Besides the moon, the night's brightest light will be the International Space Station now through early February at dusk. At the end of this article I've listed several ways you can find out exactly when and where to look for it. Even at its faintest, the ISS equals Sirius, the brightest nighttime star, and when it passes overhead it can rival Venus.

The ISS travels from west to east to take advantage of Earth's west-to-east rotation. (Bob King)
The ISS travels from west to east to take advantage of Earth's west-to-east rotation. (Bob King)

The station travels from west to east across the sky — opposite that of the stars — because it was launched in the direction of Earth's spin to take advantage of the free rotational boost the planet provides. If it traveled the other way, against the direction of rotation, it would take a lot more fuel (= money and weight) to reach the necessary speed for orbit.

The first two pieces of the International Space Station — the Russian-Zarya and US-Unity — are mated together in orbit in December 1998. (NASA)
The first two pieces of the International Space Station — the Russian-Zarya and US-Unity — are mated together in orbit in December 1998. (NASA)

The space station's been around so long we tend to take for it granted like the sun or moon. But like anything fashioned by humans it has a limited lifetime. Come to think, so do the moon and sun, but that's a different story!

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The first piece of the ISS was launched into orbit in 1998. Two years later the first crew arrived. It's been inhabited ever since. Over the years, more pieces, called modules, have been added which have expanded the space station's research capacity and population. By the time the main construction was completed in 2011 the ship had grown to the size of a U.S. football field. Its enormous girth, large and reflective solar panels and relatively low orbit make it the brightest artificial satellite in the sky.

NASA astronaut Shannon Walker tends to plants growing inside the Veggie plant growth facility a space botany study earlier this month. The investigation is cultivating Extra Dwarf Pak Choi, Amara Mustard, and Red Romaine Lettuce, which are harvested in orbit, with samples returned to Earth for testing. (NASA)
NASA astronaut Shannon Walker tends to plants growing inside the Veggie plant growth facility a space botany study earlier this month. The investigation is cultivating Extra Dwarf Pak Choi, Amara Mustard, and Red Romaine Lettuce, which are harvested in orbit, with samples returned to Earth for testing. (NASA)

Within the modules, astronauts conduct hundreds of experiments in microgravity conditions that can't be duplicated as easily and consistently on Earth. Microgravity refers to the weak gravity experienced inside an orbiting spacecraft. Although photos of astronauts gobbling globules of floating water make it look like there's no gravity on the space station, the ship is bound in orbit by the gravitational force of the Earth the same way we're bound to this earthly life.

On the ground the force of gravity is 1 g, but 250 miles (400 km) up at the ISS it's a little less, equal to 0.89 g. Remember that the farther you are from a massive body, the weaker the gravity pulling you back. 0.89g is still plenty enough gravity to feel weight and walk around, although with a lighter step.

The reason why astronauts are weightless as if there were no gravity is because the ISS is simultaneously accelerating downward at 0.89g, and the two cancel out.

Wait a minute. The space station is falling? Yep. But it doesn't hit the Earth because at the same time it's plunging it's also moving forward fast enough to round the curved globe and maintain a constant altitude. All that forward speed (17,100 mph) came during launch when powerful rockets pushed it in that direction.

The space station fades from view as it enters the Earth's shadow on its way to Orion (left) on Tuesday night, Jan. 19, 2021 over Duluth, Minn. (Bob King)
The space station fades from view as it enters the Earth's shadow on its way to Orion (left) on Tuesday night, Jan. 19, 2021 over Duluth, Minn. (Bob King)

Since there's next to no friction with the air at 250 miles altitude, once the space station achieves that speed it just keeps going except for occasional adjustments due to cumulative air resistance over time. But make no mistake. The same way your feet would leave the floor of a fast-falling elevator, the astronauts leave the floor of the ISS and float like dandelion seeds around the cabin once they've reached orbit.

Originally slated to operate for 15 years, the station has been circling the planet for more than 22. NASA expects it to remain in orbit until its 30th anniversary in 2028 and perhaps beyond. The deciding factor will be how much science can be done versus how much time and money the agency may eventually have to spend on repairs. We use the same logic when it comes time to buy a new car.

Once the decision is made to deorbit the station it will be directed to enter Earth's atmosphere and burn up as a fiery "meteor" over a remote stretch of ocean.

Here a few ways in which research aboard the ISS has benefited both people and the planet:

* Disease research with the focus on potential cures for Alzheimer's disease, Parkinson's disease and cancer.

* New water purification systems. Water treatment systems created for the astronauts have been adapted to Earth-based systems. One of the first was used in Iraq in 2006.

* Understanding how our body changes in microgravity to better understand the challenges we'll face when launching to Mars.

* Monitoring the planet's land, water and air with a suite of instruments and through the lens of astronaut photography.

* Inspired kids' interest in space and science and led them to consider careers as astronauts.

There are many more. For a more in-depth look check out these 20 breakthroughs from 20 years of science on the ISS.

OK, so how do I see the space station?

Go to Heavens Above and select your city by clicking on the blue Change your observing location and other settings link. Then return to the home page and click on the blue ISS link to see a 10-day table of passes that includes time, direction, brightness and altitude. 10° of altitude is equal to one fist held at arm's length against the sky. The higher the negative number in the brightness column, the brighter the pass. Click on any pass time and a map will appear showing the station's path across the sky.

All times shown are local times on the 24-hour clock for your location, so 18:30 = 6:30 p.m. and 19:15 = 7:15 p.m. You can also get a list of customized passes and alerts by downloading the free ISS Spotter app for iPhone and ISS Detector for Android devices.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.