If you're out watching the twilight sky in the time frame
from 45 to 90 minutes before sunrise, or 45 to 90 minutes after sunset, you'll might
see a few "moving stars." They are most likely artificial satellites.
The brightest of all
is the International Space Station, and this month provides some great opportunities
to see it from just about anywhere.
Satellites are seen at night because they are illuminated at
high altitudes by reflected sunlight and can be seen against a dark sky. A
satellite entering the Earth's shadow immediately vanishes from view and
pursues an unseen path until it again emerges into full sunlight. There are nearly
10,000 satellites now in orbit around Earth, and typically it should not take
more than 15 minutes of skywatching to spot one.
The biggest and brightest
The International Space Station (ISS) is by far the largest
and most brilliant of all the man-made objects orbiting the Earth. In early
June, the station got its biggest live-in addition
yet, a billion-dollar
Japanese lab stretching 37 feet, named Kibo, which means "hope." Currently
more than four times as large as the defunct Russian Mir space station, the International
Space Station — when fully completed — will have a mass of about 1,040,000
pounds (520 tons). It will then measure 356 feet across and 290 feet long, with
almost an acre of solar panels to provide electrical power to six
state-of-the-art laboratories.
Circling the Earth at an average altitude of 240 miles and
at a speed of 18,000 mph, it can appear to move as fast as a high-flying jet airliner,
sometimes taking upwards to four or five minutes to cross the sky.
Nominally, its visual magnitude from the ground can make it
appear as bright as the planets Jupiter and Venus, although in recent days some
observers have seen the ISS
briefly "flare" to dazzling brilliance, thanks to sunlight
glinting off one of its many solar panels. In fact, some have even been able to
glimpse the ISS while the sun was just above the horizon!
Windows of opportunity
During the next couple of weeks, North Americans and
Europeans will have many opportunities to see the ISS flying over their homes,
due chiefly to a seasonal circumstance.
Right now, the nights are still rather short and the
time that a satellite in a low Earth orbit (like the ISS) remains
illuminated by the sun can extend through much of the night, a situation that
can never be attained during other times of the year. Because the ISS circles
the Earth about every 90 minutes on average, this means that it's possible to
see it not just once, but for several consecutive passes.
Moreover, because the ISS revolves around the Earth
in an orbit that is inclined 51.6 degrees to the equator, there are two types
of passes that are visible.
In the first case (we'll call it a "Type I"
pass), the ISS initially appears over toward the southwestern part of the sky
and then sweeps over toward the northeast. About seven or eight hours later, it
becomes possible to see a second type of pass (we'll call it "Type II"),
but this time with the ISS initially appearing over toward the northwestern
part of the sky and sweeping over toward the southeast.
And between roughly July 17 through 24, thanks to the
shortness of the nights, North Americans will get a chance to see the ISS
undergoing a series of Type I passes after sunset in the evening sky, and then
see it again the following morning before sunrise, undergoing a series of Type
II passes.
For some locations, there may be as many as six
chances to see the ISS during a single night! For much of North America and Europe, the "prime viewing period" for both evening and morning passes will run
roughly from about July 17 through 21. After July 21, the window of opportunity
for the Type II morning passes will close and only Type I evening passes will
be possible, lingering into the early part of August.
Viewing Tips
Some passes are superior to others. If the ISS is not
predicted to get much higher than 20 degrees above your local horizon, odds are
that it will not get much brighter than a moderately bright star (10 degrees is
roughly equal to the width of your fist held at arm's length). In addition,
with such low passes, the ISS will likely be visible for only a minute or two. Conversely,
those passes that are higher in the sky — especially those above 45 degrees —
will last longer and will be noticeably brighter.
The very best viewing circumstances are those that
take the ISS on a high arc across the sky about 45 to 60 minutes after sunset,
or 45 to 60 minutes before sunrise. In such cases, you'll have it in your sky
upwards to four or five minutes; it will likely get very bright — perhaps even
briefly "flare" in brilliance — and there will be little or no chance
of it encountering the Earth's shadow.
While the ISS looks like a moving star to the unaided
eye, those who have been able to train a telescope on it have actually been able
to see and even photograph its T-shape as it whizzed across their field of
view. Some have actually been able to track the ISS with their scope by moving
it along the projected path. Those who have gotten a good glimpse describe the
body of the space station as a brilliant white, while the solar panels appear a
coppery red.
For the upcoming series of evening and morning
passes, take note of the fact that, for those occurring in the evening, the ISS
will usually start out rather dim, then tend to grow in brightness as it moves
across the sky. In contrast, for the morning passes, the ISS will already be
quite bright when it first appears and will tend to fade somewhat toward the
end of its predicted pass. This is due to the change in the angle of sunlight
hitting the vehicle.
Lastly, remember that in certain cases, the ISS will either quickly disappear when it slips into the Earth's shadow (during evening
passes) or quite suddenly appear when it slips out of the Earth's shadow
(during morning passes). This becomes increasingly more likely for those
predicted passes that take place more than 90 minutes after sunset or more than
90 minutes before sunrise.
When and where to look
So what is the viewing schedule for your particular
hometown? You can easily find out by visiting one of these three popular web
sites:
Each will ask for your zip code or city, and respond
with a list of suggested spotting times. Predictions computed a few days ahead
of time are usually accurate to within about a minute. However, they can change
due to the slow decay of the space station's orbit and periodic reboosts to
higher altitudes, so check frequently for updates.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other
publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.
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