Welcome to The Deep science and technology column where we cover topics from the deep sea to deep space and beyond.

Although Pluto is no longer officially recognized as a planet (www.guam.net/planet/pluto.html) we are still sending a spacecraft to examine this mysterious world.  It’s called New Horizons and it was the fastest spacecraft ever launched (36,000 mph).  It took only nine hours to reach the Moon’s orbit; it took the Apollo astronauts almost four days to make the same trip.

	But 36,000 mph still isn’t fast enough for New Horizons.  Pluto is so far away that the outbound journey would take over 14 YEARS at 36,000 mph.  So the robot spaceship’s keepers decided to go to Jupiter first and steal a little speed.  The official term is gravity assist.
New Horizons spacecraft (Credit NASA)

To do a gravity assist, you first need to approach your planet.  And something most of us forget about is that no matter which planet you’re talking about, your planet is moving.  And it’s moving VERY fast.  All the planets are supersonic; they move faster than the speed of sound on Earth.  Your personal spaceship, the planet Earth, orbits the Sun at a blistering 66,000 mph; about twice as fast as the original speed of the New Horizons spacecraft.

So as you approach your speeding planet, you begin to speed up as the planet’s gravity pulls you closer.  As you speed away, you lose speed as the planet’s gravity tries to pull you back making it appear that a ‘gravity assist’ to accelerate your spacecraft is an impossibility.

But what makes it work is that the planet is also moving.  Jupiter is farther from the Sun than Earth and its orbital speed is slower, only 48,000 mph.  If the spacecraft is moving in the same direction as Jupiter; Jupiter will ‘shove it along’ and the robot’s speed will increase by as much as 30,000 mph!  It’s not completely free though, the spacecraft will slow the planet down as it goes by.  Of course, the exchange depends on the relative masses of spacecraft and planet.  New Horizons weighs about ½ ton and Jupiter weighs as much as 318 Earths.  So New Horizons won’t slow Jupiter down much!

New Horizons whizzed past Jupiter in its speed-stealing maneuver last month and it had to get pretty close to Jupiter for the maximum acceleration.  However, it’s not traveling at Earth’s orbital speed of 66,000 mph, it’s only speeding along at 52,000 mph because some of the snatched energy was used to boost New Horizons into a higher orbit.  Pluto doesn’t orbit in the same plane as the planets (one of the reasons for its reclassification) and New Horizons had to climb up from the solar system plane in order to encounter Pluto.  Even at 52,000 mph, New Horizons won’t reach Pluto for another eight years; in 2015. But during its Jupiter flyby, New Horizons took some awesome pictures.  It flew closer than the Galileo spacecraft which circled Jupiter for four years; reaching just outside the orbit of Callisto, Jupiter’s outermost large moon. It took several pictures of Jupiter itself, including our first close-up view of ‘Little Red’ a smaller version of Jupiter’s Great Red spot that was created from the merger of two white spots two years ago.

Little Red (Credit NASA)

But its crowning glory were pictures of the Pizza Moon; Jupiter’s closest large moon, Io.  Now the first thing you have to understand is that New Horizons didn’t take pictures of Jupiter’s moon ‘Ten’.  Even though the name looks like ‘ten’ (especially if both letters are capitalized like this, IO) the moon is named for one of the Roman god Jupiter’s many conquests (as are all the major Jovian moons) and it’s pronounced eye-oh (or sometimes ee-oh).

Io, the Pizza Moon with Jupiter in the background (Credit NASA)

We discovered when the Pioneer and Voyager spacecraft also used Jupiter for gravity assist that Io is the most volcanically active world in the solar system.  The scientists who ran the Voyager mission first called Io the Pizza Moon and soon discovered that Io’s pockmarked appearance was the result of hundreds of active volcanoes.  Over the years we’ve discovered that many of them are in continuous eruption.  The solar system’s largest active volcano is located on Io and its name is Mount Pele.

Jupiter’s moon Io.  The large ‘blotch’ at left center is Mount Pele, the solar system’s largest active volcano. (Credit NASA)

Io is almost exactly the same size as our Moon and it orbits Jupiter at almost exactly the same distance as Luna orbits the Earth.  Luna and Io are a marvelous demonstration of the physics law “The closer you are to a large body; the faster you must go.”  Our moon Luna takes 28 days to orbit Earth, Io orbits Jupiter in 42 hours. 

The other large moons of Jupiter (two are larger than Mercury) push and pull on Io as it circles the largest mother world in the solar system.  This deforms Io and the planet’s surface can flex in and out as much as 300 feet.  As you can imagine, this causes heating within this icy moon and makes Io the most volcanically active world in the solar system. 

	When New Horizons whizzed past Io, scientists figured they would catch the volcano Prometheus erupting since Prometheus seems to erupt most of the time.  They did see Prometheus, but they were amazed to find that the site of a large lava lake, Tvashtar, was in eruption.  The plume from Tvashtar rose 180 miles above Io’s surface.
The Tvashtar region taken by the Galileo spacecraft in 2001 (Credit NASA)

Andy Cheng, a NASA scientist said that the structure visible in the Tvashtar plume suggests that the erupting gas is condensing into solid particles.  In other words, the gas is crystallizing in the airless space above Io to form a kind of sulfurous snow.  Snow from a volcano.  An alien world, indeed!
	Tvashtar erupts on Io (The plume from Prometheus is the smaller plume at 9:00) (Credit NASA)

Tvashtar’s plume half in sunlight and half in darkness.  The glowing spot is the top of the volcano.