Planetary Spectroscopy Interferometer

Transcription

1 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 RAS 12 th October

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

3 Key Science Drivers 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 compounds in upper atmospheres. Stratospheric circulation Spatial resolution similar to MIR on SOFIA 2.5 m telescope Temperature variability of cool rock/ice bodies in the solar system. Thermal light curves and rotational variability of TNOs, KBOs, asteroids, Pluto system, moons. Water coma/tail mapping of cometary atmospheres. Astronomical Targets Key major point: star formation offers a higher regions only poorly mapped spatial in key resolution species at terahertz Spectral resolution sufficient to see Possible option for terrestrial wavelengths limb sounding than of previously wind induced doppler shift mesosphere and thermosphere achieved, and at wavelengths J. Hurley et al., Planetary and Space Science 58 (2010) inaccessible to ALMA. RAS 12th October

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

5 Frequency Band Choice 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: cm -1, (3013 GHz) 88.0 cm -1 (2640 GHz), 92.5 cm -1 (2273 GHz), cm -1 (3135 GHz), 55.7 cm -1 (1669 GHz), Goal cover three critical bands 1667 GHz H 2 O (55.6 cm -1 ), 4.5 arcsec spatial resolution, km on Jupiter. Weak CH 4 nearby 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 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 RAS 12th October

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

7 Choice of antenna configuration is complicated Spacecraft Layout and u-v coverage Simple view is as the S/C rotates we take data and auto-correlate between antenna positions to get visibility fringes Gives good u-v coverage but data rate restriction may prevent implementation Alternatively we correlate on board Distribute the antenna slightly differently to attempt even coverage of Fourier components Many (many) ways of doing this optimum is Reuleaux triangle (Keto 1997) With 4 antenna may be overkill here s another configuration 2.54 Digitised IF Digitised IF Transmit digitised integrated signal Correlate Visibility function RAS 12th October

8 On Board Correlation Options On-board correlation easier because it reduces the problem of transmitting data, created especially by the rotation of the spacecraft. Two options under consideration: 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. 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 and filter IF Amp and filter IF Amp and filter IF Amp and filter Digital Spectral Correlator Digital Spectral Correlator Digital Spectral Correlator Digital Spectral Correlator Transmitted phase lock Digitised Amplitude and Phase of IF Digital Cross Correlator N(N-1) Cross correlation productts RAS 12th October

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

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

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

12 Attitude and Orbit Control System Pointing requirement is TBD but likely to be sub-arcmin Better than currently achieved on SSTL UoSat series but sensors and reaction wheels should be enough Sensors (SSTL) Star camera x 2 - Altair or Rigel Sun sensor x 2-2 axis DMC Rigel Mechanisms (SSTL) Wheel x 2 several options 10SP-M or 100SP-O or 200P-SP-ST Magnetorquer x 2 - MTR5 or MTR30 MTR30 RAS 12 th October

13 Booms Baseline needed for : 5 m boom length. Phase control: The receivers must be on the same plane to have the same phase. Booms must be stable so the difference remains small Small, fixed, differences can be corrected by the correlator. Vibration and flexure compensation using PZT actuators and sensors Lab scale demonstration of principle has been achieved by a number of groups. CFEsat boom (SSTL) Compact stowage: cylindrical envelope 450 x 150 mm Deployed length > 3.6 m: achieved within 5 minutes. Deployed frequency (with 3 kg payload) > 1 Hz. Stowed frequency > 150 Hz. Deployed alignment to be maintained within +/- 3. Deploy and support a 10 mm diameter payload harness. Minimise mass (target < 4.5kg). Minimise deployment power Provides indication of deployed length to +/- 10 mm. RAS 12 th October

14 Power, Mass and Bus Choice Power required ~ 500 W 28.5% efficient triple-junction GaAs cells assumed for the solar panel giving a solar panel area ~ 2.5 m² Need to establish the orbit average power requirements during the operational mode, and the orbit duty cycle for the instrument and the data downlink Need to establish several components of the mass budget Indicative mass ~ kg Platform generically on the order of SSTL UoSat-12 indicated at present RAS 12 th October

15 Operations Concept Number and Location of ground stations needs to be appropriate to provide several opportunities to communicate in one day For Sun-synch orbit latitude (only) dictates number of contact passes UK ~ 6 per day Svalbard 12 per day Contact time for one pass over a ground station ~ 7-10 min ~ 90minutes a day if Svalbard used Max data rate looking like 400 MBits/s Need to track the ground station during a pass Steerable antenna needs to deal with rotating spacecraft Implementation presents a challenge (!) Could point whole spacecraft This would limit operations but avoids antenna problem Sun-synchronous orbit coverage RAS 12 th October

16 The basic concept of a low cost heterodyne interferometer looks feasible and can be built entirely using UK technology Such a mission will deliver outstanding scientific return not only on Solar System objects but lalso on Galactic sources with spatial resolution 3x greater than Herschel and observes species inaccessible to ALMA Our short study has highlighted some critical items for further study: Detailed interferometer layout, correlation operations, pointing and data rate requirements Temperature requirements for the receivers especially stability requirements Demonstration receiver system needs to be breadboarded Can we achieve the required temperatures with low cost spacecraft? Can we achieve the required pointing requirements (APE, RPE, PRE) with a low cost spacecraft? How to downlink the data - antenna implementation on rotationg spacecraft and issue? We have done enough to show that the concept is not stupid......full Phase A study now required. Summary RAS 12th October