Treebeard's Stumper Answer 30 March 2001

Catch an ISS Pass

We've been reading science fiction in my science classes lately, but NASA has been making it science fact with the International Space Station (ISS). At just the right time and place, the big ISS is visible as one of the brightest and fastest objects in the sky. We have several opportunities to view it next week. (See below.) The challenge is to observe an ISS pass. Early fog and sunrise and city lights might be a problem. The stumper is why all the view times are in the early morning and evening. Wouldn't it be easier to catch an ISS pass late at night when the sky is darkest?

View of the International Space Station (ISS) from the approaching Space Shuttle Atlantis on February 28, 2001. Space shuttle astronauts on this mission conducted the 100th space walk by an American. That's the Rio Negro region of Argentina two hundred kilometers below.

Notes: Here's a table of the next few visible International Space Station (ISS) passes over Dunn Middle School in Los Olivos, California (34.6680°N, 120.1140°W, Epoch 1 April 2001). Pass times from Santa Barbara to Santa Maria are just seconds different. This data is from the Heavens-Above site. The predicted viewing times change slightly from day to day because of course adjustments and other variables, but you can check the site for updated info and detailed sky charts. You have to specify your location, but it's easy to select from their huge database. (Or just click here for Los Olivos, Santa Barbara, or Santa Maria.) These times are all Daylight Savings Time (PDT). Because of our usual morning fog, the best time to catch an ISS pass here in central California looks like next Wednesday evening, April 4, starting at 8:46 PM (PDT). There are many other viewing opportunities this coming week, morning and evening.

Where the ISS is now, moving to the right. Reload this page a few times to see it move! (Image from Heavens-Above.)

All times are Pacific Daylight Time (PDT). Magnitude (Mag.) is brightness. The smaller the number the brighter the ISS appears, and negative numbers are brightest of all. Altitude (Alt.) is how high to look in the sky, measured in degrees with 90° straight up. Azimuth (Az.) is the compass direction around your horizon. Note that an entire pass only lasts a few minutes, so don't be late!

Date	Mag.	Starts			Max. Altitude			Ends
		Time	Alt.	Az.	Time	Alt.	Az.	Time	Alt.	Az.
Sunday, April 1, 2001	-0.5	05:38:59 PDT 5:39 AM	12	NW	05:41:51	74	NE	05:44:58	10	SE
Monday, April 2, 2001	1.6	04:42:23 PDT 4:42 AM	26	ENE	04:42:23	27	ENE	04:44:34	10	E
Monday, April 2, 2001	0.9	06:15:16 PDT 6:15 AM	10	W	06:17:38	20	SW	06:19:58	10	S
Tuesday, April 3, 2001	-0.2	05:18:36 PDT 5:18 AM	45	S	05:18:36	45	S	05:21:04	10	SSE
Tuesday, April 3, 2001	0.9	20:11:48 PDT  8:11 PM	10	S	20:14:01	18	SE	20:14:25	18	ESE
Wednesday, April 4, 2001	-0.7	20:46:29 PDT  8:46 PM	10	SW	20:49:36	81	NW	20:50:35	40	NE
Thursday, April 5, 2001	0.1	19:46:29 PDT  7:46 PM	10	SSW	19:49:19	33	SE	19:52:10	10	ENE
Thursday, April 5, 2001	2.0	21:22:57 PDT 9:23 PM	10	W	21:25:18	19	NNW	21:26:08	17	N
Friday, April 6, 2001	0.6	20:21:35 PDT 8:22 PM	10	WSW	20:24:33	42	NW	20:27:32	10	NE
Saturday, April 7, 2001	2.6	20:58:19 PDT 8:58 PM	10	WNW	20:59:54	13	NNW	21:01:30	10	N
Sunday, April 8, 2001	1.6	19:55:58 PDT 7:56 PM	10	W	19:58:37	25	NW	20:01:17	10	NNE

Here's two more ISS stumpers I've been thinking about:

• I've wanted to do this stumper ever since the solar panels were deployed last December, since they made the ISS much brighter. But whenever I checked, there were no good viewing opportunities here in California. This week there are many opportunities. Why is this so variable?

• It's interesting that some of these ISS passes go from north to south, but others go from south to north. What kind of orbit is this? Why does it change direction? How does it change day to day?

The weather did not cooperate with my attempts to view the International Space Station this week, but there will be other times. The ISS orbits the Earth 16 times a day! But satellites are only visible in the early morning and evening when they are lit by the sun while it's dark for the observer. Think of camping at the base of a tall mountain. The sun sets and your camp is in shadow, but the peak still catches the light for a while as the sun sinks lower and the darkness moves up the summit. Late at night, the summit and the satellite far above are both in the dark.

Notes:

I first understood Earth-satellites many years ago while camped by Desolation Lake in the high wilderness basin just west of Mount Humphreys in the High Sierra Nevada mountains. The sun set early at my camp, but that great peak caught the setting sun in alpenglow for a while until the sun set for the summit too. Now imagine a mountain 200 km high! As the sun sets lower and lower, you will see the shadow line climb up until only the very summit is lit, then it too is dark. Now take away the mountain except for the very top, and that's the satellite! That's why we can usually only view Earth satellites soon after sunset and just before dawn. They're close enough to Earth to be caught in its shadow when the sun is too far below the horizen to light them, so satellites are only visible for a few hours after sunset or before sunrise. The exception is during summer at high latitudes far north or south where the sun is never too far down, and some satellites can be seen the whole night through if they pass over in polar orbits. This also explains why even very bright satellites often fade away as they move across the sky and enter the Earth's shadow.

Another view of the ISS now from www.space.com.

The real stumper for me is to understand the ISS orbit. The Space Station has been potentially visible (through the clouds) every day for the last 10 days here in Santa Barbara, but we won't get another chance until at least April 18. Why are viewing days clumped in groups? Why does the time change from morning to evening? Why does the direction change from north to south? Here are predicted viewing times for Santa Barbara over the next 90 days, as figured by Scott Hather's freeware SatScape program. This table only shows visible passes with altitude > 10° high. The data will soon be out of date as the orbital elements changes. June 2 looks remarkable with 3 visible passes. We'll be camped at our usual place at the Live Oak Music Festival for the June 15-17 views!

Date	Time	Start Az.	Alt.	Duration	End Az.
18 APR 2001	21:06:22	005	12	00:02:23	050
19 APR 2001	21:38:51	330	33	00:05:46	106
20 APR 2001	20:37:22	350	16	00:04:04	072
21 APR 2001	21:10:00	321	55	00:06:07	121
22 APR 2001	21:43:33	288	25	00:05:17	171
23 APR 2001	20:40:33	312	74	00:06:11	136
24 APR 2001	21:14:17	273	16	00:04:01	191
29 MAY 2001	04:48:53	010	11	00:01:46	046
31 MAY 2001	04:03:10	008	11	00:02:05	050
01 JUN 2001	04:26:18	333	29	00:05:11	101
01 JUN 2001	22:26:28	296	13	00:03:11	003
02 JUN 2001	03:16:30	006	12	00:02:13	052
02 JUN 2001	04:50:12	307	50	00:05:43	143
02 JUN 2001	21:13:54	251	35	00:05:26	031
03 JUN 2001	21:38:58	296	13	00:03:06	002
14 JUN 2001	22:34:01	343	20	00:04:24	086
15 JUN 2001	22:53:41	318	59	00:05:35	126
16 JUN 2001	21:39:17	347	17	00:04:02	079
17 JUN 2001	21:58:14	322	44	00:05:29	119
18 JUN 2001	22:17:32	296	33	00:05:10	159
19 JUN 2001	21:01:35	327	36	00:05:19	112
20 JUN 2001	21:20:07	302	44	00:05:23	150
21 JUN 2001	21:39:26	263	13	00:02:50	201

I think this is a strobe light effect, like how wheels seem to go backwards in film. The ISS is in a regular orbit around the Earth every 90 minutes or so. 90 minutes is exactly one-sixteenth of a day, so the Earth rotating underneath in 24 hours should bring the satellite back to almost the same place in the sky after 16 (24 / 1.5) orbits. Almost, but not quite. Playing with simulator software, it appears that the ISS orbit is a bit faster than 90 minutes, so its path seems to regress a bit westward with each orbit. In time, it moves out of that narrow window where it is in the sun and we observers are in shadow, so we can't see it again until it has regressed a full orbit. That's why viewing opportunities come in clusters. It has it's own orbital frequency, but we actually see it at a different frequency that also depends on the sun and our exact location on the Earth.

Visualizing satellite orbits in 3D is tricky. Here's an image of the April 4, 2001 pass over Santa Barbara, made with WxTrack. It shows several successive passes, so you can see how the orbit shifts to the left/west with each pass. In time, it will pass over most of the Earth.

WxTrack image (modified), April 4, 2001 at 8:46 P.M., PDT.

The sine wave-like orbit looks complicated, but note that the day-night line has a similar up-and-down shape. In fact the ISS is orbiting around the center of the Earth in an almost-circular orbit. As the Earth turns underneath, it creates this elegant curve. The ISS orbit was deliberate. The International Space Station must be accessible from all participating nations. The not-quite-polar ISS orbit swings north and south at an inclination of 51.6 degrees from the equator. This brings the path over both Cape Canaveral and the Russian Baikonur Cosmodrome in Kazakhstan (lat. 48° N) for easy access.

I tried to catch the ISS pass on the last day of our viewing cluster. It wasn't a great pass with magnitude 1.8 and a max altitude of 24 degrees. The best view was just after 8:00 pm, and it wasn't quite dark yet, and I didn't see anything. *Sigh* There will be other times. But I did log the images from Heavens-Above every 10 mnutes, and I stitched them into this animation. Note how far the ISS moves every ten minutes! This animation shows two passes over Santa Barbnara on the west coast of the US, but only the first was potentially visible. Note how the ISS orbit always looks like a straight line, though the Earth keeps turning below. It would be better to keep the ISS orbit stationary and show the Earth turn beneath it.

Here are some links for further research:

• Heavens-Above
is a great site for info on visible satellite passes. Pick your location from their huge database, and then save the link for easy access. NASA has several JAVA programs that can track satellites:
• J-Track Satellite Tracking
• J-Pass Satellite Passes
• J-Track 3D
I recommend using JTrack and JTrack-3D together to understand the orbital geometry.
• There's good satellite tracking software for Windows available on the Web as freeware: WxTrack, Satscape, and STSPLUS (DOS) all do the job. There's links to more software here and here.

• These programs all depend on current orbital data ("TLEs" or "2-Line Elements") that mathematically describe each satellite orbit in a standard way. Current TLE data is available at CelesTrak. AMSAT has a tutorial on what it means, and there's a NASA technical document (PDF).

• NASA has lots of info about the International Space Station at their Human Spaceflight site. Their International Space Station Reference offers news and many megabytes of reference materials as PDF documents. Science@NASA has interesting space science reports that you can receive by email if you subscribe. (Recommended!) They have many reports on the ISS, including Station Sightings (July 24, 2000 and A New Star in the Sky (Dec 1, 2000).

• There are many interesting amateur sites on the Web about Earth satellites, both from amateur astronomers and ham radio fans. Good starting links are the Visual Satellite Observer's Home Page and the DXZone.

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