by Darrell Heath
Hi everyone, I’m Darrell Heath with the UALR College of Arts, Letters, and Sciences; welcome to The Night Sky.
As an amateur astronomer I’m always looking up. During the day it’s possible for me to see such things as rainbows, solar halos, strange cloud formations and even the Moon. At night I have the stars, planets, meteors and even the occasional comet to look out for. All of these things are naturally occurring objects and phenomena that any of us can observe throughout the year. But ever since 1957 when Soviet Russia launched Sputnik, the first satellite to orbit the Earth, we’ve had human made objects to look for in the night sky as well.
Some estimates place the number at over 30,000 man-made objects in orbit around the Earth right now. Over 1200 of those are operational satellites, the rest is “space junk”: intact but out of control satellites, bits and pieces of broken up satellites, or the spent rocket bodies that carried them out into space. Functional satellites include those used for communications, geophysical surveys, weather monitoring, navigation, spy and military operations, space telescopes, and even those large enough to hold a crew of several humans.
How high up a satellite is depends on such things as its purpose, how long it’s meant to stay in orbit, and whether or not maintenance or servicing is required. The highest satellites are 22,223 miles above us. They travel around the globe near the equator in geosynchronous orbits: in other words their revolutions match that of the Earth’s and they’re always positioned over the same spot on the planet. Television and communication satellites use these orbital positions. From geosynchronous orbits down to about 1,243 miles you’ll find satellites traveling in Medium-Earth orbits, GPS satellites are often found here. Satellites that are meant to survey and image the Earth, or are serviced periodically, travel in Low-Earth orbits, anywhere from 111 miles to 1,243 miles above. This is the home of spy and military satellites as well as weather monitoring satellites and the International Space Station.
On any given night of the year it’s possible for you to see a satellite passing overhead, but it’s usually those in Low-Earth orbit that are seen the most often. No optical aid is needed, just use your eyes to scan the sky and look for a faint star-like object moving against the background stars. Within about 15 minutes you should see at least one or two. If you see flashing lights of different colors then you’ve spotted an aircraft and not a satellite.
As is the case with most things involving stargazing there’s an optimal time of night and year to observe satellites. Keep in mind that satellites are not producing any light of their own, at least none that you can see. The light that makes them visible to our eyes is sunlight being reflected off their body or from panels. The best time to observe them is anywhere from 45 minutes to two hours after sunset or just before sunrise. The sky has to be just dark enough to make them visible and yet not so late that your viewing location has passed within Earth’s shadow. During the early evening, or just before sunrise, there’s enough sunlight hitting our atmosphere hundreds of miles above to light up a satellite or piece of space junk. The time of year can also be an important factor. During winter the Earth is in a point in its orbit so that Earth’s shadow is already well overhead soon after sunset, not a good time to see these otherwise faint objects. During summer our orbital position is now such that Earth’s shadow is angled southwards and never quite makes it overhead, even at midnight. Summer is prime time viewing season for satellites.
So, what can you expect to see?
Well, most satellites you see are going to be faint, just at the threshold of naked eye visibility. Some may be as bright as the Moon or Venus! If your viewing location is optimal and you’re looking at the right time you can see anywhere from 10 to 20 satellites an hour.
While they don’t have blinking lights some of the out of control satellites and bits of space junk are tumbling in space. This can make them appear to flicker as they move across the sky.
Typically satellites will be traveling from a westerly direction towards an easterly direction and not the other way around. Satellites are launched this way to take advantage of Earth’s 1,040 mph spin rate. Because the Earth spins faster near the equator most launches take place as near there as possible: you get to add more payload for less fuel this way. Occasionally you’ll see satellites heading on north-south trajectories, or vice versa; be sure to smile and wave when you see one of these because it may well be one of those spy satellites I mentioned earlier.
Satellites have no way of increasing or decreasing their speeds, they are falling to the Earth in a curve just fast enough to match the curvature of the Earth and to keep themselves in orbit. Usually you’ll see them traverse the sky within a couple of minutes. If they’re traveling much faster then it’s most likely a dead satellite skimming the upper atmosphere and on its way towards a fiery death. Another spectacular display is when a Centaur, an upper rocket stage used to boost a satellite’s payload into space, vents some of its fuel, resulting in a glowing cloud of gas.
While the majority of satellite sightings are a bit on the dim side there are two that can occasionally shine as bright, or even brighter, than the planet Venus: the International Space Station and Iridium flare satellites.
At 450 tons and bigger than a football field the International Space Station is the largest man-made structure in space, it circles the globe in a Low-Earth orbit 225 miles up every 90 minutes. The ISS is a microgravity laboratory and is occupied by six people at any one time; since its construction in November of 2000 over 200 people from 15 different nations have lived and worked there. Over an acre of solar panels extend out to either side of the crew and cargo areas and when the ISS passes over at just the right time and the sunlight hits those panels it can be the brightest object in the evening sky after the Moon. Sometimes, when the ISS is angled just right towards the Sun the station can appear to flare in brightness to magnitude -8; more than 16 times as bright as Venus!
It’s orbit around the Earth is inclined by about 52 degrees with respect to the globe’s equator, this means that just about everyone on the planet gets to see it at one time or another, but you’ll have to know exactly when in order to see it. The best way to do that is to have NASA send you email notifications as to the time, date, and viewing directions. Visit spotthestation.nasa.gov to sign up. Alternatively you can visit web sites such as Heavens-Above.com for a customizable itinerary. There are even a number of apps that will notify you as to the next flyover. It’s best appearances are when it’s higher than 20 degrees above the horizon, anything below that means it will be difficult to see and rather dim to boot.
Iridium satellites are older spacecraft used for voice and data coverage to satellite phones and pagers and they orbit about 500 miles above the Earth. They are also one of the most dramatic satellite performers you will see in the night sky. Each spacecraft is equipped with three door-sized and highly reflective antenna arrays and during a favorable flyover sunlight will hit them and direct it back to an observer on the ground. This creates the spectacular sight of a dim-looking satellite suddenly flaring in brightness; so much so that at times it can become 25 times brighter than the planet Venus. Many people who happen to see one often mistake it for a fireball or UFO. Check with the Heavens Above web site for flyover times and, yes, there are even apps for that as well.
While you are gazing up and looking for these moving dots of light in the dark here is something to think about.
The furthest a human being has traveled in space is when the crew of the ill-fated Apollo 13 mission used the Moon’s gravity to help slingshot them back safely to Earth. At the furthest they were 248,655 miles away: no human has ever been more distant. The furthest a man-made spacecraft has ever been is the Voyager 1 space probe, currently 11 billion miles away, within the edge of interstellar space. The closest star system to Earth is Alpha Centauri, some 4.3 light years away. Traveling at the speed of one of NASA’s retired space shuttles, say 17,600 mph, it would take you 165,000 years to reach Alpha Centauri (and that’s with no potty breaks!). While those satellites we see appear to be very distant the truth is that we have just barely dipped our toes into the great cosmic ocean.
Until next time I encourage you to get outside and look up in both awe and wonder.