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GUY RAZ, host: Two state-of-the-art NASA spacecraft are on their way to a rendezvous with the moon. Their mission, known by the acronym GRAIL, is to make a precise map of the moon's gravitational field.

NPR's Joe Palca has been covering the mission since before it launched two weeks ago, and he stumbled across something he thought was, well, kind of weird. Here's Joe.

JOE PALCA: To help reporters out, NASA produces a press kit for most of its major missions. So I'm reading through the GRAIL press kit, learning about the size of the spacecraft - washing machine sized - and how long they'll be on the job at the moon - approximately three months - and where the mission control room - at a Lockheed Martin facility just outside of Denver. I also learn that GRAIL can launch any day between September 9th and October 19th. And then I get to this sentence.

ELIZA BARCLAY: On each day, there are two separate, instantaneous launch opportunities separated in time by approximately 39 minutes.

PALCA: That's my colleague Eliza Barclay in her best NASA press kit voice. I wasn't surprised that the mission had an extremely precise launch time. I know you have to wait until the moon's orbit around the Earth, and the Earth's orbit around the sun, and the Earth's rotation around its axis are all in exactly the right position before you can launch your rocket. But what's up with two instantaneous launch opportunities? And why 39 minutes apart?

I called the NASA press office. They were stumped, but they promised they'd find someone who could give me the answer. In the meantime, I called several universities with large aerospace programs and asked if anyone there had the answer. Nope. Nada. Nothing. So now this is beginning to bug me, and I'm also beginning to worry.

If an aerospace engineer can't figure this out, how am I ever going to understand it? While I'm pondering this, GRAIL deputy project manager Tom Hoffman called. I asked him about the two instantaneous launch opportunities. I'm guessing it's not a simple answer, but can you take a crack at giving me a simple answer?

TOM HOFFMAN: Yeah. Actually, the instantaneous launch is a pretty simple answer. That's not driven by the mission at all. It's driven by the launch vehicle.

PALCA: The launch vehicle, not orbital mechanics or a complex trajectory? But Hoffman explained. Nah-uh. The problem is slow electronics. There is actually a window of about 40 minutes or so each day. Modern computers in modern rockets can calculate the precise direction the rocket needs to launch in on a second-by-second basis. But GRAIL was launching on a Delta II rocket, and Hoffman told me the Delta II rocket is a rather old design. It's been around for more than 20 years.

HOFFMAN: And so the rocket just can't - it can't do that internally in its own computer system. It has to be basically fed the - all the information.

PALCA: And guess what? Hoffman said it takes about 39 minutes to load in the new information and then check to make sure that it was loaded correctly. Not the answer I was expecting, but there's something oddly pleasing about a brand-new space mission launching with some rather creaky old electronics. Whatever works, as the saying goes. Joe Palca, NPR News.

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