Have you looked at the night sky recently and have you seen a strange line of luminous points? Have you noticed an increase in the number of UFO reports mentioned in the news and social networks? If so, you may have already been touched by what many have come to fondly call Elon Musk's "Space Train": a line of closely grouped Starlink satellites that are making their way around the world.
Some have wondered what is special about Starlink satellites that allows them to be visible from the ground with the naked eye, but in reality that is nothing new. It's about being in the right place at the right time, both for the observer Y The spacecraft in question. The trick is to make the object in space capture sunlight when it is already established, from the point of view of the observer. It is essentially the same reason why the Moon shines at night, but on a much smaller scale.
Phenomena are known as "satellite flare", and pursuing them is a favorite pastime of the avid observers of heaven. If you know when and where to look on a clear night, you can easily detect the International Space Station as it moves through the sky thanks to this principle. NASA even offers a service that uses email or SMS to inform you when the ISS should be visible from your location.
What makes Starlink satellites unique is not that we can see them from the ground, but that there are many of them flying in a straight line. The initial launch launched 60 satellites in a much more closed formation than we have seen before; Elon even warned that collisions between individual Starlink satellites were not out of the question. The cumulative effect of these proximity satellite blasts is somewhat surprising, and it is understandable that people are concerned about the appearance of the night sky when the 12,000 Starlink satellites are in orbit.
The good news is that the effect is only temporary. As the satellites spread out and start individual maneuvers, that long line in the sky will vanish. But before Elon's "Space Train" goes away forever, let's see how it was created and how this unique phenomenon can still be glimpsed.
Like a deck of cards
As a general rule, satellites are expelled from their carrier rocket with a certain degree of force, so they will remain at a safe distance from each other. To keep complexity and reliability low, this mechanism in many cases is nothing more exotic than a large spring that can be launched remotely. Even this slight push is enough to make sure that the gap between the objects continues to expand, as they move through their now separated orbits (although still very similar).
But the launch of Starlink was unique in the sense that no motorized ejection system was used. Instead, the satellites were allowed to move away from the second stage of the Falcon 9 driven by nothing more than simple physicists. This was not only cheaper and easier than creating an ejection system for 60 satellites, but keeping them together at launch also simplifies the orbital maneuvers necessary to move them to their final orbits.
So, how was this achieved? We can get our first clue by looking closely at the live coverage of the SpaceX mission. Before the satellites are deployed, the movement of the Sun and Earth in the background shows that the second stage of Falcon 9 was in a flat turn. If you've ever seen live coverage of previous SpaceX releases, or really any launch, you'll know how unusual this is.
Normally, the launch vehicle would be stabilized so that the only impulse imparted to the payload is that of the ejection force. But here, SpaceX intentionally accelerated the second stage of the vehicle so that when the satellites were launched, they were essentially "thrown" by the centrifugal force. Speaking about the deployment at a pre-launch press conference, Elon Musk said that "it will look like extending a deck of cards on a table."
To help visualize this, we can recreate the implementation on a smaller scale with everyone's favorite rocket simulator, the Kerbal Space Program:
Seen from this perspective, it is clear how the "Space Train" was initially formed by this unusual method of satellite deployment.
See them before they leave
If that was the end of the story, we might expect to see a relatively narrow line of Starlink satellites in the sky for quite some time. But these satellites have a job to do, and their own individual propulsion systems. Once the inertia imparted by the rotation of Falcon 9 gave them a little space to breathe, they could activate their thrusters and begin to perform the necessary maneuvers to place them in their final orbits.
While reading this, the "Train" is breaking. The low-thrust ion engines used by Starlink satellites mean that their separation will not occur overnight, but with each pbading day there is more distance between this group, which was once extremely close. No one outside of SpaceX knows exactly how long this process will take, but a few weeks is probably not an unreasonable estimate given the movement we've seen since launch.
That means there is still time to see them for yourself. The N2YO website is currently tracking the mbad of Starlink satellites and will generate a list of dates and times in which they should be visible from their location. Keep in mind that, since these satellites are actively maneuvering, the exact time at which they make their pbad could vary slightly from these calculations. To get the best results, adjust your eyes to the dark and start looking a few minutes before the planned pbad.
A potentially more tenuous future
Logically, it could be badumed that a "Space Train" should be visible during the first weeks after each new batch of Starlink satellites is launched. So, even if you end up losing it due to cloud coverage or the fact that you do not want to get up before dawn, there should literally be hundreds of opportunities to do so in the next few years.
But we already know that these satellites were just one step above the prototype stage, and that SpaceX anticipates that they will go through several design changes before they actually start to increase production. Elon has also commented that he has directed the Starlink team to look for ways to reduce the reflectivity of satellites. It is possible that hardware adjustments to the next set of satellites to launch reduce or completely eliminate the flash effect.
One thing is for sure: whether the next generations of Starlink satellites manage to eliminate the effect of the "Space Train" after launch, there is no reason to worry that the night sky of the future is crossed by dotted lines illuminated. Once all maneuvers have been completed and deposited in their final orbits, the Starlink satellites will no longer be visible in the night sky than the thousands that are already flying around the head.
Images of Starlink satellites in orbit taken from a video posted on YouTube by Movie Vertigo.