NASA’s Juno Spacecraft carries a science payload consisting of nine instrument packages to provide unprecedented data on Jupiter’s magnetic environment, its gravitational field, the incredibly dense atmosphere & cloud cover, the interior of the planet and Jupiter’s puzzling aurora.
Juno uses it instruments to look for clues about Jupiter’s formation which will allow scientists to infer details on the solar system’s formation since Jupiter maintained its current state since the early stages of the solar system. Also, the mission sets out to determine whether Jupiter has a solid core, find out how much water is present within the planet’s dense atmosphere, & study winds that can reach more than 600 Kilometers per hour.
Juno is carrying the following scientific instruments that are explained in detail below:
- Gravity Science – GS
- Magnetometer – MAG
- Microwave Radiometer – MWR
- Jupiter Energetic Particle Detector Instrument – JEDI
- Jovian Auroral Distributions Experiment – JADE
- Radio and Plasma Wave Sensor – Waves
- Ultraviolet Spectrograph
- Jovian Infrared Auroral Mapper – JIRAM
To reveal the interior structure of Jupiter, Juno makes detailed measurements of the planet’s gravitational field which will point to internal structures that are hidden by the planet’s dense atmosphere.
The experiment is a radio science experiment that involves X-Band and Ka-Band ranging from ground stations on Earth to follow the spacecraft in its orbit around the planet and detect even minute changes in the motion of the spacecraft. Local variations in gravity can act on the spacecraft in orbit and cause it to speed up or slow down – those changes in spacecraft motion can be detected using the Doppler Shift in the X and Ka band transponders used by the radio sub-system.
For the gravity experiment, the High Gain Antenna needs to be pointed directly at Earth so that Ka-Band Ranging Signals and X-Band Signals can be sent and received. The Deep Space Network has only one Station capable of providing Ka-Band uplink which is Deep Space Station 25 at DSN Goldstone.
Turnaround ranging using the Deep Space Network involves the DSN station that sends an Ka-Band signal to the spacecraft containing ranging tones that it imposes on a carrier using phase modulation. When the spacecraft receives the tones, it sends them right back via X-Band downlink. The DSN station records the timing of the ranging tones uplink and the timing of the tone’s reception order to calculate the line-of-sight distance to the spacecraft.
After processing of the data taking into account delays by the electronics on the spacecraft and the ground, atmospheric and ionospheric properties, interplanetary plasma, and relativistic effects, the ranging method has an accuracy of about one meter in the outer regions of the solar system.
Following corrections for radio signal distortion in Earth’s atmosphere, scientists will be able to use ranging data to map the gravity field of the planet and identify internal features.
The MAG instrument of Juno measures Jupiter’s magnetic field to create a detailed three-dimensional map of the Gas Giant’s magnetic environment.
Juno uses a fluxgate magnetometer developed at NASA’s Goddard Spaceflight Center that is installed on one of the three solar arrays of the spacecraft to move the instrument as far away from the spacecraft platform to avoid false readings caused by Juno’s own magnetic emissions.
MAG uses dual-fluxgate magnetometers to measure the magnetic field vector and a 3-cell scalar Helium magnetometer sensor provided by JPL is used to measure the strength of the field. An Advanced Stellar Compass provides precise attitude data for each of the sensors.
Two fluxgate magnetometers are installed on the magnetometer boom that is installed on the solar array – one is installed 9.8 meters from the spacecraft structure and the other resides 11.8m from the S/C bus and is rotated 180 degrees relative to the other sensor. The scalar Helium magnetometer is located inboard, 8.8m from the platform.
The two fluxgate sensors use the nonlinearity of magnetization properties for the high permeability of easily saturated ferromagnetic alloys to serve as an indicator for the local field strength. The entire Magnetometer instrument weighs 15.25 Kilograms.
Having the two fluxgate sensors installed at different distances will allow scientists to determine Juno’s magnetic field that is subtracted from the data to achieve high-precision measurements of Jupiter’s magnetic environment.
“Juno’s magnetometers will measure Jupiter’s magnetic field with extraordinary precision and give us a detailed picture of what the field looks like, both around the planet and deep within,” says Goddard’s Jack Connerney, the mission’s deputy principal investigator and head of the magnetometer team. “This will be the first time we’ve mapped the magnetic field all around Jupiter-it will be the most complete map of its kind ever obtained about any planet with an active dynamo, except, of course, our Earth.”
The MWR instrument will study the hidden structure beneath Jupiter’s cloud tops – capable of determining on the structure, movement and chemical composition to a pressure of 1,000 atmospheres which corresponds to a depth of 550 Kilometers below the cloud cover. The instrument will help determine the abundance on Water and Ammonia in the Jovian atmosphere.
MWR consists of six separate radiometers each with its own antenna and receiver that measure the radiation at six different frequencies along the orbital track of the spacecraft (600 MHz, 1.2 GHz, 2.4 GHz, 4.8 GHz, 9.6 GHz & 22 GHz). The receivers are fed by a combination of patch & slot array antennas as well as horn antennas optimized for the different wavelengths.
The MWR antennas are mounted on the outside of the Juno spacecraft. The 600 MHz antenna occupies one entire side of the hexagonal spacecraft body and is directly installed on the spacecraft platform. The 1.2, 2.4, 4.8 & 9.6 GHz antennas are installed on a separate support panel attached to another panel on the vehicle’s body. The 22 GHZ antenna is installed on the upper deck of the vehicle. All antennas are connected to the MWR electronics unit via coaxial cables or rectangular waveguides.
The 22 GHz antenna is a profiled corrugated horn with a circular-to-rectangular transition. It is made of solid aluminum and shows a low side lobe performance. The 2.4, 4.8 and 9.6 GHz antennas are waveguide slot array antennas as part of a low-volume, low-mass system. Each antenna has 8×8 slots divided into four 4×4 arrays. The two low-frequency antennas are 5×5 patch array antennas.
The MWR electronics unit has a modular design, consisting of five individual slices: a Power Distribution Unit (2 slices), a Command & Data Handling Unit, and the Housekeeping Unit (2 slices).
The Power Distribution Unit includes six power converters that are connected to the 28-volt spacecraft bus and generate voltages needed by the MWR instrument. The Command & Data Handling Unit includes a 8051 microcontroller based system that processes and executes spacecraft commands and telemetry, controls all data provided by MWR including science & housekeeping data. The system is connected to the main data system of Juno via a redundant RS-422 bus with a transfer rate of 57.6Kb/s. MWR includes a master crystal oscillator to provide precise timing data.
The Housekeeping Units make temperature measurements for radiometric calibrations and they monitor the instrument’s health & bus voltages. A total of 128 multiplexed channels are used to monitor temperature (112 channels) and bus voltages (16 channels). Inside the electronics vault, thermistors are used for temperature measurements while data on the outside of the vehicle is acquired by platinum resistive thermometers that can withstand the extreme radiation environment at Jupiter.
Jupiter Energetic Particle Detector Instrument
JEDI installation locations on Juno – Image: NASA/JHU
The JEDI instrument will measure energetic particles and their interaction with Jupiter’s magnetic field, investigating Jupiter’s polar space environment with special focus on the physics of the intense Jovian auroras. JEDI measures the energy, spectra, mass species (H, He, O, S), and angular distributions of the higher energy charged particles. The JEDI instrument weighs 6.4 Kilograms including 5 Kilograms of shielding material.
The instrument consists of three nearly identical sensors – each with six ion and six electron views that are arrayed in 12 by 160 degree fans with six 26.7° look directions. Two of those units are installed in a way so that nearly a complete 360-degree coverage normal to the spacecraft spin axis can be achieved in order to get complete pitch angle snapshots. The other sensor is aligned with the spin axis to gather complete sky-views over one spin period of 30 seconds. Each of the JEDI-270/90 units measures 23.3 by 15.9 by 16.1 centimeters while the single JEDI-180 unit is 23.3 by 16.9 by 12.8cm. JEDI sensors are self-contained, they have no additional hardware inside the electronics vault of the spacecraft.
Each JEDI Sensor includes the electron and ion sensors as well as detector preamplifiers. The sensor heads and main electronics are integrated as a single unit installed on the Juno spacecraft. The sensor heads have separate data and power interfaces with the spacecraft and run independently of each other.
Ions are examined by compact time-of-flight (TOF) by energy and TOF by MCP-Pulse-Height spectrometers that determines three TOF parameters and the energy of ions to identify Hydrogen, Oxygen, Sulfur and other ions. JEDI measures ions at energy ranges of 10keV (kilo electronvolt) to 10 MeV. Electrons from 25 keV to 1 MeV are measured using collimated solid-state detectors that provide energy data and directional distributions.
The JEDI sensor heads consist of an aperture opening, electron deflectors, start foils and anodes, a microchannel plate detector, stop anodes and foils, solid state detectors and pre-amplifiers as well as supporting electronics.
The JEDI sensor heads include TOF sections 6 centimeters across that feed the silicon solid-state detectors. The SSD array and the individual pre-amplifiers are connected to an Event Board that determines particle energies.
JEDI Sensor Design
As an ion enters the instrument, it first passes through a thin foil in the collimator (350A Aluminum) before reaching the start foil (carbon-polyamid-carbon) and generating secondary electrons. These electrons are then directed from the primary particle path to the microchannel plate detector where the Start Signal is generated for the Time of Flight measurement. A 500-Volt potential between the foil and the MCP directs the secondary electrons to the TOF detector with high accuracy (1ns dispersion in transit time). The segmented MCP anodes with two start anodes for each of the six angular segments provide data on the direction of travel of the ion.
Secondary electrons that are created as a result of the ion passing through the stop foil are again directed to the MCP and cause a Stop Signal. The time-difference between the two signals represents the time it took the ion to pass through the 6-centimeter TOF instrument.
After the stop foil, ions impact the Solid State Detectors that consist of electron and ion pixels. The SSD determines ion energy which coupled with the TOF measurement delivers ion mass and particle species data. The collimator foil is installed on a high-transmittance grid supported by stainless steel frames. The start/stop foils use a tungsten-copper frame.
Electrons entering the instrument are first decelerated by a 2.6kV potential which is part of the TOF system for ion measurements.
After passing the stop foil, the electrons are again accelerated by a 2.6kV potential. Reaching the SSD detectors, the electrons are detected in the electron pixels that can measure electrons at energies of 25 keV to 1 MeV. The electron detectors are covered with 2-micrometer aluminum metal flashing to reject protons at low energies. Electron measurements do not require a TOF measurement because direction is directly measured by the detector.
The detector system has to be time-multiplexed and can either measure electrons or ions. Three species modes (electron energy, ion energy & ion species, all coupled with direction measurements) are cycled every 0.5 seconds. The six physical SSDs provide a total of 24 SSD pixels (every SSD has 2 electron and two ion pixels – one large pixel of 6.2 by 6.5mm and a small pixel in the center of 1.3 by 1.6mm). Each SSD is connected to a Preamp Board that is part of the sensor assembly.
The electronics box of each sensor holds the event board, power supplies and support electronics. The Event Board interfaces with the sensor to receive TOF signals, SSD data, and MCP pulse heights that are processed by a RTAX2000 16-bit processor. A dedicated Low Voltage Power Supply delivers the low-voltage buses for the various electronics of the sensor while the high-voltage is provided to the sensor head via a High Voltage Supply and Monitor Unit. Data and commands between the instrument and the spacecraft are exchanged via an RS-422 link. Overall, JEDI can process 30,000 events per second.
Each JEDI Sensor head is protected by a cover that is deployed after launch as part of instrument commissioning.
Radio and Plasma Wave Sensor – Waves
The Waves instrument measures radio and plasma waves in the Jovian magnetosphere to help understand interactions between Jupiter’s magnetic field, the magnetosphere and the atmosphere. It measures the electric and magnetic field components of in-situ plasma waves and freely propagating radio waves.
The instrument consists of a V-shaped antenna that measures four meters from tip to tip – a dipole antenna to measure electric fields, and a magnetic search coil to measure the magnetic component. Waves electronics are installed inside the Radiation Vault.
The dipole antenna and its electronics are built to analyze electric fields in the frequency range of 50Hz to 40MHz. The antenna consists of two elements each 2.8m in length. These elements are extended in a plane rotated 45 degrees to the aft of the aft deck with a subtended angle of 120 degrees. The antenna is installed aft of the solar panel wing that features the Magnetometer Boom to be symmetric with the wing.
The magnetic search coil consists of a fine copper wire wrapped 10,000 times around a 15-centimeter mu-metal core (77% nickel, 16% iron, 5% copper 2% chromium) – an alloy that has a very high magnetic permeability. The search coil is installed on the aft flight deck parallel to the spacecraft z-axis which itself is parallel to the spacecraft spin axis. This is done to minimize the effects of the very strong magnetic field of Jupiter rotating as the vehicle spins at 2rpm near perijove. The magnetic search coil measures wave magnetic fields from 50hz to 20kHz.
The Waves sensor electronics consist of two receivers – a low and a high frequency receiver. The low frequency receiver includes two channels that are covering the frequency range of 50 Hz to 20 kHz. The system operates in two configurations – one allows for simultaneous sampling of electric and magnetic sensor data and the other configuration uses a signal from the Juno Power Distribution Unit to reflect voltage fluctuations on the bus to be used in a noise cancelling mode with either the electric or the magnetic signal being analyzed in the second channel. The third channel of the Low Frequency Receiver is a high band that covers frequencies of 10 kHz to 150 kHz used for electric signals only with noise cancelation capability.
The High-Frequency Receiver is comprised of two nearly identical units – one used to analyze data from 100 kHz to 40 MHz and the other allows for high-resolution waveform measurements in a 1-MHz band.
The baseband receiver includes a variable gain amplifier, a 100 kHz to 3 MHz bandpass filter and a 12-bit analog-to-digital converter. The second receiver is a double sideband heterodyne receiver detecting the amplitude of signals in 1-MHz bandwidths from 3 to 40 MHz as a swept frequency receiver.
The Waves Data Processing Unit consists of two field programmable gate arrays.
The first is responsible for Waves instrument operations including command execution, data output functions, observation scheduling and on-board analysis. The second FPGA is optimized to carry out Fourier transforms and other signal processing operations to move signal analyses from the analog to the digital domain – performing spectrum analysis, spectral binning and averaging, and noise cancellation.
(for complete article and additional information on the Juno spacecraft, visit https://spaceflight101.com/)
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