe this occultation, and/or are interested in participating in the IOTA expedition, please contact me; see the bottom of this page. John Moore wrote: David Dunham/Roger V. - anyone else headed to Oakdale, LA? I just spoke to Joel Johnson who owns and lives on the Allen Parish Airport that is just south of Oakdale off 165 and directly under the current path (I think) that I just downloaded from John John Irwin -click here for your chart showing the lines over the airport. Joel has graciously agreed to allow us to set up whatever we need between the taxiway and the runway and other sites on the airport property. It appears that the airport property could possibly accommodate about 16 chords per the map I just downloaded. I would like to use this site for my 6 planned stations but there are a lot of additional chords for someone else to occupy. I told Joel Johnson that we would call prior to driving onto the property. The Allen Parrish Airport number (rolls to his cell) is 318-215-0090. See ya in LA! PS Joel is now helping us with local logistics. The area is wet and muddy from last night's rain; we can't use dirt (now muddy) roads. We will seek resident's permission to set up our scopes in or near their driveways. _ _ _ And Jon Giorgini wrote about the radar update expected on March 5: [this has been superceded with the successful orbit #204 update on Mar. 5] As a heads up for the occultation track, there will probably be an Apophis orbit update from radar in the coming UTC morning of March 5. Summation of two days of data produced a probable (5-sigma) detection in delay data. It should be possible to confirm that with an additional measurement during the next (longer) track starting in several hours. Unless those measurements disagree, the orbit solution will be updated in Horizons and I'll send a confirmation e-mail. ------------------------------------------------------------------------------- Jon Giorgini | Navigation & Mission Design Section Senior Analyst | Solar System Dynamics Group Jon.D.Giorgini@jpl.nasa.gov | Jet Propulsion Laboratory ------------------------------------------------------------------------------- An Arizona "dry run" for the March 6/7 Apophis event On the morning of Feb. 21, there was an occultation of a 10.5-mag. star in Sagittarius by the 14-km asteroid (1224) Fantasia at 5:30 am MST that might have been visible from the northern suburbs of Phoenix, but more likely from areas farther north and east, where the occultation was successfully recorded from two remote pre-pointed stationary telescopes. Due to the asteroid's relatively small size and fast motion, the occultation lasted only 0.4s. The orbit was recently updated by Davide Farnocchia at JPL with Gaia data, and Steve Preston followed his update, increasing the event's rank from 17 to 63. That's likely about what we'll have for Apophis on March 6/7, after the radar update. So we planned to deploy 5 stations for this event, and others also observed from the northeastern part of the greater Phoenix area, to cover the 1-sigma uncertainty zone, to check out our equipment that we'll also use on March 6/7. Observers in the northern and eastern parts of the Phoenix region were encouraged to try this event, to cover the possibility of a 2-sigma south shift, but the actual shift was only about 1/3rd path-width to the south. Steve Preston's Web page for the event was here, including his usual finder charts of different scales to locate the star. If you don't have Occult Watcher, an interactive Google map of the path is here. The star was at J2000 RA 17h 48m 08.6s, Dec -24 deg. 47' 03", northwest of the "Teapot" and about 6 deg. east of 3rd-mag. theta Ophiuchi, which served as a pre-point opportunity at about 5:04am MST. _ _ _ OTHER NEAR EARTH ASTEROID OCCULTATIONS A link to predictions for some fainter occultations in January, now past, is here. It includes some predictions for occultations by Sisyphus, a relatively large NEA, that will occur in April, and during 2022. There is another occultation by Apophis, of a 9.3-mag. star, on March 11, visible fom Europe (n. France, s. Germany), Israel, Jordan, and Saudi Arabia. Steve Preston's prediction page for the event is here. More will be added here later, after the March 7th occultation. Occultations by the 6-km asteroid (3200) Phaethon were successfully observed in July, September, and October, 2019, and in October 2020, enabling a substantial improvement of Phaethon's orbit and a determination of its small non-gravitational acceleration (apparently due to shedding of Geminids meteroids from its surface caused by the extreme thermal stress of its close perihelion passes) to an accuracy of 10 sigma, three times better than previous determinations. Here is a press release about the Phaethon occultations, the first observed occultations for a relatively small near-Earth object (NEO). Links to presentation and other files giving much more information about the Phaethon occultations, and results from observations of them, are given on this Web page. David Dunham, 2021 March 6 e-mail: dunham@starpower.net; cell phone: 301-526-5590