How NASA ‘Captured The Moment’ Of Cassini’s Last Dive

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How NASA 'Captured The Moment' Of Cassini's Last Dive

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When NASA’s Cassini spacecraft made its fiery plunge into Saturn’s cloud tops, it shot back photos and telemetry data that scientists, and the public, will pore over for years to come.

But even as we ogle the photos in our news feeds and anticipate revelations that will surely come from our first-ever sniffs of Saturn’s atmosphere, another question comes to mind: How, on a spinning Earth 746 million miles from a tiny craft moving at 42,000 miles an hour, did NASA connect with Cassini and bring home that precious data?

Courtesy NASA/JPL-Caltech

In this illustration, Cassini plunges into Saturn’s atmosphere. Using its control thrusters, Cassini works to keep its antenna pointed at Earth while it sends its final data, including the composition of Saturn’s upper atmosphere.

To gather the data from Cassini, NASA’s Jet Propulsion Labs (JPL) relied on its Deep Space Network (DSN). The network provides unbroken communications with all spacecraft exploring beyond Earth’s orbit. JPL communicates with about 40 spacecraft altogether, launched by NASA as well as space agencies in Japan, Europe, and India. Some are driving across Mars; others are circling Jupiter. Still others, in the case of Voyager 1, are moving beyond our solar system into interstellar space. All are streaming data—the most precious data ever created—toward Earth. NASA works with Oracle to gather that data safely and share it with the world.

Here’s how it works:

The Deep Space Network consists of four sites: one in Spain, one in Australia, one in the desert of California, with central control at JPL in Pasadena, California. With communication dishes that can be nearly the size of a football field, the sites send commands and retrieve telemetry back from the spacecraft.

As the Earth rotates, JPL can see any spacecraft from at least one of the antennas at one of those sites. Members of the DSN team are responsible for pointing the antenna dishes at a spacecraft, sending commands and receiving data, and then prepping photos and sensor readings to deliver to the scientific community and the public.

A 1-Centimeter Target, 30 Light Hours Away

That mission is anything but straightforward when you consider what’s behind the phrase “pointing antenna at a spacecraft” that’s hundreds of millions of miles from Earth.

In the case of Voyager 1, which is 30 light hours from Earth, the aim must be accurate to the centimeter level. That’s especially hard when microsecond variations in the Earth’s spin can translate into pointing errors in the antenna.

The DSN team must model the rotation rate of the Earth to get the most accurate position to point the antennas. Planetary scientists and mathematicians calculate the position and velocity of a spacecraft relative to an antenna.

8 Gigabytes in Four Months

Once the calculations are right and the data is streaming, JPL’s software and hardware take over, sending commands and receiving the faint signals as they reach Earth.

The data rates from the spacecraft to Earth are very slow. When the New Horizons spacecraft flew by Pluto last year, the DSN received only a thousand bits per second from Pluto down to Earth. In all, the hours-long Pluto encounter resulted in about 6.25 gigabytes of images and data, which took the team 469 days to downlink.

The DSN systems takes the signal from the antenna, transforms it into bits, then into frames, and then processes the frames into pictures. Other software translates the signal into spectrometry readings, voltage levels, temperatures, and pressure readings.

JPL’s Deep Space Network places compute power at each of the four antenna sites around the world. Until recently, that meant more than 300 Sun servers spread among them. They are now consolidating that number to just six Oracle Private Cloud Appliances (PCAs), one at each site and a few at JPL headquarters in Pasadena. Due to the unusual nature of JPL’s work, the DSN team writes most of their own software. Each PCA will run a number of virtual machines tuned to the Deep Space Network’s various homegrown applications.

The new machines are designed to lower the cost of administration. For example, a single Oracle patch updates all the firmware and operating systems on the PCA without affecting the applications. That way, JPL can upgrade their infrastructure while tracking spacecraft—important when can’t-miss data is always arriving from ambitious missions throughout the solar system and beyond.

The Deep Space Network team shares the images and telemetry they gather with the public. You can find it on the Planetary Data Systems website.

Jeff Erickson is Oracle’s editor at large.

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