Is it a bird? Is it a plane? No, it’s the ISS

by Dr Josef Borg

An image of the International Space Station, captured by the crew of STS-132, on May 23, 2010. Photo: NASA

This article appeared on the Times of Malta on April 11, 2021. It is being reproduced here with the consent of the author.

Flying about 400km above the earth’s surface at a speed of 28,800km/h, the International Space Station (ISS) completes one revolution around the planet in approximately just 92 minutes, with astronauts on board the station therefore experiencing 16 sunsets and sunrises every earth day. The ISS represents one of the largest multinational projects ever embarked upon, seeing the collaboration of five different space agencies – NASA, ESA, CSA, JAXA and Roscosmos – a true testament to what can be achieved through global collaboration and unity.

The ISS follows several other previous earth-orbiting stations, being the ninth such station around our planet to be inhabited. It is also the largest artificial satellite, currently measuring 73m in length and 109m in width. Its in-orbit assembly started in 1998 with the Russian Zarya module, followed closely by the American Unity module. The Zarya-Unity modules remained unin­habited for two years, during which time the Russian Mir station was still in operation, until the Zvezda module was added to Zarya-Unity in July 2000, making it fully habitable. Further modules, such as Destiny, Pirs and Poisk, Tranquility and Columbus, among many others, were added later on. The station serves as a unique laboratory operating in microgravity, with research carried out over the years spanning numerous fields in astronomy, astrobiology and meteorology.

Being in low earth orbit and of significant size, the ISS is easily visible from several locations on the earth’s surface as it passes overhead close to sunrise and sunset. The ISS does not generate any significantly bright light of its own, so much like all other satellites and the planets, it is visible from the earth’s surface only due to surface reflection of light originating from the sun. During the day, any passes of the ISS are unfortunately not visible to the naked eye due to the sun’s brightness outshining it. During late night hours, on the other hand, the ISS would be in the earth’s shadow, and thus no sunlight would reach the ISS for it to reflect and be visible. The only moments, therefore, where sunlight would reach the ISS and the sky would be dark enough for the ISS not to be outshone by the sun are indeed the hours close to sunrise and sunset.

When such moments occur, the view of the ISS can indeed be rather striking. Depending on the observing position and particular ISS pass, the ISS can be seen passing overhead at different angles from different locations. The best passes to observe would be those where the ISS would pass directly overhead, at a 90-degree angle to the horizon, also known as zenith. The ISS can reach peak brightness as high as the brightest planet, Venus, when directly overhead (magnitude of around -4), and it is indeed the brightest artificial object in the sky, aside from particular satellite flares.

So how can one tell when a fortuitous pass of the ISS will occur overhead from Malta? Several mobile and web applications exist to provide details of such passes, where the user need only indicate the country/location of observation and detailed information about the next ISS passes from that location is provided. The next bright pass, visible from Malta, will be on April 24 at 5:47am.

Dr Josef Borg completed a PhD in Astronomy at the Institute of Space Sciences and Astronomy, University of Malta, and is currently a researcher at the Faculty of Health Sciences at the University of Malta.