Why would the Chinese government be flying a large stratospheric balloon?

On Thursday, US officials confirmed that a high-altitude balloon, launched days ago by the Chinese government, has been flying over the northern United States. This has since become an international incident and led the US secretary of state, Antony Blinken, to delay a high-profile visit to China to meet with the nation's president, Xi Jinping.
The balloon's flight raises several questions--such as, just what the heck is it doing there? This story will address what is known and not known about the flight. The information below is based on public statements, other news reports, and an interview with a stratospheric balloon expert, Andrew Antonio, whose company, Urban Sky, is developing the world's first reusable stratospheric balloons for remote sensing.
How big is the balloon?
This information is not precisely known. Some media reports have suggested it is about the size of "three school buses." It is clearly a very large balloon, however, much bigger than would be used typically to gather data for meteorological purposes.
How high was it flying?
At an altitude of about 60,000 feet, according to the Pentagon. This is in the lower stratosphere but above protected air space for commercial airliners. Even though it is above commercial air space, the United States still considers this region of the atmosphere above its borders to be sovereign air space--far below outer space, which begins at 328,000 feet (or 100 km) and does not belong to any one nation.
Where was it going?
The balloon was observed over Montana on Thursday but appears to have drifted further to the southeast, over Kansas, on Friday.
What was it doing?
According to a statement from a Chinese Foreign Ministry spokesperson on Friday, flying the balloon over the United States was an accident: "It is a civilian airship used for research, mainly meteorological, purposes. Affected by the Westerlies and with limited self-steering capability, the airship deviated far from its planned course. The Chinese side regrets the unintended entry of the airship into US airspace due to force majeure," the statement said.
The true purpose of the balloon, be it this or something else, is not presently known. But again, given its large size, the balloon likely was not being used to gather meteorological data.
What other reason might a large balloon be in the stratosphere?
The obvious, and probably correct answer, is spying. China, of course, has excellent observation satellites in low-Earth orbit, just like the United States. However, Antonio said, stratospheric balloons have some key advantages. By raising and lowering itself in the atmosphere, a stratospheric balloon can maintain its position over an area for hours, days, or even weeks. This provides higher-resolution imagery, with a persistence that satellites cannot match, at a far lower price. Urban Sky, for example, is developing small stratospheric balloons that can be used to monitor the progress of wildfires and efforts to contain them.
So the Chinese launched this balloon to spy directly on the United States?
Probably not. The Chinese would have known that sending a clearly observable balloon into the US heartland would be a provocative action, and they are unlikely to have done so on purpose.
The most likely scenario, Antonio believes, is that the termination mechanism, which is used to bring down a balloon at the end of its desired flight time, failed. Typically a stratospheric balloon will have one or more backup termination mechanisms, but a technical problem would explain why a balloon launched in China days or weeks ago could have eventually drifted into the United States. (The Chinese government may not want to admit this technical failure publicly.) The prevailing currents in the stratosphere would appear to support this theory of a drifting balloon the Chinese government had lost control of.
The time to fly such a balloon, for spying purposes, would be during the summer months, Antonio said. That's because during the winter the winds throughout the stratosphere are much more uniform in the Northern Hemisphere. This means that raising and lowering the balloon would provide very little steering capability. "Controlled stratospheric flight is a thing, but it's not something you can really do over the United States at this time of year," Antonio said.
Some photos showed large solar panels. What are those for?
They're likely being used to power the onboard instrumentation, such as powerful cameras or telescopes, as well as a mechanism to raise and lower the balloon, if one exists on board.
If this were a spy balloon, what is there to look at in Montana?
There are some Air Force facilities with nuclear capabilities in the state, but there is no information to be gleaned by this balloon that has not already been collected by satellites. US military officials do not believe there is any strategic advantage for China to make observations with a balloon. This lends more credence to the notion that it has gone off course, perhaps due to the failed termination system.
Does the US military have strategic capabilities in the stratosphere?
Yes. Working with companies such as World View and Aerostar, the US military is funding the development of so-called ISR, or intelligence, surveillance, and reconnaissance capabilities in the stratosphere. Urban Sky does not have military contracts for these kinds of services, Antonio said.
So what does this all mean?
In recent years we have seen the increased militarization of space. It therefore should not be a huge surprise that we are also seeing such capabilities developed in the stratosphere, which lies above the troposphere and extends several tens of kilometers above the surface of the Earth.