SCOTT SIMON, HOST:
What goes up must come down. When we toss something to the heavens, we tend to have some idea of where it'll land, but that's not the case with a giant rocket launched last week from China. It's expected to come tumbling down to Earth late tonight somewhere. Joining us now is NPR's Joe Palca. Joe, thanks so much for being with us.
JOE PALCA, BYLINE: You're welcome.
SIMON: So where's it coming down? OK, I won't put you on the spot.
PALCA: (Laughter).
SIMON: Why don't we know, in this case, where it's coming down?
PALCA: Well, you know, usually when they send a rocket into space, it kind of - it sends its payload off, and then it drops back down to Earth, and you can plan it so that it drops back over the ocean, or if it does happen to go into orbit, you can save a little fuel and fire rockets and bring it back down to Earth. But in this case, neither of those things seem to have happened.
SIMON: Well, what do we know about this rocket?
PALCA: Well, I'm glad you asked that. It's a Chinese Long March 5B. It weighs about 22 1/2 metric tons. And last week, it launched an element of the Chinese space station into orbit. But as I say, it didn't fall out of orbit immediately. It went into orbit, and there didn't seem to be a plan to bring it down in a controlled fashion.
SIMON: So why not just keep it in orbit? Why doesn't it just stay in orbit?
PALCA: Yeah, it is low enough that on the low passes as it goes around the Earth, it touches the top of the atmosphere. And the atmosphere, which is very thin, still has molecules in it. And those molecules cause friction, and the friction slows down the spacecraft. And occasionally, it slows down enough that it no longer can stay in orbit.
I talked with Moriba Jah, an aerospace engineer at the University of Texas at Austin. And he said it's hard to predict where it will come down.
MORIBA JAH: Basically, it could be anywhere. That's because there's so much ambiguity and uncertainty.
PALCA: So if you knew the exact size and shape and orientation of the rocket as it circled the Earth, and if you knew the precise characteristics of the atmosphere it was flying through, you could make an estimate where the rocket would come down. But Jah says we just don't know those things.
SIMON: It is possible, Joe - isn't it? - that it will come down - I don't want to, you know, arouse unnecessary fear, but that it will come down on an area that's populated.
PALCA: Yes, possible, but probably not. Here's Moriba Jah again.
JAH: In all likelihood, it'll be an ocean because most of the Earth is covered by water. So my guess is that, you know, high likelihood that it could hit the water. But that's just based on, you know, just kind of brute force statistics, not based on, like, any modeling of, you know, the physics in high detail because there's a lot of unknowns, you know?
SIMON: So odds are it won't hit a city, maybe. But will it even come down in a single piece?
PALCA: No. Most of the time, these break apart into multiple pieces. It's possible - again, possible - that you could get a single large chunk, and that could be dangerous. But as Jah says, the most likely place it'll come down is over water.
SIMON: And as of now, as we speak, what is the best estimate, according to experts, of where it may come down?
PALCA: Well, the U.S. Space Command is pointing to an area in the Indian Ocean, but really, it could be anywhere along a long swath of places. But, yeah, the Indian Ocean at the moment.
SIMON: Well, it's a very beautiful place...
PALCA: Yup.
SIMON: ...To end your voyage, isn't it?
PALCA: (Laughter) Well, I hope not. But, yes, I suppose so if you're a rocket, yeah.
SIMON: Joe, thanks very much for enlightening us.
PALCA: We'll be safe.
SIMON: NPR's Joe Palca.
PALCA: Thank you, Scott.
(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.
