A dance of planets and probes (Science Advent 11)

(Every day until December 25, I’m posting a science-related image or video and description.)

Day 11

Like so many of us, I love science fiction movies and TV, but for the sake of narrative, they make difficult things simple. In particular, spaceships move in and out of “standard orbit” (to use Star Trek‘s terminology) effortlessly. Sure, sometimes orbits fail to give the main characters a chance to pull off daring escapes or last-minute restorations of power, but watching these shows might give you the impression orbital mechanics are simple. (Even Earth satellites stay in orbit because of an odd balance of effects. One of these days, I’ll write about that.)

The truth, as usual, is a bit more complex. Consider the Juno mission, which is slated to arrive at Jupiter in July 2016, but was launched from Earth on August 5, 2011. It’s not taking nearly five years to reach Jupiter because the probe is moving really slowly, but because it needs to have precisely the right speed and direction to insert itself in the correct orbit. That orbit is a polar orbit, sweeping over both north and south poles of the planet, for the sake of measuring the gravitational and magnetic fields in unprecedented detail. A spacecraft on a different trajectory might slingshot past Jupiter (as did the Pioneer, Voyager, and Cassini probes).

The trajectory of the Juno space probe through the Solar System. To gain the correct speed and direction, the probe followed a looping path, passing by Earth again this fall. [Credit: NASA/JPL]

The trajectory of the Juno space probe through the Solar System. To gain the correct speed and direction, the probe followed a looping path, passing by Earth again this fall. [Credit: NASA/JPL]

To get the right speed, Juno is following a circuitous path through the Solar System. From Earth, it swooped out past Mars before plunging back toward the Sun, accelerating all the while. On October 9, 2013, Juno passed by Earth again, resulting in the video you see above. During this passage, the probe increased its speed by about 7.3 kilometers per second (4.5 miles per second, or roughly 8,800 miles per hour) — a very fast rate. In fact, Juno’s looping trajectory was the reason for its high speed: it used the Sun’s and Earth’s gravity to gain speed and steer it. Sure, you could technically accelerate the spaceship using rocketry, but every kilogram of fuel or engine increases inertia, making it harder to move the ship. It’s more efficient to use gravity, even though it extends the time the probe is in transit.

In the video above, Juno collected a series of low-resolution images of Earth and its Moon. Since the probe is spinning rapidly (to better measure Jupiter’s magnetic field), the images were corrected to produce a stable film. Since the cameras weren’t designed for pretty pictures, we’re not seeing “blue marble”-quality images here. However, this kind of flyby has its own beauty. We are witnessing the dance of Earth and Moon, and by extension the pirouette of probe and planet.

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