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).
_ _ _


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 
_ _ _


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.
_ _ _


(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.
_ _ _


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
_ _ _


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.
_ _ _


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.
_ _ _ _ _ _


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)

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
_ _ _ _ _ _


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.


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:  dunham@starpower.net
cell phone:  301-526-5590