Subscribe to our free daily newsletters
. 24/7 Space News .




Subscribe to our free daily newsletters



SATURN DAILY
Reconstructing Cassini's Plunge into Saturn
by Staff Writers
Pasadena CA (JPL) Oct 17, 2017


"To keep the antenna pointed at Earth, we used what's called 'bang-bang control,'" said Julie Webster, Cassini's spacecraft operations chief at NASA's Jet Propulsion Laboratory, Pasadena, California. "We give the spacecraft a narrow range over which it can rotate, and when it bangs up against that limit in one direction, it fires a thruster to tip back the other way." (This range was indeed small: just two milliradians, which equals 0.1 degree. The reconstructed data show Cassini was subtly correcting its orientation in this way until about three minutes before loss of signal.

As NASA's Cassini spacecraft made its fateful dive into the upper atmosphere of Saturn on Sept. 15, the spacecraft was live-streaming data from eight of its science instruments, along with readings from a variety of engineering systems. While analysis of science data from the final plunge will take some time, Cassini engineers already have a pretty clear understanding of how the spacecraft itself behaved as it went in. The data are useful for evaluating models of Saturn's atmosphere the team used to predict the spacecraft's behavior at mission's end, and they help provide a baseline for planning future missions to Saturn.

Chief among these engineering data, or telemetry, are measurements indicating the performance of the spacecraft's small attitude-control thrusters. Each thruster was capable of producing a force of half a newton, which is roughly equivalent to the weight of a tennis ball on Earth.

During the final moments of its plunge, Cassini was traveling through Saturn's atmosphere, which was about the same density as the tenuous gas where the International Space Station orbits above Earth. In other words, there's barely any air there at all.

Despite the fact that this air pressure is close to being a vacuum, Cassini was traveling about 4.5 times faster than the space station. The higher velocity greatly multiplied the force, or dynamic pressure, that the thin atmosphere exerted on Cassini. It's like the difference between holding your hand outside the window of a car moving at 15 mph versus one moving at 65 mph.

Data show that as Cassini began its final approach, in the hour before atmospheric entry it was subtly rocking back and forth by fractions of a degree, gently pulsing its thrusters every few minutes to keep its antenna pointed at Earth. The only perturbing force at that time was a slight tug from Saturn's gravity that tried to rotate the spacecraft.

"To keep the antenna pointed at Earth, we used what's called 'bang-bang control,'" said Julie Webster, Cassini's spacecraft operations chief at NASA's Jet Propulsion Laboratory, Pasadena, California. "We give the spacecraft a narrow range over which it can rotate, and when it bangs up against that limit in one direction, it fires a thruster to tip back the other way." (This range was indeed small: just two milliradians, which equals 0.1 degree. The reconstructed data show Cassini was subtly correcting its orientation in this way until about three minutes before loss of signal.)

At this point, about 1,200 miles (1,900 kilometers) above the cloud tops, the spacecraft began to encounter Saturn's atmosphere. Cassini approached Saturn with its 36-foot-long (11-meter) magnetometer boom pointing out from the spacecraft's side. The tenuous gas began to push against the boom like a lever, forcing it to rotate slightly toward the aft (or backward) direction. In response, the thrusters fired corrective gas jets to stop the boom from rotating any farther. Over the next couple of minutes, as engineers had predicted, the thrusters began firing longer, more frequent pulses. The battle with Saturn had begun.

With its thrusters firing almost continuously, the spacecraft held its own for 91 seconds against Saturn's atmosphere - the thrusters reaching 100 percent of their capacity during the last 20 seconds or so before the signal was lost. The final eight seconds of data show that Cassini started to slowly tip over backward.

As this happened, the antenna's narrowly focused radio signal began to point away from Earth, and 83 minutes later (the travel time for a signal from Saturn), Cassini's voice disappeared from monitors in JPL mission control. First, the actual telemetry data disappeared, leaving only a radio carrier signal. Then, 24 seconds after the loss of telemetry, silence.

These data explain why those watching the signal - appearing as a tall green spike on a squiggly plot of Cassini's radio frequency - in mission control and live on NASA TV - saw what appeared to be a short reprieve, almost as though the spacecraft was making a brief comeback. The spike of the signal first began to diminish over a few seconds, but then rose briefly again before disappearing with finality.

"No, it wasn't a comeback. Just a side lobe of the radio antenna beam pattern," Webster said. Essentially, the reprieve was an unfocused part of the otherwise narrow radio signal that rotated into view as the spacecraft began to slowly tip over.

"Given that Cassini wasn't designed to fly into a planetary atmosphere, it's remarkable that the spacecraft held on as long as it did, allowing its science instruments to send back data to the last second," said Earl Maize, Cassini project manager at JPL. "It was a solidly built craft, and it did everything we asked of it."

SATURN DAILY
Intense storms batter Saturn's largest moon, UCLA scientists report
Los Angeles CA (SPX) Oct 16, 2017
Titan, the largest of Saturn's more than 60 moons, has surprisingly intense rainstorms, according to research by a team of UCLA planetary scientists and geologists. Although the storms are relatively rare - they occur less than once per Titan year, which is 29 and a half Earth years - they occur much more frequently than the scientists expected. "I would have thought these would be once-a- ... read more

Related Links
Cassini at NASA
Explore The Ring World of Saturn and her moons
Jupiter and its Moons
The million outer planets of a star called Sol
News Flash at Mercury


Thanks for being here;
We need your help. The SpaceDaily news network continues to grow but revenues have never been harder to maintain.

With the rise of Ad Blockers, and Facebook - our traditional revenue sources via quality network advertising continues to decline. And unlike so many other news sites, we don't have a paywall - with those annoying usernames and passwords.

Our news coverage takes time and effort to publish 365 days a year.

If you find our news sites informative and useful then please consider becoming a regular supporter or for now make a one off contribution.

SpaceDaily Contributor
$5 Billed Once


credit card or paypal
SpaceDaily Monthly Supporter
$5 Billed Monthly


paypal only

Comment using your Disqus, Facebook, Google or Twitter login.

Share this article via these popular social media networks
del.icio.usdel.icio.us DiggDigg RedditReddit GoogleGoogle

SATURN DAILY
Russia launches cargo ship to space station

US spacewalkers install 'new eyes' at space station

NASA May Extend BEAM's Time on the International Space Station

USNO Astronomers Measure New Distances To Nearby Stars

SATURN DAILY
First Four Space Launch System Flight Engines Ready To Rumble

Rocket motor for Ariane 6 and Vega-C is cast for testing

ASPIRE Successfully Launches from NASA Wallops

RS-25 Engines Ready for Maiden Flight of NASA's Space Launch System

SATURN DAILY
What NASA's simulated missions tell us about the need for Martian law

Debate over Mars exploration strategy heats up in astrobiology journal

Opportunity Feeling the Chemistry

Mimetic Martian water is under pressure

SATURN DAILY
China launches three satellites

Mars probe to carry 13 types of payload on 2020 mission

UN official commends China's role in space cooperation

China's cargo spacecraft separates from Tiangong-2 space lab

SATURN DAILY
Eutelsat's Airbus-built full electric EUTELSAT 172B satellite reaches geostationary orbit

Lockheed Martin Completes First Flexible Solar Array for LM 2100 Satellite

SpaceX launches 10 satellites for Iridium mobile network

GomSpace and Luxembourg to develop space activities in the Grand Duchy

SATURN DAILY
Space radiation won't stop NASA's human exploration

Oculus unveils standalone virtual reality headset

Microlasers get a performance boost from a bit of gold

Students, researchers turn algae into renewable flip-flops

SATURN DAILY
Biomarker Found In Space Complicates Search For Life On Exoplanets

Are Self-Replicating Starships Practical

New telescope attachment allows ground-based observations of new worlds

The Super-Earth that Came Home for Dinner

SATURN DAILY
Ring around a dwarf planet detected

Helicopter test for Jupiter icy moons radar

Solving the Mystery of Pluto's Giant Blades of Ice

Global Aerospace Corporation to present Pluto lander concept to NASA




Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News






The content herein, unless otherwise known to be public domain, are Copyright 1995-2017 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. All articles labeled "by Staff Writers" include reports supplied to Space Media Network by industry news wires, PR agencies, corporate press officers and the like. Such articles are individually curated and edited by Space Media Network staff on the basis of the report's information value to our industry and professional readership. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement