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The Chinese successfully launched a Long March 5b... that apparently fell off the West Coast of Africa

On Monday, a massive, out-of-control Chinese rocket fell out of the sky off the west coast of Africa, becoming one of the largest human-made objects ever to make an uncontrolled descent to Earth from space. At first, the rocket seemed to harmlessly slam into the Atlantic Ocean. But now it seems that some pieces of debris may have hit solid ground, according to local reports from Côte d’Ivoire describing metallic objects that apparently fell from the sky.

Ars Technica billed it as Large chunks of a Chinese rocket missed New York City by about 15 minutes with this paragraph

It is perhaps worth noting that before it entered Earth's atmosphere, the core stage track passed directly over New York City. Had it reentered the atmosphere only a little bit earlier, perhaps 15 to 20 minutes, the rocket's debris could have rained down on the largest metro area in the United States.

  1. Was there a large risk of debris damage from this rocket?
  2. Was this really as close as call as they make it out to be if it fell just off the west coast of Africa?
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    $\begingroup$ Close call -- missed NYC by only 4000 miles! $\endgroup$ – Russell Borogove May 15 at 18:59
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    $\begingroup$ What missile defense? $\endgroup$ – Organic Marble May 15 at 19:22
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    $\begingroup$ Low Earth orbits are about 90 minutes period, 15 to 20 minutes are a lot more than "only a little bit earlier". $\endgroup$ – Uwe May 15 at 19:29
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    $\begingroup$ @OrganicMarble US NMD etc en.wikipedia.org/wiki/United_States_national_missile_defense . At this level of risk they usually issue some kind of statement afterwards. $\endgroup$ – user3528438 May 15 at 19:49
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    $\begingroup$ @RussellBorogove - The vehicle was in an uncontrolled, decaying low Earth orbit for six days or so. The uncertainty in when reentry would occur was about one day just a couple of days prior to reentry. A fifteen minute delta in the time at which reentry did occur was well within reason. $\endgroup$ – David Hammen May 16 at 22:28
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This comment is too long to fit as a comment, so I'll write it as a partial answer.

  1. Was this really as close as call as they make it out to be if it fell just off the west coast of Africa?

Definitions of "just missed" and "close call" are subjective, but we can think about the sensitivity of the final state to the initial state. A simulation would show that a very very small difference in velocity initially would have produced a large shift in entry point.

If a reaction control thruster had fired a little differently or if the rocket body had tumbled a different way, it's certainly possible that the effects of drag over the days that the rocket body experienced gradual lowering and slowly spiraled towards the denser layers of the atmosphere would have been different enough to slightly change the orbit's phase, and therefore time to reentry by a fraction of a part per thousand, which is all that would have been necessary to move the point by thousands of kilometers.


Related factoid; Space News' Bridenstine criticizes China for uncontrolled rocket reentry quotes the current director of NASA:

“It was seemingly a successful launch, until we started getting information about a reentry of a rocket body, a reentry that was really dangerous,” Bridenstine said at the committee meeting. “It flew over population centers and it reentered Earth’s atmosphere. It could have been extremely dangerous. We’re really fortunate in the sense that it doesn’t appear to have hurt anybody.”

A slightly earlier reentry could have posed a risk to the United States. The core stage’s trajectory took it over the U.S., from Los Angeles to New York, before going over the Atlantic. A reentry just a half-hour earlier could have resulted in debris landing on U.S. soil.

and then goes on to highlight how Bridenstine's earlier position on India's shooting down of its own satellite and production of space junk was amended by the White House a few days later. Without saying so directly, the article leaves us to imagine that the criticism of China might also have been shaped to some extent by the current US administration.

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I would say that it is exactly as close a call as they make it out to be, except that they seem to think "missing by fifteen minutes" is a close call when it is not. At orbital speeds, fifteen minutes is a sixth of the way around the earth.

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    $\begingroup$ Fifteen minutes is a very close call when the uncertainty is half a day. $\endgroup$ – David Hammen May 17 at 1:11

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