On November 17 around 12h UT, less than 30h from now, the two STEREO spacecraft will pass through perigee, and will be visible for an hour or two before they will pass through the Earth's shadow. The best areas from which the satellites can be observed are around the locations under where they enter or exit the shadow. Observers in southern and eastern Asia, and New Zealand and Australia have a chance to observe them with telescopes; they are unlikely to be bright enough to be seen with binoculars. The table below gives the shadow entrance and exit times and locations: Spacecraft Ahead Behind Shadow Event Entrance Exit Entrance Exit Universal Time 13:35:31 13:56:54 11:10:33 11:31:52 Altitude, km 2506 3506 2180 3274 longitude, deg. 103.2 E 164.8 E 136.9 E 154.7 W latitude, deg. 27.8 N 16.8 S 26.0 N 15.8 S Best areas India New Taiwan New for observation s.e.Asia Zealand Japan Zealand China Australia Philippine (low in Islands east) China Delta-V Maneuver Start End Start End Universal Time 12:56:00 12:59.0 10:00:00 10:05.0 In addition, observers in the areas under the shadow entrance might see the spacecraft brighten a little as they fire their thrusters for velocity-change (Delta-V) maneuvers. The times of these are also given above. The start times are exact, but the end times are uncertain by several seconds since the maneuvers are terminated by on-board accelerometers that sense when the Delta-V target has been achieved. About 20 minutes before the maneuvers, and a few minutes after them, the spacecraft change their orientation, so a change in apparent brightness might be seen then as well. The satellites are thousands of kilometers apart near perigee. They spend most of their orbit near apogee, near the Moon's distance; the direction of the apogees is near the direction to the Sun, so they can not be observed then, and can only be seen within a few hours of perigee. They will probably be about 9th magnitude, but could be as faint as 10th or even 11th. When Greg Roberts observed them shortly after launch, they were about 8th mag., with some brightenings to perhaps 6th mag., but the two spacecraft were too close to resolve then (so twice as bright) and they were also lower than they will be at this perigee. Observers towards the Sun from the shadow points (that is, west of the entrance points and east of the exit points) will have better (smaller) phase angles so the satellites will appear brighter for them, but they will have less time in a reasonably dark sky to see them. You can generate detailed path predictions (RA, Dec, alt., az., distance) in the sky for your location using the The JPL Horizons ephemeris generator Web site at http://ssd.jpl.nasa.gov/horizons.cgi . To get predictions at less than a minute interval, specify "equal intervals" for the "units" of the step size, and then if you specify a time range of 20 minutes, specifying an "equal intervals" value of 600 will give you data at 2-second intervals. The JPL Horizons site uses orbits for the satellites determined before we performed large maneuvers Tuesday morning, Nov. 14. Those maneuvers were included in the orbital data supplied to JPL so their effects are included, but although the maneuvers were performed well, they did have errors of about 1% which cause them to deviate a little from the JPL ephemeris, which can not be updated before this perigee passage. We have compared it with a new orbit determined after the Nov. 14th maneuvers, and find the following offsets that you will probably see relative to the Horizons prediction: Ahead: Path 0.3 deg. south and 48 seconds late. Behind: Path 0.07 deg. south and 12 seconds late. These were calculated for locations right under the shadow entrance and exit points; for other locations where the spacecraft will be lower in your sky, the path offset will be a little smaller. The maneuvers mentioned above are smaller than the Nov. 14th maneuvers and have been factored into the predictions. The Nov. 17th maneuvers could have errors as well, but there will not be sufficient time from when they occur to the perigee passage to notice any significant differnce that might be caused by that. This is the 2nd perigee passage of these satellites. The only inhabited area from which the 1st perigee passage was visible on November 6 was from part of Latin America; I have heard of no successful observations. The purpose of the two maneuvers this week is to target each spacecraft to the first lunar swingby on December 15th, and the Behind spacecraft to the right conditions at the first swingby so that it will have a second lunar swingby on January 21st. The first maneuvers were successful for this purpose; if the maneuvers on November 17th are as successful, the targeting of the lunar swingbys should largely be complete, with only one or two additional very small maneuvers needed later on to refine the trajectories. Much more about STEREO is on the main STEREO Web site at http://stereo.jhuapl.edu and on my STEREO Web site at http://highorbits.jhuapl.edu/stereo.htm . David Dunham, IOTA and STEREO mission design with much help from Jose Guzman, Peter Sharer, and many others on the STEREO team -- No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.430 / Virus Database: 268.14.6/535 - Release Date: 15/11/2006 3:47 PM ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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