Solar-Powered Spacecraft Juno Uses Gravity Assist Technique to Continue its Jupiter Exploration Mission

Juno, a probe on a mission to explore Jupiter, was back above our heads on Wednesday October 9^th after one wide turn around the Sun.

The probe’s trajectory passed only 500 km from Earth. But why did this spacecraft, launched by NASA in August 2011, come back so close to its initial place? In fact, the rocket could not give enough energy to the probe to fly directly to the fifth and biggest planet of our solar system, so it came back to pump the power from our own planet.

“It is a manoeuver called Gravity assist or flyby,” explains Muriel Richard, senior scientist at the Swiss Space Center. “It is a way to change the trajectory and speed of a spacecraft using the gravity forces of a planet. The flyby is a smart solution that needs to be planned with extreme precision, but it allows saving a lot of fuel and expenses. This is how the two Voyager probes travelled through space, alternatively using the gravity forces of Jupiter, Saturn, Uranus and Neptune, to finally get out of our solar system a few months ago, after a 35-year trip.”

As two celestial objects get close to each other, they will exchange energy proportionally to their masses, further describes Muriel Richard. “So, approaching Earth yesterday, Juno was pulled into the planet’s gravity and actually decreased Earth’s orbital energy by a very little amount. In this exchange, Juno gained a very large momentum that propelled it to deep space. It is as if Juno “bounced back” on a rapidly moving Earth with respect to the Sun, as a tennis ball would “bounce back” with even more speed on a racquet that hits it.” This manoeuver give it enough power to reach Jupiter in July 2016. It is now travelling at a speed of 140’000 km/h.

Juno is the first solar-powered spacecraft to explore an outer planet. Its main goals are to understand how Jupiter was initially formed. It will study its dynamical and structural properties, core, gravitational and magnetic fields, atmospheric composition, and the three-dimensional structure of the planet’s polar magnetosphere.