Table of contents

Occultations by asteroids

 (Last updated: 2013 March 15)

1.0 Introduction

Information on observing occultations can be found on the Introduction and Circulars pages on the EAON web site. The EAON site should be considered the prime source of information on all matters relating to occultations. If there are differences between this site and theirs then please assume they are correct.

From time to time during the course of its orbit around the Sun, an asteroid will appear to pass in front of a star or another Solar System body as seen from Earth. Such a passage is referred to as an occultation. As the asteroid passes in front of the star the latter will appear to dim or disappear altogether. The track width of such an occultation is very narrow – approximately the same width as the asteroid eg; no more than 100 kilometres or so. When the track intersects the Earth there will be only very few potential observers situated along it. Every observation whether positive or negative, is important in defining the limits of the track.

Occultations are the only way, apart from spacecraft missions to asteroids and radar observations of nearby objects, to determine the approximate size and shape of those bodies and are, of course, much cheaper.

The International Occultation Timing Association (IOTA) web site can be found here and that of the International Occultation Timing Association/European Section (IOTA/ES) web site can be found here.

2.0 Predictions

Predictions of possible occultations are available from the European Asteroidal Occultation Network (EAON) , Euraster and Steve Preston’s web site. Predictions are necessarily only approximate as the orbits of most asteroids and the positions of most stars are not known to the required level of precision to accurately define occultation tracks well ahead of time. The margin of uncertainty thanks to the Hipparcos astrometry is probably better than +/-50 km so that the probable area of visibility is accurate in general terms. But the uncertainty is now unlikely to be greater than +/-100 km in most cases now. The predicted times will be more accurate and the probable error is of the order of +/-2 minutes.

Astrometry is sometimes requested prior to the event to allow the predicted track and time of occultation to be refined.

Here are the predictions as listed in the current 2013 Handbook of the BAA:

OCCULTATIONS OF STARS BY ASTEROIDS AND DWARF PLANETS

Occultation events for 2013 were selected by Tim Haymes from a world list of events computed and published annually by Edwin Goffin of the VVS (Flemish Astronomical Association).  The BAA is very grateful to Mr Goffin for permission to publish his predictions in the Handbook.

In the table of predictions below:

Time = UT of closest geocentric approach.

Region of Visibility codes (RoV):

1 = North and Central America,

2 = South America,

3 = Europe, North Africa and the Middle East,

4 = South Africa,

5 = Russia,

6 = Pakistan, India, and SE Asia,

7 = Japan, China and Taiwan,

8 = Australia and New Zealand.

Date	2013					Diam		V
month	day	hr	m	Number	Name	km	Star ID	Mag
1	27	3	4	735	Marghanna	74	HIP 50857	7.2
1	21	6	1	679	Pax	51	HIP 51406	7.9
1	10	3	46	500	Selinur	43	PPMX 6161150	9.0
1	14	15	36	120	Lachesis	174	PPMX 6817736	9.7
2	9	12	58	564	Dudu	50	HIP 33133	6.5
2	22	2	13	469	Argentina	126	HIP 25363	6.8
2	13	17	49	392	Wilhelmin	63	HIP 28190	7.9
2	3	21	15	1243	Pamela	70	PPMX 10412652	8.4
2	4	3	25	100	Hekate	89	HIP 48151	8.8
2	26	8	32	336	Lacadiera	69	PPMX 12611813	9.7
2	20	0	26	121	Hermione	209	HIP 80269	9.7
3	3	1	48	729	Watsonia	49	HIP 53417	4.3
3	16	23	44	230	Athamantis	109	PPMX 9435248	7.8
3	24	9	55	375	Ursula	216	HIP 67006	8.3
3	7	9	38	329	Svea	78	HIP 71779	8.4
3	27	16	53	521	Brixia	116	PPMX 5016917	9.0
3	20	18	34	63	Ausonia	103	PPMX 7220607	9.6
3	15	3	26	192	Nausikaa	103	PPMX 7213040	9.7
3	7	13	55	255	Oppavia	57	HIP 52938	9.7
3	17	11	50	206	Hersilia	113	PPMX 8345008	9.8
3	8	2	44	478	Tergeste	79	PPMX 7019118	9.8
4	20	17	30	1	Ceres	952	PPMX 5028406	7.8
4	22	12	25	470	Kilia	26	PPMX 10531857	8.5
4	3	16	46	348	May	83	PPMX 11651748	9.1
4	5	21	60	70	Panopaea	122	PPMX 8385078	9.2
4	15	1	29	480	Hansa	56	PPMX 10665999	9.4
4	22	20	54	859	Bouzaréah	74	PPMX 13819464	9.6
4	13	21	57	225	Henrietta	120	PPMX 9635194	9.6
4	15	5	36	578	Happelia	69	PPMX 10486962	9.6
4	17	21	40	351	Yrsa	40	PPMX 5176564	9.8
5	27	1	14	169	Zelia	34	PPMX 13627531	7.7
5	5	21	22	23	Thalia	108	PPMX 11881636	8.2
5	23	19	52	258	Tyche	65	HIP 72566	8.8
5	30	0	38	153	Hilda	171	HIP 47396	9.0
5	9	17	9	470	Kilia	26	PPMX 9546532	9.5

6	12	9	49	332	Siri	40	HIP 84478	6.4
6	14	20	1	510	Mabella	57	HIP 105800	7.7
6	10	15	31	407	Arachne	95	HIP 83871	8.0
6	23	9	19	403	Cyane	49	HIP 83813	8.8
6	14	18	11	760	Massinga	71	HIP 99148	8.9
6	18	10	35	133067	2003 FB128	140*	HIP 73880	9.4
6	5	7	12	348	May	83	PPMX 11640493	9.4
6	4	22	1	75	Eurydike	56	HIP 50225	9.5
6	21	2	58	1032	Pafuri	55	HIP 91804	9.6
6	29	8	36	479	Caprera	73	PPMX 8966996	9.8
7	26	8	47	576	Emanuela	85	HIP 94645	6.4
7	10	10	31	238	Hypatia	148	HIP 116004	7.4
7	2	18	6	455	Bruchsalia	84	HIP 97523	8.5
7	18	14	32	42	Isis	100	HIP 8098	9.0
7	2	23	6	409	Aspasia	162	HIP 64900	9.4
7	21	10	45	307	Nike	55	PPMX 7873090	9.5
7	12	4	51	618	Elfriede	120	PPMX 11673156	9.6
7	14	8	42	66	Maja	72	PPMX 12767993	9.6
7	7	12	56	580	Selene	46	PPMX 12748529	9.8
7	2	15	22	387	Aquitania	101	PPMX 10702419	9.8
7	30	9	20		2002 CT154	90**	PPMX 8358502	9.8
8	6	16	8	302	Clarissa	39	HIP 116060	6.6
8	13	9	34	120	Lachesis	174	HIP 27312	7.9
8	16	19	48	1306	Scythia	67	PPMX 11639805	8.5
8	25	0	20	160	Una	81	PPMX 11857084	8.6
8	28	20	54	776	Berbericia	151	PPMX 5993607	8.7
8	8	11	50	425	Cornelia	64	PPMX 6857518	9.5
8	15	19	58	774	Armor	50	HIP 111336	9.6
9	16	17	30	2595	Gudiachvili	32	HIP 19718	6.6
9	11	1	17	196	Philomela	136	HIP 2038	7.6
9	28	8	21	339	Dorothea	38	PPMX 6963994	8.8
9	3	21	51	71	Niobe	83	PPMX 2038697	9.0
9	14	13	14	225	Henrietta	120	PPMX 9627059	9.4
9	5	14	20	1241	Dysona	83	PPMX 10833844	9.4
9	8	1	8	3451	Mentor	145***	PPMX 7945527	9.6
9	18	20	36	436	Patricia	60	PPMX 3850868	9.7
10	26	15	22	516	Amherstia	73	HIP 8175	8.0
10	22	1	14	1021	Flammario	99	HIP 34828	8.1
10	5	16	14	128	Nemesis	188	HIP 4516	8.9
10	13	6	56	94	Aurora	205	PPMX 7835764	8.9
10	17	1	58	173	Ino	154	PPMX 11732935	9.4
10	21	6	44	617	Patroclus	141	PPMX 6855021	9.6
10	6	20	27	2	Pallas	545	PPMX 10301227	9.7
11	8	23	16	283	Emma	148	HIP 88541	7.3
11	10	13	16	17	Thetis	90	HIP 22505	7.9
11	28	10	44	599	Luisa	65	HIP 711	8.9
11	11	9	27	438	Zeuxo	61	PPMX 5135778	9.0
11	5	18	27	2207	Antenor	85	PPMX 6851414	9.0
11	7	5	23	493	Griseldis	46	PPMX 3006432	9.1
11	18	6	49	751	Faïna	111	PPMX 5174295	9.3
11	19	16	25	18	Melpomene	141	PPMX 7175068	9.4
11	2	6	58	2	Pallas	545	PPMX 11319493	9.6
11	16	23	19	279	Thule	127	HIP 22291	9.7
11	26	10	17	134	Sophrosyne	108	PPMX 2950921	9.8
11	1	18	48	511	Davida	326	PPMX 7979446	9.8

12	15	20	24	1254	Erfordia	45	HIP 38601	6.9
12	26	5	21	733	Mocia	89	HIP 17548	7.2
12	21	13	34	2731	Cucula	51	HIP 30485	7.6
12	19	15	21	350	Ornamenta	118	HIP 22021	7.7
12	29	2	31	954	Li	58	HIP 59979	8.4
12	5	19	42	166	Rhodope	~40	PPMX 9033190	8.6
12	9	3	52	2320	Blarney	39	HIP 12443	8.7
12	28	19	12	67	Asia	58	PPMX 6989437	8.8
12	18	20	46	51	Nemausa	148	PPMX 8131707	9.5
12	9	15	44	120	Lachesis	174	HIP 32056	9.5
12	23	16	12	335	Roberta	89	PPMX 6856981	9.7

Diameters not in the IRAS catalogue:
* Plutino, (A = 0.25 assumed)
** TNO, (A = 0.25 assumed)
*** Jupiter Trojan, (A = 0.05 assumed)
A = geometric albedo

An example of an EAON chart is shown below. It includes star and asteroid data, duration of occultation, drop in magnitude of the star, time of the occultation, the track and finder charts.

I would strongly recommend obtaining the latest orbital elements for the asteroid from the Minor Planet Center’s Minor Planet and Comet Ephemeris Service and plotting its predicted track on a planetarium program such as Megastar. Using a program with which you are familiar greatly reduces the chances of observing the wrong star. The example below shows the track of the asteroid from 10 – 15 Dec with the asteroid occulting the target star on 12 Dec.

3.0 Observing and timing the occultation

Spending several minutes staring at a star field through a telescope can be quite tiring and the eyes start to play tricks. This is particularly the case if the target star is difficult to see due to for example; its faintness, thin cloud or a bright sky background.

Practicing before the event is therefore to be recommended. You may find out that some obstacle is blocking your view of the event particularly if the target star is low down. It will also help you to identify the correct star field and, if the asteroid is bright enough, see it moving towards the target star. Such practice will also enable you to set the eyepiece in such a position that the event can be viewed from a comfortable sitting position if at all possible.

An alarm clock with two alarms is extremely helpful. Set the first alarm to one minute before the predicted time and the second for the end of the observing period. This way you can increase your concentration nearer the predicted time and know when to stop.

At the telescope, monitoring of the target star should commence some minutes before the predicted time and continue for at least the same period after it (the occultation prediction data will usually include a suggested time period). At the instant of occultation the brightness will reduce by the predicted amount, which depends on the relative brightness of the asteroid and star being occulted. Beware also when making an occultation observation of so-called secondary events which may happen some time either side of the primary occultation . Although termed `secondary', such an event may also produce a dip in brightness virtually identical in magnitude to that of the primary event. Any secondary event may represent an occultation by a satellite of the minor planet.

See EAON Circular #5, Parts two, three and four for detailed explanations of various methods of recording the time of the occultation.

4.0 Time keeping

A radio controlled clock which uses the broadcast time signals to update the display will indicate the time to the necessary degree of accuracy. One’s error, or Personal Equation (PE), can be calculated and the occultation timings modified accordingly.

See also EAON Circular #5, Part Five, Short notes about time-keeping sources.

4.1 Timing example

An example of timing using a stop watch (with lap timing feature) and a radio controlled clock and assuming a ‘Positive’ result.

Activity	Stopwatch	Stopwatch time	Time of occultation	Time with PE applied
Star dims or winks out completely	Press Start	0	20:59:48 (21:06:00 – 6 mins 12 secs)	20:59:47.7 (21:59:48 – 0.3)
Star returns to normal brightness	Press Lap	6.5 secs	20:59:54.5 (20:59:48 +6.5 secs)	20:59:54.2 (20:59:54.5 – 0.3)
Radio controlled clock indicates a complete minute eg; 21:06:00	Press Stop	6 mins 12 secs

4.2 Personal Equation

One’s reaction time or Personal Equation (PE) should be calculated and subtracted from the times reported. The PE can be estimated using a stop watch as follows;

- cover the seconds, tenths and hundredths of second’s digits

- start the stopwatch

- press stop when the next full minute is indicated

The indicted seconds and fractions of seconds will be your reaction time. Repeat the exercise say thirty times and take the average (mine was 0.3 secs). See EAON circular #5, Part one, Personal equation and accuracy for a more detailed explanation with examples.

A simulation of an occultation allowing the observer to determine their 'Personal Equation' can be found here. A more amusing example can be found here.

5.0 Reporting

By joining the PLANOCCULT mailing list (use link on the Euraster ) observers can see the occultation reports submitted by other observers. Reports should be sent to this list and copied to Gilles Regherre at gillesregheere@yahoo.fr and Jan Manek at jan.manek@worldonline.cz . The ARPS Director is a member of PLANOCCULT and will thus see all reports submitted.

Information about the sizes, shapes and positions of asteroids comes from the occultation observations. Each positive observation gives a chord length across the minor planet and its precise position at that time. It is well worth attempting to observe the occultation if your site is close to but not on the predicted track. Such negative observations (ie; no occultation observed) help to set limits on the size of the asteroid. If poor weather prevents observation then a report should not be submitted.

Below is an example of how the approximate shape of an asteroid was derived from a number of observations. The importance of ‘Negative’ results are clearly illustrated. Observers 2 and 20 (and some others) didn’t see the occultation but their results put limits on the size of the asteroid.

A reporting form is available from EAON . Report summaries can be found on the EAON and Euraster web sites.

6.0 Results

European reports can be found on the Euraster website. Mike Kretlow’s database can be found here. This database was launched on 12 November 2007 and, at that time, was still under test.

7.0 Conclusion

You have to be fairly dedicated to do this stuff. Remember the ‘Rule of Ten’ ie; for every ten occultations you prepare for the weather will wash out nine of them and for every ten you actually observe you may get one ‘Positive’ . Remember however that, for those observing close to the edge of the predicted track, ‘Negatives’ are just as important as ‘Positives’. Now that predictions are improving the ratio of ‘Positives’ to ‘Negatives’ may be improving so go out and prove me too pessimistic!!!

Having put you off observing occultations for ever just remember that the greater the effort put in the more rewarding is the actual success when it comes – so do give it a try.

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