Here's Why It Was So Hard to Track That Chinese Rocket That Fell to Earth
- Earlier this month, part of China’s Long March 5B rocket reentered Earth’s atmosphere.
- As the spacecraft tumbled toward our planet’s surface, experts raced to accurately calculate its trajectory. It wasn’t an easy task.
- A number of variables—including Earth’s gravity, weather conditions, and an object’s surface finish—make reentry predictions difficult to calculate.
In early May, the 23-ton core stage of a Chinese rocket, the Long March 5B, hurtled uncontrolled back toward Earth. The world held its breath as experts and amateurs alike toiled away, plotting the trajectory of the rocket as it plunged to Earth’s surface.
Though the rocket stage ultimately broke apart over the Indian Ocean, its reentry caused quite a stir, because projections revealed there was a minuscule chance it could have landed in a densely populated region. (In a similar situation last year, the tumbling rocket stage of another Long March 5B rocket missed New York City by 30 minutes.)
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While the U.S. Space Force’s 18th Space Control Squadron and other entities do a great job of tracking these fast-moving objects, “what’s very difficult is when an object is coming back down re-entering through the atmosphere in an uncontrolled fashion,” Carlos Niederstrasser, an aerospace engineer at Northrop Grumman, tells Pop Mech.
Analysts rely on a number of computer models to make predictions about where objects will land, but that’s a tough task, even for those with access to sensitive information and high-tech equipment. Many of these models provided differing estimates of when China’s Long March 5B rocket would reenter, sparking confusion on social media and beyond.
Niederstrasser likens generating reentry models to crafting a weather report: Not all models are equal, and there are a lot of uncertainties that, when compounded, could result in vastly different predictions.
“You hear in weather reports that the European model says we might get 4 inches of snow, while the North American model says we might get only 3 inches of snow,” Niederstrasser explains. “Beyond that, the models need certain data and certain assumptions feeding into them. That’s where you end up getting additional significant variability.”
Tracking and predicting the reentry of these objects is a tough task—one that’s only going to become more critical as we continue to crowd Earth’s low orbit with additional satellites.
What a Drag!
Objects that circle Earth typically do so in a circular or elliptical orbit. However, when an object draws nearer and nearer toward Earth’s atmosphere, it begins to experience air resistance, or aerodynamic drag, and slows down. This marks the final stages of its life in Earth’s orbit.
When analysts make estimates regarding the drag an object experiences as it begins reentry, they must account for an important variable: the density of Earth’s upper atmosphere, or the ionosphere. Changes in atmospheric density influence how much drag an object will experience as its orbit around Earth decays.
Everything from global weather patterns to Earth’s day and night cycles influences atmospheric density. Solar activity also plays an important role. “The density of the atmosphere, it turns out, is highly affected by our sun,” Niederstrasser explains.
Solar weather can heat up molecules in the ionosphere, causing this atmospheric layer to expand and, in some cases, contract. And while we’re able to reliably predict the larger cycles that influence our sun’s solar activity, smaller, day-to-changes in solar weather are harder to predict.
In addition to the density of Earth’s atmosphere, there are other variables—like gravity, for example—that feed into these models. “Earth’s gravitational field is not perfectly even,” Niederstrasser says. “We have mountains. We have oceans. That all affects the gravitational field a little bit.” These fluctuations can minutely influence a de-orbiting object’s path as it slowly tumbles toward the surface.
A Whole Lot of Junk
“On top of nature’s variability, you have the unknowns of the rocket body itself,” says Niederstrasser. And the more information analysts have about the rocket, the better. An object’s size, shape and finish influence how it will reenter Earth’s atmosphere.
More massive objects are difficult to slow down. This means they’ll likely stay in orbit a little longer. Smaller, less massive objects have an easier time re-entering Earth’s atmosphere.
Rivets, weld lines, and other seemingly minor features can impact how much drag is exerted on an object and, in turn, how fast it enters Earth’s atmosphere. “Clearly, we make rockets to be as smooth and aerodynamic as possible, but there is some metal, paint and other material on the surface that does impact that surface finish,” Niederstrasser says.
Calculate It Yourself!
Niederstrasser breaks down an equation that analysts use to track objects reentering Earth’s atmosphere. This one shows the deceleration force on an object due to atmospheric drag.
ad = (1/2)(CdA/m)ρv2
ad represents the acceleration (or deceleration force).
Cd is the coefficient of drag. According to Niederstrasser, it “captures the overall shape (aerodynamic or not) and the surface finish of the object.”
A represents the cross sectional area of the object facing the wind. If objects are tumbling uncontrolled along their orbit, he explains, this variable will be constantly shifting.
m is the mass of the object. “An interesting tidbit is that if you are observing the object from the ground and don’t have ‘inside’ information, it is impossible to separately estimate Cd, A, and m,” Niederstrasser explains.
Rho (ρ) represents the density of Earth’s atmosphere and is determined by mathematical models.
v is the velocity of the object.
The size and shape of an object in relation to its flight direction can also provide important clues about how much drag it’s experiencing.
Imagine sticking your hand out of a car window. When you angle your palm parallel to the road, air easily moves around it. When you angle your palm perpendicular to the road, you feel the wind press against it and push it back. “At some points, its skinny edge is going to be toward the wind and at other points, its big side edge is going to be against the wind,” Niederstrasser explains.
During an uncontrolled entry, the side of the object facing the wind will change. In the case of the Long March 5B rocket, he says, “we knew it was tumbling because optical observations showed it was flashing brighter and dimmer brighter and dimmer.”
But there are other considerations, too, including what’s on board. Spent rocket stages, for example, may leak leftover propellants or gases that have the potential to push them into different orbits.
In most cases, the analysts running these models have information from the manufacturer that can shed light on the variables with which they’re working. But sometimes, as with the reentry of China’s Long March 5B rocket—they don’t have that information and have to estimate.
Not This Again
This certainly isn’t the first time space junk has reentered Earth’s atmosphere. In 1979, NASA’s 84-foot-long space station, Skylab, reentered over the South Pacific, spraying a field of debris over western Australia. And just this month, a SpaceX Falcon 9 rocket stage made an uncontrolled reentry over Washington State. Several farmers in the area found bits of the rocket on their property.
China has developed a reputation for dropping rocket stages where they shouldn’t. Last year, a similar core stage fell back to Earth after the country launched yet another Long March 5B rocket. Debris from that rocket body reentered Earth’s atmosphere over the Atlantic Ocean and sprayed debris across western Africa.
There’s a lot of junk in low-Earth orbit, and we’re bound to see chaotic reentries again, says Niederstrasser. There are over 34,000 objects larger than 10 centimeters orbiting Earth—ranging from things like spatulas to 100-foot-long rocket bodies. With the satellite constellations from companies like SpaceX and OneWeb, we’re launching more objects into orbit than ever before. These objects and their orbits will eventually decay.
To mitigate the amount of material that could fall back to Earth, many private companies and several of the world’s space agencies are starting to think ahead. They’re either developing elaborate plans to capture space junk in orbit, or they’re designing rockets and satellites that sustainably remove themselves from orbit.
Ultimately, these events captivate the entire world because their trajectory is almost entirely unpredictable. Statistically speaking, it’s highly unlikely any piece of debris that plummets to Earth would crush you.
But on the extremely remote chance a piece of space junk does happen to fall in your yard, don’t get too close—it could release noxious fumes like nitrogen tetroxide. (This was one of the concerns NASA had when boaters got too close to SpaceX’s Demo-2 capsule last year.) At least you won’t have to worry about the damage: Per the 1967 Outer Space Treaty and the 1972 Liability Convention, the country responsible for launching and operating the spacecraft is liable for any damage it causes.
In the meantime, we’ll keep our eyes trained on the skies in preparation for the next time a piece of space junk makes a grand reentry back through Earth’s atmosphere.
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Published at Thu, 27 May 2021 17:10:00 +0000