Juno Earth Flyby

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Juno Earth Flyby - Oct. 9, 2013

Artist's concept of Juno's Earth flyby
Artist’s rendering of the Juno spacecraft approaching Earth on Oct. 9, 2013. Credit: NASA/JPL-Caltech

NASA’s Jupiter-bound Juno spacecraft will perform a close flyby of Earth on Oct. 9, 2013. The time of closest approach is approximately 19:21 UTC (3:21 pm U.S. Eastern time). During the flyby Juno will come to within 347 miles (559 kilometers) of Earth.

The Juno spacecraft was launched from Kennedy Space Center on August 5, 2011 toward Jupiter. Juno’s rocket, the Atlas 551, was only capable of giving Juno enough energy or speed to reach the asteroid belt, at which point the Sun’s gravity pulled Juno back toward the inner solar system. The Earth flyby gravity assist was planned as part of Juno’s trajectory to increase the spacecraft’s speed relative to the Sun so that it is sufficient to reach Jupiter. (The spacecraft’s speed relative to Earth remains constant.) Because of the flyby, Juno’s velocity relative to the Sun increases from 78,000 miles (126,000 kilometers) per hour to 93,000 miles (138,000 kilometers) per hour.  Juno is moving much faster than satellites that orbit the Earth because Juno is orbiting the Sun, not Earth.

Juno will receive a huge boost from Earth’s gravity equivalent to about 70 percent of the total change in velocity, or delta-v, provided by the Atlas V 551 rocket. Thus, the boost from the flyby is almost as powerful as a second rocket launch.

The spacecraft passes over the ocean off the coast of South Africa at the point of closest approach. About two minutes before this point, Juno will pass into Earth’s shadow for about 20 minutes. Juno emerges from the planet’s shadow when it is about 5,400 miles (8,700 kilometers) above Earth at approximately 19:39 UTC when it is over the east coast of India. 

Juno arrives at Jupiter on July 4, 2016, to study the giant planet’s interior, atmosphere and giant magnetosphere. Its main objective is to improve our understanding of how Jupiter formed. What we learn from Juno will teach us about the early stages of our solar system and how Earth and our neighbor planets formed. Juno will even help us understand how the planets around other stars form. 

On the map & geometry images provided below, the red segments of the spacecraft’s path indicate the time when Juno is within Earth’s shadow.

Map showing Juno’s ground track during the Earth flyby
Map showing Juno’s ground track during the Earth flyby.  Credit: NASA/JPL-Caltech Larger view

The geometry of Juno’s Earth flyby near the rime of closest approach
The geometry of Juno’s Earth flyby near the rime of closest approach. Credit: NASA/JPL-Caltech Larger view

How to See Juno

Viewers near Cape Town, South Africa will have the best opportunity to view the spacecraft traveling across the sky near closest approach (weather permitting). While the spacecraft may not be visible to the unaided eye, a pair of binoculars or a small telescope with a wide field of view should help. Viewers in India may be able to spot the spacecraft after it emerges from eclipse. Viewers in other parts of the world, including the United States, might be able to image the spacecraft using a telescope the evening following the flyby, as a faint object moving against the background stars. (Estimated visual magnitudes for South Africa and India, will be posted here as soon as they are available.)

For amateur astronomers with telescopes, Juno ephemeris information is available from the NASA/JPL

HORIZONS website >

In addition, information about Juno’s position on the sky during and after the flyby, including sky charts, has kindly been made available by the satellite spotting website Heavens Above. IMPORTANT: you must specify your viewing location in order to see accurate predictions for your area. (Note that this is not a NASA website, and by linking to it, NASA is not endorsing the site or any advertising presented there. The site is hosted by the German aerospace center, DLR.) Visit Heavens Above website >

Earth-approach movie and science during the flyby

The close flyby provides the opportunity for a trial run of science operations at Jupiter. The Juno team will use this occasion to exercise Juno’s science instruments and sample a planetary magnetosphere to get a preview of what to expect from the spacecraft once it arrives at the giant planet. Most of Juno’s science instruments have observations planned for the encounter, except for the exquisitely sensitive Microwave Radiometer, which will remain powered off as a protective measure.

Planned observations include:

  • - The Advanced Stellar Camera, part of Juno’s Magnetometer experiment, will acquire a movie sequence of images on approach to Earth. The sequence begins on Oct. 5 and continues through closest approach.
  • - As Juno passes the moon, its infrared spectrometer instrument, called JIRAM, will acquire images and spectra of our natural satellite.
  • - Juno’s imaging camera for public engagement, called JunoCam, will take a series of color images of our home planet beginning near the time of closest approach.
  • - The radio and plasma wave instrument, called Waves, will listen to the transmissions of Earth’s magnetosphere – possibly detecting signals of human origin as well.

Once processing of these data products is completed, these visuals will be published at www.nasa.gov and missionjuno.swri.edu.

Ways to Participate

The Juno mission team welcomes images from amateur astronomers who attempt to photograph the spacecraft during and after the flyby. (See links to spacecraft sighting and ephemeris data on this page.) Observers may send images to Juno’s public engagement team via email to be considered for sharing via web and social media.

Amateur radio operators are invited to join in sending a coordinated Morse code message that the spacecraft’s radio and plasma wave instrument may be able to detect. Learn more >

* Additional information about the flyby will be posted here as it becomes available.

View of Juno’s Earth flyby from NASA’s Eyes on the Solar System
The geometry of Juno’s Earth flyby near the rime of closest approach. Credit: NASA/JPL-CaltechView of Juno’s Earth flyby from NASA’s Eyes on the Solar System

Useful links

Flyby videos

Why fly by?

Ion Propulsion System Hot Fire Test for Deep Space 1
Juno's trajectory to Jupiter. Credit: NASA/JPL/Caltech
› Larger view

Why does Juno not travel directly to Jupiter? To launch a spacecraft to another planet, a rocket is used to escape Earth’s gravity. Once a spacecraft is away from our planet, it is still in orbit around the Sun (just like Earth). The velocity or speed of an orbit around the Sun dictates the distance an object can reach with respect to the Sun. It would take a more powerful rocket to send a spacecraft as massive and capable as Juno directly to Jupiter.  Therefore the Juno team uses a technique called a gravity assist to increase Juno’s speed so that it can reach Jupiter.  

The spacecraft first loops around the inner solar system, returning to the original distance from the Sun from which it was launched (Earth’s orbit). The trick is to time the flyby so that when Juno returns to Earth’s orbital distance, our planet is there. Then Juno can use Earth’s orbital momentum to increase its own momentum – in effect, stealing a small amount of Earth’s orbital energy. Chemical propulsion provided by Juno’s launch vehicle gave the spacecraft a bit more than half the boost it needs to get to Jupiter; the Earth flyby provides the rest. So the Earth flyby is like having almost a second rocket for free! It takes about two years for the Juno spacecraft to travel around the sun once and come back to Earth for the flyby, and then three more years to coast out to Jupiter.

In 2006, the Pluto-bound New Horizons spacecraft was sent on a direct flight past Jupiter (its own gravity assist), reaching the giant planet in just one year. New Horizons used the same rocket as Juno (the Atlas V 551), however New Horizons was far less massive than Juno, and didn’t need to stop upon reaching Jupiter. *

Mission milestones in 2013

  • - Aug. 5: Juno’s two-year launch anniversary (Aug. 5, 2011)
  • - Aug. 12: Juno reaches halfway point (distance-wise) in its trek to Jupiter
  • - Aug. 31: Spacecraft reaches perihelion – the closest it ever gets to the sun – at a distance of 0.88 Astronomical Units
  • - Oct. 9: Earth flyby gravity assist maneuver
  • - Oct. 11: One billion miles traveled since launch

How a gravity assist works:

The Juno spacecraft received a little over half of the boost it needs to get to Jupiter from its launch vehicle. The Earth flyby gravity assist maneuver provides the rest. Gravity allows the spacecraft to steal a very small bit of our massive, moving planet’s great momentum.

The key to understanding how Juno and other spacecraft use gravity assists is recognizing that the spacecraft is in orbit around the Sun, not the Earth. A spacecraft’s velocity consists of two things: a magnitude (or speed) and a direction. Both of these components change during a gravity assist to produce a new course, and it is easiest to see each effect by looking at the flyby from two different frames of reference.

Flyby diagramDiagram illustrating the two important components of Juno’s gravity assist. Earth’s point of view is represented on the left, and the sun’s point of view is represented on the right. Image credit: NASA/JPL-Caltech.

    • - From Earth’s point of view (scientists call this in the Earth’s “frame of reference”) : To Earth, the approaching spacecraft is like a cyclist heading downhill into a perfectly symmetrical valley - the speed increases as the cyclist approaches the planet (or goes down into the valley). As the cyclist emerges from the valley, the speed is reduced (because they are going back up the valley hill).  Their speed (relative to the valley) is the same as it was before going into the valley, but the direction has changed. The spacecraft’s direction, but not speed relative to Earth, will similarly be altered by the gravity assist.
    • - From the Sun’s point of view (scientists call this in the Sun’s “frame of reference”): Earth is a very massive object moving around the Sun. Earth’s orbital speed around the Sun can be considered as momentum, which is something that any moving massive object carries (for example, a speeding truck has a lot of momentum). When a spacecraft flies closely past the moving planet, the planet’s tug on the spacecraft has a profound effect. Earth loses an immeasurable amount of momentum to Juno. Because Juno has much less mass compared to Earth, this small amount of momentum results in a relatively large boost to Juno.

* At launch, Juno weighed about 8,000 pounds (3625 kilograms), while New Horizons weighed about seven times less.




Page Last Updated: September 30th, 2013
Page Editor: Tony Greicius