Planetary Spectroscopy Interferometer A low cost mission to explore - PowerPoint PPT Presentation

Planetary Spectroscopy Interferometer A low cost mission to explore planetary atmospheres and surfaces of our solar system Design study and Science Objectives by: Bruce Swinyard - UCL/RAL Space Leigh Fletcher, Neil Bowles – Oxford University Stuart Eves – SSTL Brian Ellison – RAL Space Bastien Rouquie – SSTL (Intern summer 2012) Craig Underwood – University of Surrey PSI PSI PSI RAS 12 th October 2012 1 1

PSI - Conceptual Design Ø Heterodyne THz interferometer for planetary science Ø Low Earth orbit – passively cooled Ø “Room temperature” technology – Schottky diode mixers Ø 4 receivers with ~60 cm antennae to ensure phase reconstruction and efficient of u-v plane filling Ø THz spectral range – precise frequencies TBD Ø 10 m maximum baseline Ø At the 179 µ m water line (1667 GHz) à 4.5 arcsec resolution Ø Complementary to ALMA (400-3000 µ m or 100-750 GHz.) Key point: High TRL technology Ø We undertook a short study in Summer 2012 and low cost spacecraft solution to identified critical issues key to delivering cost efficient science. PSI PSI RAS 12th October 2012 2 2

Key Science Drivers Ø Dynamics and Composition of Middle Atmospheres of all the planets in our solar system – Middle atmospheres are underexplored, winds have never been measured. – Planetary water cycles, origin of oxygen Spatial resolution similar to MIR on compounds in upper atmospheres. SOFIA 2.5 m telescope – Stratospheric circulation Ø Temperature variability of cool rock/ice bodies in the solar system. – Thermal light curves and rotational variability of TNOs, KBOs, asteroids, Pluto system, moons. Ø Water coma/tail mapping of cometary atmospheres. Key point: PSI offers a higher Ø Astronomical Targets – major star formation Spectral resolution sufficient to see spatial resolution at terahertz regions only poorly mapped in key species wind induced doppler shift wavelengths than previously Ø Possible option for terrestrial limb sounding of mesosphere and thermosphere achieved, and at wavelengths J. Hurley et al., Planetary and Space Science 58 (2010) inaccessible to ALMA . PSI PSI RAS 12th October 2012 3 3

Planetary Spatial Scale Ø A 3.1 THz channel would have 2.4” spatial resolution, compared to 4.5” for the 1.7 THz channel. Ø Assuming a 4.5" resolution, this spatially resolves Jupiter (average 48" size); and allows 4-5 spots across Saturn's 20" disc for similar purposes. Ø You still only get disc-averaged spectra of Uranus (3-4"), Neptune (2") and Titan (0.9"). Ø At opposition, Mars can be up to 25" in size, so you'd get 5-6 spots across the disc. Ø Venus can be up to 66" at opposition, 14-15 FOVs across. However, PSI will be unable Key point: Although horizontal to view Venus due to the need for a sunshield. resolution on these planets is limited, heterodyne spectral resolution permits excellent vertical coverage, resolving structures throughout planetary atmosphere. PSI PSI RAS 12th October 2012 4 4

Frequency Band Choice Ø Key species: CH 4 (temperature sounding), H2O, CO, HCN and isotopes. – CH 4 lines get stronger with higher frequencies. – CO peaks near 50 cm -1 (1498 GHz) – HCN peaks near 45 cm -1 (1350 GHz) – HCl, HI, HBr, HF all have lines in the sub-mm – H 2 O strongest lines are: 100.5 cm -1 , (3013 GHz) 88.0 cm -1 (2640 GHz), 92.5 cm -1 (2273 GHz), 104.6 cm -1 (3135 GHz), 55.7 cm -1 (1669 GHz), Ø Goal – cover three critical bands – 1667 GHz H 2 O (55.6 cm -1 ), 4.5 arcsec spatial resolution, 13000 km on Jupiter. Weak CH 4 nearby. – 2200 GHz H 2 O (and CH 4 at 73.2 cm -1 ) – 2500 GHz OH (and CH 4 near 83.6 cm -1 ) – 3100 GHz (CH 4 , 104 cm -1 , H 2 O at 100.5 cm -1 , 2.4” spatial resolution) or 3400 GHz (CH 4 , 115 cm -1 ) Key point: Some design freedom here – options are: i)Tunable system with sub-harmonic mixing – all three bands ii) Two bands, one near 1.7 THz, one near 3.1 THz. iii) Single band at 2.5 THz Science/technical trade off to be done – assumed 2.5 THz single band for study PSI PSI RAS 12th October 2012 5 5

Receivers Based on operation at 2.5 THz: Ø Room temperature or passively cooled Schottky diode mixer ( RAL ). Ø Quantum Cascade Laser (QCL) as Local Oscillator (LO), offers good potential > 2 THz but current technology requires operation at < 100 K ( Leeds University ). Ø Translate incoming frequency, in THz, into a lower Intermediate Frequency (IF), in the GHz range. Ø Alternative design: use sub-harmonic mixer and more standard LO design. Ø Overall system sensitivity at 150 K ~2500 K and at 250 K ~4500 K Key development areas over a 5-year development timescale: Ø Improvement in LO efficiency, power output and higher operating temperature . Use of QCL extremely promising, particularly for a passively cooled system Ø Improvements in room temperature and passively cooled Schottky diode performance. Ø System integration (mixer, LO, IF) for reduction in mass and power. Ø Requirement for sideband separation, either to reduce noise or spectral contamination. Quasi-optical filtering and/or sideband separating mixers. Key Point: Technical Readiness Level Ø Enhancement of backend spectrometer – broader bandwidth, lower mass and power. • Heritage from past missions: UARS, MLS, EOS, MLS, Odin, SWAS  TRL 8 • 2.5 THz receiver in operation in Earth orbit (using gas laser LO) for ~ 3 years (EOS Aura mission)  TRL 8 • > 1THz solid state LO for Herschel HIFI  TRL 7 Quantum Cascade Lasers integrated in a micro machined (development required for J-E mission) Example pictures of RAL fabricated air-bridge planar Schottky diodes – waveguide - Sandia Labs. balanced diode (left) and fully integrated structure, i.e. including filtering • Sensitivity equal to or exceeding SWAS and ODIN (right). Anode sizes are in the region of 1 to 2 µm. – a factor 7-10x worse than Herschel PSI PSI PSI RAS 12 th October 2012 6

Spacecraft Layout and u-v coverage Ø Choice of antenna configuration is complicated Digitised IF Ø Simple view is as the S/C rotates we take Transmit data and auto-correlate between antenna digitised integrated signal positions to get visibility fringes Digitised IF Ø Gives good u-v coverage but data rate Correlate restriction may prevent implementation Visibility function Ø Alternatively we correlate on board 5.00 Ø Distribute the antenna slightly differently 10.00 to attempt even coverage of Fourier 6.42 3.72 components 6 5 5 . Ø Many (many) ways of doing this – 2 . 5 4 optimum is Reuleaux triangle (Keto 1997) 8.66 Ø With 4 antenna may be overkill – here’s another configuration PSI PSI RAS 12th October 2012 7 7

On Board Correlation Options Ø On-board correlation easier because it reduces the problem of transmitting data, created especially by the rotation of the spacecraft. Ø Two options under consideration: Ø 1. Intermediate frequency (IF) from the mixers is transmitted to the central hub via co-axial cables, where either they are fed into a digital correlator. Ø 2. Use the Wide Band Spectrometer ( Star Dundee/Astrium) to digitise and spectrally deconvolve the signals providing digitised amplitude and phase. The digitised information is transmitted to a central spatial digital correlator. LO LO LO LO IF Amp IF Amp IF Amp IF Amp and filter and filter and filter and filter Digital Spectral Digital Spectral Digital Spectral Digital Spectral Correlator Correlator Correlator Correlator Transmitted phase lock Digitised Amplitude and Phase of IF Digital Cross Correlator N(N-1) Cross correlation productts PSI PSI RAS 12th October 2012 8

Orbit and Mission Concept 800 km circular sun-synchronous In practice, observations of a orbit with an inclination of 98.6 particular target object will be degrees, and a “dawn-dusk” local conducted over a period of time time of ascending node. when the Earth orbits between the target and the Sun Solar panels and sun-shield continuously pointed toward the Sun. PSI PSI PSI RAS 12 th October 2012 9

Satellite configuration issues Ø Central hub with four deployed booms Launch configurations – pre-attached to Ø Remote antenna attached at booms or free flyers launch or free flying and attached in orbit? Ø Need to balance centre of mass – restricts configuration options? Ø Single sun-shield to protect receivers and booms Ø Where to put the solar panels and comms? Ø Or multiple smaller shields protecting each receiver Ø Keeping the booms thermally stable? PSI PSI RAS 12th October 2012 10

Thermal control Ø Temperature required for current QCL technology is ~150 K Ø Mixers are better colder but will operate at room temperature Ø Thermal environment provided by passive radiators and deployed sun-shield Ø Two possibilities (see previous): Ø over the whole structure Ø only over the booms and the detectors (preferred from communications point of view) Ø TDS-1 drag sail concept ( Cranfield) looks like a possible solution but some issues to address Ø Reflectivity of the sail (multiple layers) Ø Robustness to deal with manoeuvres Ø RF transparency PSI PSI PSI RAS 12 th October 2012 11