Pentagon officials announced on Thursday that they had detected a Chinese “surveillance balloon” flying over Montana. On Friday the Pentagon’s press secretary said that the balloon is now over the central U.S. and moving eastward at an altitude of about 60,000 feet. Observers on the ground have been able to snap photographs and videos of the object, and the incident has prompted Secretary of State Antony Blinken to cancel a planned trip to China.

Although China’s Ministry of Foreign Affairs quickly claimed that the object is merely a civilian weather observatory blown off course, later on Friday, the Pentagon press secretary, Air Force Brigadier General Patrick Ryder, held a press briefing where he stated, “We know that it’s a surveillance balloon.... We know this is a Chinese balloon and that it has the ability to maneuver.”

This maneuverability is beyond the capabilities of most high-altitude balloons, says John Villasenor, director of the Institute for Technology, Law and Policy and a professor of electrical engineering, law, public policy and management at the University of California, Los Angeles. “The only balloons I’ve ever heard of are the ones that can go up and down or the ones that don’t do anything—they just go completely at the mercy of the winds,” he says. “But the phrasing from these spokespeople seems to suggest some greater degree of control than that. I don’t know what that means, but I think it’s notable.... It adds some more complexity to the whole thing.” In addition to its maneuverability, the surveillance balloon differs from a typical weather balloon in other ways, according to the Weather Channel. First, it has been airborne for days, but weather balloons typically remain up for only a couple of hours. The Chinese balloon is also roughly the size of three buses, whereas weather balloons typically expand to only about 20 feet across.

Scientific American spoke with Villasenor about why the aircraft’s maneuverability is so unusual and how such surveillance balloons compare with satellites.

[An edited transcript of the interview follows.]

Is it possible to steer or otherwise control a typical high-altitude balloon?

It can be controlled, but let me be careful about what I mean by that. Balloons go with the wind. And so the only control that a balloon has is: in some balloons, you can control the altitude—you can make it go higher or lower. To the extent that the wind speed and direction varies with altitude, you can change the altitude, within some limits. And that can give you some measure of control—but not anywhere near the level of control that you have [with an airplane]. You can’t turn around and go the other direction; you can never go upwind in the balloon. You’re always going to be moving with the atmosphere that surrounds it.

Given existing knowledge of wind patterns at different altitudes, would it be possible to launch a balloon with the goal of having it end up in a specific location—such as in the air over Montana, where the recently discovered Chinese one was when the Pentagon announced it had detected the object?

The general prevailing wind patterns are well known. I’m sure almost all Scientific American readers know it takes longer to fly from the East Coast to the West Coast than from the West Coast to the East Coast. The flights are longer because you’re generally flying against the wind going west instead of with the wind going east. Those prevailing winds are the same reasons why a balloon launched in China can end up in the U.S., whereas if we launched one from Washington State, it wouldn’t be able to go west. So yes, if you want a balloon to end up in a certain place, you can strategically choose where you launch it from and you can have some reasonable degree of control [over where] it’s going to end up. But you can’t control it within, you know, one mile. If you wanted it to end up over Montana and not over, let’s say, Texas, then you might launch it from a particular place.

Just how big does a balloon like this have to be for the Pentagon to pick it up?

It’s not only the size of it but what it’s made of. Different materials will reflect radar differently, for example. How visible it would be would depend on how big it is, what it’s made of and also the detection technology being employed to track it. And I will also say that once you know something is there, it’s a lot easier to find it. Part of finding things is knowing where to look, and [once] you know roughly where something is, and you’re looking very carefully, you’re going to be able to track it.

How can the Pentagon tell it is actually a spy balloon, as opposed to a civilian project for meteorology research, as the Chinese Ministry of Foreign Affairs claimed?

If you took it out of the sky and examined what was on it, that would probably answer the question! If it’s a weather balloon, then it would be equipped with sensors to measure things like temperature, and so on. If it’s a spy balloon, then it would be equipped with, perhaps, high-resolution cameras or equipment to detect electromagnetic signals, things like that. So if you actually physically had access to it, it would be pretty easy to assess whether it was really simply a weather balloon that had blown off course or whether it was created with the idea of surveillance. But without access to it, I don’t know how you would get that information.

As a platform for surveillance, how does a balloon compare with a satellite?

A balloon is not a particularly good platform for a couple of reasons. One is that you can’t steer it, really, and it only passes over the place that you’re looking at—if you even get lucky and are able to place it correctly—once. Whereas the satellite is in orbit, so it goes around and around. Also, the U.S. would be perfectly within its rights ... to take down a balloon that was launched by a foreign country ... and that was flying in the U.S. airspace. Countries have sovereignty over their airspace, but outer space is different. You can launch a satellite, and it can fly 150 miles over any other country, and most countries can’t—and even those who can, will not—take it down. There’s a norm that satellites operate in a zone that everyone has access to, whereas balloons operate in the sovereign airspace of the particular country. That’s a huge difference right there.

What advantage might a balloon have?

If you’re trying to get high-resolution imagery of something, distance matters. For example, if you’re in a balloon that’s 12 miles or 15 miles up, that’s an order of magnitude closer to the surface than a satellite. All else being equal, you’ll have that much higher resolution, so that could be an advantage of imaging from a balloon. But again, it’s a very blunt instrument to use because of this very poor degree of control—almost none—that you have over where this thing is going to end up.

When you’re at 50,000 or 60,000 feet, then you can see, but you can’t see [far]. If you’re 60,000 feet over Montana, you can’t see Texas, right? Whereas if you’re a satellite that’s hundreds of miles high, then you can see a lot farther. Balloons are much, much closer to the surface. So on the one hand, you’re closer to the things you might be looking at, but you have a much narrower view of the Earth’s surface.

The Pentagon has stated it does not currently plan to shoot the balloon down. Why not?

They say they don’t want to hurt people or damage property on the ground. For whatever reason, they—at least now—are stating publicly that they have elected not to. In terms of the information [the balloon] was gathering, they seemed to suggest that it wasn’t [a threat]. China already has capabilities like satellites, and they didn’t seem to think the balloon was going to be dramatically different. But I’m quite sure that they could take it down if they wanted to.

If it’s not shot down, what will happen to the balloon?

Most of the time, these things eventually come down somewhere—well, they all eventually come down somewhere. The question is where. Maybe it ends up in the Atlantic Ocean somewhere—without knowing more information about it, it’s really hard to know.