NASA Spacecraft Zips By Earth En Route To Jupiter

Oct 14, 2013
Originally published on October 9, 2013 3:16 pm

A NASA spacecraft bound for Jupiter will swing by Earth on Wednesday to get the boost it needs to arrive at the giant gas planet in 2016.

Using Earth as a gravitational slingshot is a common trick since there isn’t a rocket that’s powerful enough to catapult a spacecraft directly to the outer solar system.

Launched in 2011, the Juno spacecraft zipped past the orbit of Mars and fired its engines to put it on course for a momentum-gathering flyby of Earth. During the maneuver, Juno will briefly pass into Earth’s shadow and emerge over India’s east coast.

At closest approach, Juno will fly within 350 miles of the Earth’s surface, passing over the ocean off the coast of South Africa shortly before 12:30 p.m.

The rendezvous was designed to bump Juno’s speed from 78,000 mph relative to the sun to 87,000 mph – enough power to cruise beyond the asteroid belt toward its destination.


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From NPR and WBUR Boston, I'm Meghna Chakrabarti, in for Jeremy Hobson. It's HERE AND NOW.

NASA's Juno mission to Jupiter has been flying through space for about two years and has already traveled more than a billion miles. But today, it's basically back where it started, meaning here, near Earth. So why is the Juno mission making such a close flyby of our blue planet on its way to the great gas giant?

Joining us to explain is NPR science correspondent Joe Palca. Joe, come on, tell us what's the story behind this close flyby.

JOE PALCA, BYLINE: Well, the answer is that the rocket that launched Juno two years ago didn't quite have the oomph to send it on a direct course. And so what happened is it sent it on a path out past the orbit of Mars, and it kind of swung back to Earth. And now this gravity of Earth is going to kind of use it - use the gravity as a slingshot to fling Juno off toward Jupiter. So after today, it's going to be heading in the right direction.

CHAKRABARTI: So does that happen often, the use of the Earth's gravity as a slingshot for interplanetary satellites?

PALCA: Yeah, it's been used for quite a while. I mean, sometimes they used it for just a nudge in one direction or another. But I was trying to figure - going back to about - there was a mission called Galileo to Jupiter about 20 years ago, and it used an even wilder trajectory. It went first to Venus, then back to Earth, then back to Earth again, and then off to Jupiter.

So these things have gotten more and more complex partly because they want to build bigger satellites. But they don't have bigger rockets, so they use the gravity to give them a hand.

CHAKRABARTI: It takes a lot of power to just get off the Earth's surface. So what's the closest Juno will come to Earth today?

PALCA: Three hundred and forty-seven cozy miles.


CHAKRABARTI: That's only about 100 miles further than - or farther out than the orbit of the International Space Station. So pretty close. Now, is it going to have to fire its rockets in addition to the gravity assist?

PALCA: Nope, nope. It's done - that rockets have done their bit for now. It's just going to swing by. It does have - it's not going to be completely passive, though. It's going to switch on some of its instruments. Actually, the joke is that it's going to see if it can detect life on Earth as it comes by.


PALCA: But they do have an experiment with some ham radio operators that are going to be sending very, very slowly - in the 10-meter band, they're going to be sending the Morse code for the word hi, H-I. And they're going to send it over two hours and 40-minute period 24 times. So it's - hi takes 10 minutes to send. So you have to be very patient to be part of this experiment.

CHAKRABARTI: A ham radio in the digital world?

PALCA: Yeah, yeah. It's just - well, the cool thing is it's a way of engaging the public in this process. I mean, Juno does have instruments that are supposed to detect radio waves and plasma waves, and so this is a way of checking the instruments are working OK. But one of the things I've learned in this project I've been working on called Joe's Big Idea about, you know, what motivates scientists is they really like sharing. And this is a way of getting people on board and getting them to participate in this kind of cool mission.

CHAKRABARTI: Well, speaking of looking for life on Earth, I was looking for more information about the Juno mission on NASA's website, and I got the government shutdown page, of course. So is the shutdown going to have any effect on this mission?

PALCA: Well, first of all, Meghna, I have to chide you for not coming to me first. I know the super-secret passwords and Web pages, so I could have clued you in on that. But having failed to use the most appropriate way, which is checking with me, yes, the websites, a lot of them are down. Some of them are still up. But as I've been telling people, the laws of physics are going to guarantee that Juno will reach Jupiter with - actually, it's going to happen in July 4, 2016 whether the government reopens or not.


PALCA: So it's going to happen. What might - what has been questioned is whether or not the people who are supposed to be keeping an eye on it and recording data and what have you are going to be inconvenienced. And the answer is, well, some of these missions are considered absolutely essential, and so they keep the money going for those kinds of things. And so I've talked to the mission managers - or at least I've talked through them, and they seem to be on the job.

CHAKRABARTI: Well, Joe Palca is NPR science correspondent and our source of information regardless of the government shutdown. Joe, always great to talk to you.

PALCA: You bet.

CHAKRABARTI: This is HERE AND NOW. Transcript provided by NPR, Copyright NPR.