On August 24, 2017, a SpaceX Falcon 9 rocket carried the FORMOSAT 5 satellite to an altitude of 720 km. This was Taiwan’s first indigenously built remote-sensing satellite, and its space journey was unique.
Most rocket launches follow a curved trajectory after launch, where the rocket reaches a predetermined altitude through a curved path and then skirts horizontally over Earth before dropping the payload off. The payload is then manoeuvred to its final destination using onboard thrusters.
The FORMOSAT 5 launch didn’t happen like this.
The rocket was originally scheduled to launch in 2016, but an explosion at a SpaceX site delayed the event. As a result, FORMOSAT 5 ended up being the sole payload for a future Falcon 9 launch. Usually, each Falcon 9 typically carries payload weighing as much as 17 FORMOSAT 5s.
Faced with having to launch a single and much lighter payload, SpaceX realised that the rocket needn’t follow the usual curved route. Instead, SpaceX engineers figured it would be more efficient for the FORMOSAT-only Falcon 9 to lift off almost completely straight up.
According to a paper published earlier this year, this unusual flight path was accompanied by some strange effects in Earth’s ionosphere.
The ionosphere is an intermediary layer of the planet’s atmosphere. It is composed of a peculiar state of matter called plasma – essentially a sea of free electrons that are knocked off gas atoms by energetic ultraviolet radiation from the Sun.
The motion of a rocket through the ionosphere generates large disturbances in the layer that propagate faster than the speed of sound, generating shockwaves. Scientists created a map of the total electron content (TEC) in the ionosphere to track these disturbances.
Previous studies have observed that rocket launches induce V-shaped shockwaves. The shape of the shockwave depends on the trajectory of the rocket, which is horizontal in most cases. However, the FORMOSAT 5 launch was special because the Falcon 9 flew straight up through the ionosphere and thus created a different shape: circular.
To visualise the difference in shapes between the two kinds of shock waves, imagine V-shaped waves as the ones generated by a ball dragged on the surface of water and circular waves as those by a ball dropped into the water from a height.
According to the study’s authors, the circular shockwaves generated by the launch were the first ones of their kind recorded as well as the largest rocket-induced shockwave of any kind, reportedly as big as Maharashtra, Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Chandigarh, Delhi, Uttar Pradesh and Bihar combined.
The appearance of a circular shockwave five minutes after launch was followed by a giant ‘hole’ in the ionosphere, around 13 minutes after lift off. This ‘plasma hole’ was basically a vast expanse in the ionosphere that had had its electron content depleted by 10-70% (in comparison to days without rocket launches) over its three-hour lifetime. Rocket flights before have been found to have a similar impact on the ionosphere but not at such scale.
When burning through a stage, a rocket expels a large amount of exhaust that contains chemicals that have a tendency to react with each other. These reactions produce hydrogen and water vapour among other by-products; all of them have been found to consume electrons in the ionosphere. As it turns out, Falcon rockets use the Merlin engine, which in turn uses liquid oxygen and rocket-grade kerosene as propellant. These two react to form water vapour, and the latter can combust to produce hydrogen.
While both the giant circular shockwave and the ionospheric hole caused large variations in plasma content, the gradients caused by the latter could actually have thrown off GPS signals to the extent of approximately one metre, the authors claim.
This error is hardly significant. In fact, larger errors can be caused by other sources. Charles H.C. Lin, the lead author of the paper, admitted to Ars Technica,”Without considering the rocket launch effects, there are errors from the ionosphere, the troposphere and other factors that will produce up to 20-meter errors or more.”
However, in a statement issued by the American Geophysical Union, the body that publishes the journal Space Weather, Lin says that such rocket-induced errors could be amplified in the event of high solar activity. Such larger errors could in turn affect aviation and military applications, even self-driving cars back on Earth.
It is important to document the possible effects of rocket launches on the temporary and local makeup of the ionosphere from a public information standpoint as well. Jim Oberg, a former NASA space engineer, is noted for documenting sky-based light apparitions and giving physical explanations for claimed UFO sightings. He has been actively tracking such events associated with SpaceX launches.
As recently as two months ago, a spiral glow observed in the sky above Khartoum (possibly a result of a Falcon launch) led to a wave of media outlets claiming a UFO sighting. But Oberg commented thus: “… it reminds us again of the world-spanning, globe-girdling scope of human activity in outer space which is redefining pre-spaceflight concepts of proximity and distance, time and speed, light and dark.”
Lin echoed this sentiment, telling Ars Technica, “Human are entering an era that rocket launches are becoming usual and frequent due to reduced cost by reusable rockets. Meanwhile, humans are developing more powerful rockets to send cargoes to other planets. These two factors will gradually affect the middle and upper atmosphere more, and that is worthwhile to pay some attention to.”
Ronak Gupta recently completed his masters in fluid mechanics from the Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru.