What’s the Difference Between Spying by Balloon Versus by Satellite?

Posted in: International Law

The Chinese government has a credibility problem with respect to the surveillance balloon that the United States shot down after the balloon completed its trans-continental flight earlier this month. On the one hand, China contends that the U.S. shot down a weather balloon that had drifted off course. On the other hand, China has accused the U.S. of conducting its own surveillance-by-balloon program that has repeatedly invaded Chinese airspace—a claim that the U.S. dismisses as false whataboutism.

To be sure, China’s claims are not logically inconsistent. It is possible that the apparent fleet of Chinese surveillance balloons is merely making meteorological observations while the U.S. is the real offender. But the data that the U.S. has announced it will declassify will likely render the first half of that claim impossible to maintain. You don’t need a weatherman to know which way the wind blows.

Meanwhile, what about China’s whataboutism? Let us grant that China is lying (or mistaken) and that the U.S. does not operate surveillance balloons over the People’s Republic. So what? No one denies that the U.S. and other countries with the capability to do so operate surveillance satellites that peer into the territory of foreign sovereigns. What’s the difference between a balloon and a satellite?

The Science of Balloons and Satellites

For anyone who paid attention in high school science classes, that question is fairly easy to answer. Balloons—whether or not they include a means of controlled navigation and propulsion, as the Chinese surveillance balloon apparently did—stay aloft because they are filled with gases that are lighter than the surrounding air. As a balloon rises, the outside air becomes thinner, so that the balloon and its payload eventually outweigh the surrounding atmosphere. The lightest balloons can make it into the lower stratosphere but not higher. The Chinese surveillance satellite the U.S. shot down was flying about 11 miles from the Earth’s surface—in the lower reaches of the stratosphere.

By contrast, satellites maintain their orbit through momentum. An object can remain in a stable orbit around the Earth if its “horizontal” momentum matches the Earth’s gravitational pull so that instead of falling down to Earth it “falls” around the Earth. If the object’s momentum overcomes the Earth’s gravity, it will either end up in a more distant orbit or achieve escape velocity and venture off into outer space.

We can use Newton’s law of universal gravitation and some fairly simple calculations to show that the closer a satellite is to the Earth, the faster it must move to maintain a stable orbit. (The required speed varies inversely with the square root of distance from the Earth’s center.) However, even a fast-moving satellite cannot maintain a stable orbit within the Earth’s atmosphere because the drag from the surrounding air would slow the satellite and lead its orbit to decay. Accordingly, satellites even in so-called low-Earth orbit are hundreds of miles above the Earth’s surface, with a theoretical lower limit of about a hundred miles.

We thus have a rough and ready answer to the scientific question: satellites orbit the Earth at ten or more times the distance from the surface compared to the height at which balloons fly. They also move much faster than balloons. Satellites in geostationary orbit—that is, “parked” over a fixed position by matching the Earth’s rotational speed—move at thousands of miles per hour.

That scientific distinction illustrates the attraction of surveillance balloons. Modern satellites can capture remarkably clear images of Earthbound sites but a comparably equipped surveillance balloon, in virtue of being ten or more times closer to the Earth’s surface, can necessarily capture even greater detail. Moreover, a balloon in the troposphere (the lowest layer of the atmosphere) or lower stratosphere can intercept radio signals revealing phone or internet communications that are too faint for a satellite to detect.

The Legal Boundary Between the Atmosphere and Space

When a nation sends a balloon or other flying object into the territorial airspace of another nation without consent, the former violates the latter’s sovereignty, potentially endangering lives due to the possibility of colliding with airplanes and risking the outbreak of war. By contrast, a satellite that passes over a sovereign country does not infringe its sovereignty, because a nation’s airspace does not extend indefinitely up and out into the cosmos. Satellites in orbit are, so far as international law is concerned, like ships on the high seas. Just as no nation has sovereignty over the oceans, so no nation has sovereignty over space.

Where is the line between domestic airspace and international outer space? Oddly, although various multilateral treaties set forth various duties regarding the signatories’ conduct in and respecting space, none includes a definition of space. It is possible that that curiosity could someday lead to conflict. After all, even with a clear definition of territorial waters (up to 12 nautical miles) in Article 3 of the United Nations Convention on the Law of the Sea, disputes sometimes arise over the validity of various nations’ claims about the extent of their territorial waters.

For present purposes, however, we need not worry about the boundary between sovereign territorial airspace and outer space. The Chinese balloon and the various unidentified objects the U.S. has shot down in the last week were already in or about to enter the airspace of the U.S. or Canada (which has cooperated with the U.S.). Meanwhile, satellites operate in what clearly constitutes outer space. They do so only somewhat constrained by law.

Article IV of the Outer Space Treaty (to which the U.S., China, and many other countries are parties) forbids placing “nuclear weapons or any other kinds of weapons of mass destruction” “in orbit around the Earth.” That same provision forbids “install[ing] such weapons on celestial bodies, or station[ing] weapons in outer space in any other manner.” The next paragraph of the same article requires parties to use “the Moon and other celestial bodies . . . exclusively for peaceful purposes.”

Parsing that language, it is clear that it forbids state weaponization of the Moon and planets (other than Earth). However, by expressly forbidding placing in orbit nuclear weapons and other weapons of mass destruction but not all other weapons, the Outer Space Treaty appears to allow satellites armed with conventional weapons. That gap has led to calls for additional limitations on the weaponization of space—especially since 2021, when Russia used a missile to destroy one of its own defunct satellites. In addition to endangering other satellites by creating hundreds of pieces of space junk, the test could be a portent of a future war in orbit.

Preventing hostilities in orbit will pose at least a three-fold challenge. First, there is a drafting challenge. Satellites already play a substantial role in terrestrial warfare. The U.S. has provided Ukrainian forces with information about Russian troops’ movements based in part on images obtained from satellites. Meanwhile, every time a combatant uses GPS technology to navigate, satellites (which make GPS possible) are weaponized. Drafters of a new treaty placing limits on the weaponization of orbital space would need to carefully consider whether to forbid even these sorts of indirect military uses.

Second, there must be the political will for any new agreement. At the moment, that seems like a tall order in light of the frosty relationship between the U.S. and its military rivals Russia and China. A multilateral treaty further limiting space weaponization would not be very valuable if the leading military powers did not sign on.

Third, as with all treaties, there would be a question of compliance. Sending surveillance balloons into other nations’ sovereign territory without their consent clearly violates international law; yet China appears to have concluded it was worth trying to get away with doing so. Even more tellingly, for the last year, Russia has been flagrantly violating the most basic principles of the law of war—first by launching an illegal war for the purpose of territorial expansion and then by targeting civilians in waging that war.

Space may be the final frontier but in venturing past the Earth’s atmosphere, we seem unlikely to leave behind our conflicts and human failings.

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