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These are multiple satellites chasing each other around the world at the same altitude—in the case of Landsat 7 and Landsat 8, that’s 705 kilometers (438 miles) above the Earth. Here’s a NASA description of one of those constellations—what’s called the A-Train.

Satellite constellations can be used to provide television service, mobile communications, even the GPS systems that help us navigate our cars and track our exercise. Constellations, similarly, play a big role in helping us to study the Earth through satellite remote sensing.

What follows a few excerpts from that conversation with Lacasse.

So, the two Landsat satellites orbit at 438 miles. There are other satellites in that same orbit?

“Well, I’m aware of about 10. The majority of them are NASA, and other international NASA partners. I also found one ... the Peruvian Space Agency has one up there.”

And why are they all sharing the same space with us?

“They all have similar requirements in terms of Earth remote sensing. You know, not only looking at the land, but some of them are also looking at the atmosphere and the ocean.”

How close are they to each other? I mean, are they right on each other’s bumpers?

“Well, for the A Train, the afternoon constellation that we mentioned, they’re as close to 40 seconds to 400 seconds is the range.”

Do they need to be that close?

“Yes, they’re trying to get concurrent observations of different phenomena. If you fly too far apart, things that you want measured simultaneously will actually change in time. So, you want to have those measurements as close as possible together to remove the effects of change.”

But they all have different things they want to acquire as far as information, right?

“They have different sensors that measure different wavelengths for different purposes. Some are capturing their reflected light off the surface of the Earth, or long wavelength temperatures, or they’re sounding for particles in the atmosphere column below them. Some of them can even measure wave heights out on the open ocean.”

And yet collectively, all that data is providing a picture of what’s happening on the Earth’s surface, or happening in the atmosphere?

“That’s the chief goal of all this data collection, and over time, it’s really matured to where the systems are being designed to do exactly that. In the case of the A-Train, the afternoon constellation, those instruments, when they put them together, the data was meant to be included in the algorithms of the other instruments. And that’s used in a modeling of the Earth’s processes. Those models are quite complex and have a lot of data inputs.”

If Landsat has to get out of the way of space junk, do we communicate with the other systems to let them know what we’re doing and why?

“Yeah, each satellite, each operator, is concerned with what’s in their path. We do get common information from the Air Force, whose charged with monitoring space debris, and the orbit of all satellites, and then providing notification to satellite operators when a conjunction is predicted.”

It’s up to each operator to make a determination of what they want to do about it?

 “They tell us when a predicted conjunction is going to happen, the probability of collision, and basically the quality of data that went into calculating those numbers. And then the operator has to make a decision based on that. And it can be a difficult decision. The satellites only have a limited amount of propellant on board. Every maneuver reduces that propellant and basically takes away from the length of the mission. You can’t maneuver for every predicted collision. It is a very difficult decision sometimes.”