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Cassini probe plummets into Saturn's atmosphere after 20 years and 4.9 billion miles traveled

Spacecraft revolutionized our understanding of ringed planet, included instrument designed and built in part by scientists at JHU's Applied Physics Laboratory

Video: Deirdre Hammer

This morning, after two decades in space, NASA's Cassini spacecraft ended an incredible journey of exploration. With the spacecraft's fuel spent, operators deliberately plunged Cassini into Saturn—which it had orbited for 13 years —to make sure the planet's moons remain pristine for future exploration.

A small but significant piece of the Johns Hopkins Applied Physics Laboratory, the Magnetospheric Imaging Instrument, or MIMI, went with Cassini on its fiery descent. In the early 1990s, APL partnered with six other institutions to design and build MIMI, which studied the charged particles trapped in Saturn's magnetosphere and also made global images of fast neutral atoms.

"I feel sad in a way that Cassini and MIMI have to end in this fashion, but I understand the necessity to do so," said Ray Thompson, who served as the instrument's principal technician. "This will be the second instrument that I worked on to go into a neighboring planet. ... I feel like a part of me goes with it."

APL built one of MIMI's three sensors—the Ion and Neutral Camera, or INCA—which produced the first-ever images of a planet's magnetosphere using an APL-developed technique known as energetic neutral atom imaging.

Before INCA, spacecraft obtained what is known as fields-and-particles data from its local surroundings—the area in which the spacecraft was then operating. But INCA, essentially a pinhole camera, makes images out of fast neutral atoms instead of photons. With this technique, instruments were no longer restricted to the local environment of the spacecraft, but instead could produce global images that illuminate the structure and dynamics of different aspects of the entire magnetospheric system.

"The capability of 'seeing' the global magnetosphere and creating movies of its dynamics was a brand new capability at any planet other than Earth and has led to many insights and discoveries that would have been either impossible, or only accomplished through inference without the INCA ENA imager," said Don Mitchell, MMI's principal investigator and a member of APL's Space Exploration Sector. "The science community will be analyzing these observations for many years."

Artist's rendering of the Cassini spacecraft as it orbits Saturn

Image caption: Artist's rendering of the Cassini spacecraft as it orbits Saturn

Image credit: NASA Jet Propulsion Laboratory

Plenty of scientific analysis has already occurred since Cassini arrived at Saturn in 2004. More than 175 refereed journal articles and book chapters have been published by MIMI team lead authors, said MIMI Deputy Principal Investigator Chris Paranicas, with hundreds of others that use MIMI data and include MIMI team co-authors.

Mitchell proudly checks off several scientific discoveries coming from MIMI and INCA:

  • Characterizing the radiation environment of the whole Saturnian system
  • Measuring particles that power auroral signatures in various ways, and those that modify the surfaces of satellites and rings
  • Boosting understanding of global dynamics, including large-scale disruptions of the system and periodic phenomena

INCA's charged-particle measurements were central in identifying a noon-to-midnight planetary electric field whose origin is not known, in revealing ring arcs that were not imaged, and in linking solar wind perturbations to system changes. The instrument also imaged Jupiter on Cassini's path to Saturn, beginning in 2000, from more than half an astronomical unit—about 50 million miles—away. And it looked beyond the Saturn system, helping to redraw the shape of our solar system by gathering data that suggested our heliosphere—the area of the sun's influence that surrounds the solar system—may not have the comet-like shape predicted by existing models.

MIMI wasn't the only vehicle for APL scientists to make discoveries during the Cassini mission.

Icy moons expert Zibi Turtle led a team that mapped Titan's surface in the near-infrared and followed its weather patterns as they changed with the seasons over 13 1/2 years of Cassini observations. On two occasions, they discovered that clouds had produced extensive methane rainfall, once at the south pole during Titan's late southern summer and later in the equatorial deserts during northern spring—the first time scientists obtained current evidence of rain soaking Titan's surface at low latitudes.

"It's amazing to watch such familiar activity as rain and seasonal changes on a distant, icy satellite," she said.

Ralph Lorenz, one of the world's leading experts on Saturn's largest moon, Titan, has worked on Cassini for his entire 27-year career. His first book—Lifting Titan's Veil, published in 2002—covered what we knew before exploration by Cassini and its Huygens probe, which landed on Titan in January 2005. In 2010 he wrote about what the mission found in Titan Unveiled, and this year he captured what he described as "all the nerdy details of the spacecraft development and operation" in the Cassini-Huygens Owners Workshop Manual.

Mark Perry, an APL planetary scientist, was part of a team that found hydrogen gas—which could potentially provide a chemical energy source for life—pouring into the subsurface ocean of the icy moon Enceladus from hydrothermal vents on the sea floor. The presence of ample hydrogen in the ocean means that microbes, if any exist there, could use it to obtain energy.

"This is the first time that we have found so many of the essential components for life in one place beyond our own planet," Perry said. "And it is one of the least expected places, too: a small moon far from the sun. This remarkable discovery gives us great hope that we will eventually find life somewhere besides Earth."