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			25 Birch Street, Cambridge, MA 02138 USA
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		Tel. 617-354-0484	FAX 617-354-0665

		AAVSO ALERT NOTICE 175 (August 4,1993)


The launch of NASA's space shuttle Discovery carrying the ORFEUS mission was canceled 
on July 24, 19 seconds before the launch, due to "abnormal behavior by a hydraulic pump 
controlling thrust vectoring of the right Solid Rocket Booster." The launch date for 
Discovery has been moved to August 12. The decision to move the launch to August was 
due to the upcoming Perseid meteor shower in Earth's upper atmosphere, which is 
predicted to take place during the evening of August 11-12. This year is one of the rare 
times that the activity is expected to be extremely high as the Earth passes through the 
thickest part of the dust cloud of Comet Swift-Tuttle: The maximum, if there is one, is 
expected to last about an hour, but the uncertainty of time of arrival is about 4 hours. At 
present, the best guesses are that Western Asia, Eastern Europe, and possibly the east 
coast of the United States will be facing the stream when it arrives. The guesses assume 
that the maximum will occur before the Earth passes through the plane of the comet's 
orbit, which will occur at Ol hour Universal Time on August 12. Since this year's Perseid 
activity is a unique event, it is not completely predictable. Increased chances of a 
spacecraft in Earth orbit being damaged by a piece of debris led the shuttle managers to 
decide to wait until after the Perseid meteor shower event to launch Discovery. Our 
assistance has been asked by NASA Astrophysics Division to monitor the Perseid meteor 
activity closely, starting August 9. Pleasce call in to AAVSO Headquarters, using the 
charge-free number 800-642-3883, if the meteor activity is very high, i.e., more than 2 
Perseid meteors per minute, on the evening of August 11-12. I will be checking the 
AAVSO answering machine regularly that evening for your messages. (For a detailed and 
very informative article on the Perseids, see Sky & Telescope, August 1993, pp. 43-49).

Once Discovery is launched, the ORFEUS mission will be deployed from the shuttle and 
the observations of cataclysmic variables with ORFEUS will start 2 to 3 days after the 
launch (see AAYSD Alert Notices 173 and 174).

Please continue to monitor closely the stars that are the primary observing targets during 
the ORFEUS mission, i.e., 0058 + 40 RX And, 0409-71 VW Hyi, 0814 + 73 Z Cam, 0813 + 49 
AM Her, 2138 + 43 SS Cyg, and 2209 + 12 RU Peg. Monitor them between now and the end 
of the 9-day mission, and call in your observations of them to AAVSO Headquarters. 
Please also remember to call in if any of the brighter dwarf novae go into outburst, i.e., 
magnitude brighter than 12.5 at outburst, as these stars may be observed during the 
ORFEUS mission.

Your observations of the Perseid meteor shower and also of cataclysmic variables, and your 
phone calls to AAVSO Headquarters of their behavior, are vital to this mission. 
Astronomers at NASA Astrophysics Division appreciate and extend their sincere thanks to 
you for your valuable astronomical contributions to this and other NASA missions.


The dwarf nova V503 Cyg, varying between photographic magnitudes 13.4 and 17.0, is a 
very interesting and unusual system, in which humps in the continuum are observed both at 
quiescence and during outbursts. While the amplitude of the humps is as much as 1 
magnitude during quiescence, during outburst it decreases to 0.2 to 0.3 magnitude. 
Another interesting feature is that the period of the photometric humps is a few percent 
longer than the radial velocities of the emission lines and fluxes.

Humps seen in the optical both during quiescence and during outbursts show different 
behavior in the ultraviolet region. Thus, in order to investigate the hump temperature and 
location and the overall disk structure, astronomers at the University of Washington and 
NASA Goddard Space Flight Center have an observing run with the IUE satellite to 
observe V503 Cyg during outburst. They have asked our assistance to keep a close eye on 
this system and inform them when V503 Cyg goes into outburst.

Accompanying is an AAVSO preliminary chart of V503 Cyg. Please note that this chart, 
issued 6/89, has the position of V503 Cyg corrected (after the star was observed in outburst 
in August 1988 - see Letter to the Editor by J. Griese and C. Scovil, JAAVSO, Vol. 17, p. 
148; 1988). Please use this chart and not the AAVSO charts issued at earlier dates. 
Observers with moderate- to large-aperture telescopes are requested to monitor this star 
closely, and phone in to AAVSO Headquarters when the star goes into outburst. The 
observations in the AAVSO International Database indicate that the maximum outburst 
brightness is between visual magnitudes 13.4 and 13.8, and at minimum the star is fainter 
than 16.0.


Please continue to monitor VW Hyi and SS Cyg throughout the rest of 1993 and inform us 
of their outbursts (see AAVSO Alert Notices 173 and 174).

Please keep us informed of the brightness and behavior of AM Her (see AAVSO Alert Notices 
173 and 174).

Also, continue to monitor V348 Sgr and inform us when it becomes brighter than 13.5 (see 
AAVSO Alert Notices 173 and 174).

Thank you very much for your close monitoring and notification of your observations to 
AAVSO Headquarters, and for your valuable contributions to variable star research.

Clear skies and good observing,

Janet A. Mattei 

AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484