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SYFY WIRE Bad Astronomy

Juno Spacecraft Shaves the Earth on Its Way to Jupiter

By Phil Plait

The Juno spacecraft was launched on August 2011, and itâs taking the long road to Jupiter. Weighing an impressive 3600 kilograms (8000 pounds) at launch, its Atlas V launch rocket couldn't fling it directly to the King of the Planets. Instead, it has to be sneaky: It took a long elliptical flight outwards, past the orbit of Mars, then came back to Earth to steal some of our energy.

This is called a gravity assist, or more colloquially (if less accurately) a slingshot maneuver. On Oct. 9, 2013 it practically took the whiskers off Earth, passing a mere 560 kilometers (350 miles) from our planetâs surface! Compare that to the Earthâs diameter of just under 13,000 km (8000 miles) and youâll get an idea of how close this was.

As it passed us it took a series of lovely pictures, including this portrait of our world:

It got close enough to Earth that it was easily visible via small telescopes, too. A fleet of such instruments were aimed upward for the event, capturing images of the spacecraft as it zipped through our skies. The best Iâve seen so far is from Nick James of the British Astronomical Association. He used a 28 cm (11â) telescope to take a series of five second exposures, creating this very cool sequence:

This is pretty impressive work on his part, given how fast Juno was traveling relative to the Earth. NASA websites are still down due to the government shutdown, but I found a cached NASA site that gave Juno's speed as 138,000 km/hour relative to the Sun. Oddly, I found a site or two claiming this is the fastest human-made spacecraft in history, but the solar Helios probes were certainly moving faster than that relative to the Sun.

Still, 138,000 km/hour (93,000 miles/hour) is fast enough to get from the Earth to the Moon in three hours. Holy yikes.

To pick up this extra velocity, the spacecraft approached Earth from behind and used our planetâs gravity to tug the spacecraft along, accelerating it. That description cuts out a lot of physics, though; in the end, the way it actually works is that the probe steals a bit of the energy of Earthâs motion around the Sun. The energy exchange depends on the ratio of the masses of the Earth and Juno â a factor of a billion trillion â so the spacecraft gets a mighty kick, while the Earth slows only a teeny tiny bit in its yearly path. You could do this all day every day and the Earth would never notice, but the probe increased its velocity by 26,000 km/hour.

Hereâs an animation put together by NASA to show the path Juno has and will take:

The closer the probe gets to Earth, the more efficient the gravity assist is. Thatâs why Juno came so close to us. Itâs amazing to think we can fling a spacecraft hundreds of millions of kilometers, only to rendezvous (briefly!) with its home planet and miss it by only a few hundred. Weâre really, really good at this.

The encounter was marred by one problem: Juno went into âsafe modeâ as it passed us. That means it detected some anomaly and put itself to sleep until engineers back home could figure out whatâs what. Emily Lakdwalla from The Planetary Society talked to the Juno Project Manager, who gave some details. The good news is that on Friday (Oct. 11) Juno exited safe mode, and is awake once again. Since the gravity assist was passive â the spacecraft was just cruising with no rocket burns â itâs on the right path to Jupiter no matter what.

So once again we have an extension of humanity on its way to the outer solar system. Next stop, Jupiter!

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