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).
_ _ _


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. 

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. 
_ _ _


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.
_ _ _


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.


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.
_ _ _


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.
_ _ _


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.
_ _ _


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.
_ _ _


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.
_ _ _


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
_ _ _ _ _ _


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)

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,


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