A Cygnus cargo spacecraft was launched to the International Space Station (ISS) on May 21, 2018. Among the supplies that it carried were nine small educational and commercial satellites know as CubeSats. One of the satellites is named EQUiSat. It was designed and built by Brown Space Engineering (BSE) undergraduate students.
Four of these satellites were released on July 13, 2018 from the ISS by astronauts using a device called the NanoRacks CubeSat Deployer. They were ejected at about 1.5 m/.s (3 mph.) Initially they were spaced about 10 to 50 cm (4 to 20 inches) apart. They quickly moved ahead of the ISS together in a cluster as the distance between them more gradually increased over time. EQUiSat is the second cube from the right.
Below are maps showing the ground track of the small satellites as they orbit 400 km (250 miles) above the Earth. Also shown are the paths they take across the sky as they pass above the radio ground station at Ladd Observatory. Traveling at a speed of 27,600 km/h (17,100 mph) it takes only 92 minutes to orbit the Earth.
- The satellites are identified by a two letter tracking code:
- PC is RadSat-g
- NZ is EnduroSat
- PA is EQUiSat
- PB is MemSat
The first pair of diagrams shows the distance between the “flock” of satellites and the ISS on July 16th. This was the first day that orbital elements were published for the CubeSats. It took 3 days before they were separated from the ISS by enough distance that their position could be measured. They lead the ISS by about 400 km (250 miles. ) But they are too close together to be distinguished from each other and each individual satellite can not yet be tracked accurately enough for long term predictions. The tracking is good enough to calculate when they pass above a ground station during the next day or so.
The next pair shows the dispersion on August 23rd. This is when they were far enough apart to easily distinguish between them. They are all above the ground station at the same time but with a slight delay in rise and set times. The first one sets about 3 minutes before the last. The Doppler shift of the radio transmissions is also slightly different for each one. The distance between the first and last satellite is about equal to the distance between Providence and Chicago. The ISS is now half way around the world from them – over Australia.
The final pair shows the dispersion of the satellites on October 7th. They are still in very similar orbits but are no longer passing over the ground station at the same time. One is setting below the horizon as the next one rises. The distance between the leading and trailing satellites is now about equal to the distance between the BSE ground stations in Providence and Rome. It takes EQUiSat only 19 minutes to cover the distance between the stations.