Instrumentation for Interstellar Exploration Mike Gruntman Department of Aerospace Engineering University of Southern California Los Angeles, California Houston, October 2002 World Space Congress II 1/31
Interstellar Exploration First step into galactic environment Modest Concentrate on Reaching the unperturbed, virgin interstellar medium and exploring it in situ Exploring the distant frontier of the solar system Houston, October 2002 World Space Congress II 2/31
Sun s Galactic Neighborhood α Centauri λ = 239.4 deg β = - 42.5 deg Earth on May 21 Interstellar Wind (heliocentric V) V = 26 km/s λ = 252 deg β = + 7 deg Earth on June 3 Solar Apex V = 19 km/s λ = 270 deg β = + 54 deg Earth on June 22 Earth on March 21 Ecliptic Plane Vernal Equinox Houston, October 2002 World Space Congress II 3/31
Motivation The physics and state of the local interstellar medium (LISM) are not known in many important details Experimental data on the heliospheric interaction with the LISM are scarce, mostly indirect, and often ambiguous Houston, October 2002 World Space Congress II 4/31
Unknown or poorly known: LISM Magnetic field Number densities and ionization states of interstellar H and He Abundance of low-energy cosmic rays Interstellar dust (< 1 micron) Interstellar plasma: subsonic or supersonic Houston, October 2002 World Space Congress II 5/31
Unknown or poorly known: Interaction How does the solar wind expansion terminate? Is there a shock? Strong or weak? How is the energy divided among the ions and electrons? What happens to pickup ions at the shock and beyond? How are the ions accelerated? Is the heliopause stable? Is there the bow shock? Houston, October 2002 World Space Congress II 6/31
Interstellar Mission - Interaction Interstellar mission will provide ground truth measurements for the emerging remote techniques ENA imaging of the heliosphere EUV mapping of the heliopause Houston, October 2002 World Space Congress II 7/31
Which direction should we go? The heliopause could be 50% farther away in the sidewind direction The upwind (interstellar wind) direction is preferable Houston, October 2002 World Space Congress II 8/31
Interstellar plasma is disturbed by energetic neutral atoms ENAs (1982) Even supersonic interstellar wind would learn about an obstacle (the heliosphere) ahead Cosmic rays are disturbed Unperturbed LISM is at 350-400 AU How far should we go? Houston, October 2002 World Space Congress II 9/31
Spacecraft Velocity Mission duration ~ 20 years Escape velocity 75 km/sec or 16 AU/yr Velocity 86 km/sec at 1 AU Velocity increment (added to the orbital velocity around the sun) 56 km/sec Mass ratio R = 340,000 for specific impulse Isp = 450 sec Chemical propulsion not sufficiently efficient Houston, October 2002 World Space Congress II 10/31
Propulsion Nuclear Electric and Sails Technology rapidly approaching maturity Nuclear (fission) electric May disturb the immediate spacecraft environment Could be jettisoned at ~ 50 AU Spacecraft will be powered by a separate RTG Solar sails Will disturb the immediate spacecraft environment Could be jettisoned at ~ 5 AU (as Interstellar Probe) Spacecraft will be powered by a separate RTG Houston, October 2002 World Space Congress II 11/31
Measurements Objectives UNIQUE mission Focus on the measurements that cannot be performed anywhere else in the solar system Three types: In situ exploration of unperturbed ISM In situ exploration of the interaction region Remote observations at large heliocentric distances Houston, October 2002 World Space Congress II 12/31
Parameter Variations - Orders of Magnitude Examples: Plasma density Magnetic field 1 nt 0.01 nt at TS 1 nt at HP 0.1 nt in the LISM Houston, October 2002 World Space Congress II 13/31
Type I: Unperturbed LISM Prevented from entering the inner heliosphere Interstellar plasma Interstellar dust grains (<0.1 micron; <2 micron) Dust from the Edgeworth-Kuiper Disk (~80% excluded by giant planets) Low-energy cosmic rays Complex organic molecules (amino acids; polycyclic aromatic hydrocarbons - PAHs) Houston, October 2002 World Space Congress II 14/31
Type II: Interaction Region Ground truth measurements, complementing remote techniques (ENA imaging; EUV mapping) Solar wind plasma Solar wind pickup ions Solar and interstellar magnetic fields Anomalous and galactic cosmic rays Details of particle acceleration Low-frequency heliospheric radio emissions Houston, October 2002 World Space Congress II 15/31
Type III: Remote Observations Take advantage of large heliocentric distances H Lyman-α (121.6 nm) Interstellar hydrogen number density Scattering of the solar radiation ( glow ) Multiple scattering dependence on optical depth Independent of the detector absolute calibration Interstellar hydrogen number density Attenuation of the solar Lyman-α line Independent of the detector absolute calibration Houston, October 2002 World Space Congress II 16/31
Type III: Remote Observations Take advantage of large heliocentric distances H Lyman-α (121.6 nm) Galactic Lyman-α background radiation Glow would be < 5 Rayleigh at 300-4000 AU Upwind direction is 18 off the direction toward the Galactic center Upwind direction is 83 off the direction toward the Galactic North pole Sensor on a spinning spacecraft anisotropy of the Galactic background radiation Houston, October 2002 World Space Congress II 17/31
Type III: Remote Observations H Lyman-α Hydrogen Wall structure Houston, October 2002 World Space Congress II 18/31
Type III: Remote Observations Stereoscopic imaging of the heliosphere (interaction region) in ENA fluxes Imaging essentially involves averaging over the line of sight Sheer size of the heliosphere requires the second vantage point at 100-300 AU Houston, October 2002 World Space Congress II 19/31
Type III: Remote Observations Take advantage of large heliocentric distances Cosmic infrared radiation background Infrared emission of zodiacal dust prevent measurements from the inner heliosphere Traditional optical remote sensing instruments Survey heliocentric distribution of the Edgeworth- Kuiper Belt bodies Unique platform for optical parallax measurements from several hundred AUs Houston, October 2002 World Space Congress II 20/31
Interstellar Spacecraft Environment Debye length Doppler shift 0.3 Å for H Lyman-α 1.5 Å for sodium lines (5890 Å) Velocity 100 km/sec relative LISM 52 ev/nucleon Oxygen 830 ev Electron 0.03 ev Houston, October 2002 World Space Congress II 21/31
Measurement Concepts and Instruments Interstellar Probe study (1999) Magnetic fields Plasma and radio waves Solar wind plasma Pickup ions Suprathermal ions and electrons Interstellar plasma Interstellar neutral gas Cosmic ray ions and electrons Anomalous cosmic rays Mass distribution of dust grains and composition Lyman-α glow Infrared radiation Complex organic molecules.. Houston, October 2002 World Space Congress II 22/31
Instrumentation Challenges Long mission duration and costly DSN resources spacecraft and instrumentation autonomy Mass, power, telemetry constraints Miniaturization Not everything can be miniaturized For example, an accumulation of the statistically significant number of counts requires certain effective sensitive areas and solid angles Houston, October 2002 World Space Congress II 23/31
Instrumentation Challenges Spacecraft: likely a spinner pointed at the sun No mechanically moving (with respect to the spacecraft) sensors necessary Sample distributions of particles Field measurements Sky scans by remote sensing instruments Challenge in pointing the instruments Oxygen atoms/ions within 3 angle for 7500 K LISM May miss the instrument field of view Houston, October 2002 World Space Congress II 24/31
Instrumentation Challenges Two categories: 1. Improving existing instrumental concepts by better components Smaller Lighter Less power consuming More capable 2. New instrumental concepts Houston, October 2002 World Space Congress II 25/31
Instrumentation Challenges: Category 1 Improving existing instrumental concepts - example Low-energy particle solid-state detector Direct counting particles (ions, electrons, neutrals) with E=100 ev (and perhaps lower) Direct measuring energy of particles without preacceleration Can be used in TOF instruments No high voltages Robust Superior to MCPs Houston, October 2002 World Space Congress II 26/31
Instrumentation Challenges: Category 2 New instrumental concepts needed - example Search, identification, and detection of complex organic molecules in the LISM Most common polycyclic aromatic hydrocarbons (PAHs) are coronene (C 24 H 12 ) and hexabenzocoronene (C 48 H 18 ) Corresponding energies 15.5 kev and 30.9 kev Energies insufficient for analysis by TOF thin-foil instruments Energies too high for capture (and adsorption) on surfaces without destroying molecular bonds Houston, October 2002 World Space Congress II 27/31
Instrumentation Challenges: Category 2 New instrumental concepts needed - example Search, identification, and detection of complex organic molecules in the dust grains Collision velocity ~ 100 km/sec Higher than Comet Halley flybys by Giotto and VEGAs No laboratory accelerators achieve such velocities Interpretation would rely on theoretical modeling of high-velocity impact without experimental verification Organic material will be entirely dissociated and fragments ionized Houston, October 2002 World Space Congress II 28/31
Instrumentation Challenges: Opportunities Non-Traditional Measurements Use the entire spacecraft as a detector Dust impacts create plasma effects that are detected by plasma wave experiments (Gurnett et al., 1983) Statistically characterize dust population Precise tracking of the spacecraft Pioneer unexplained acceleration (Anderson et al., 2002) Plan in advance Houston, October 2002 World Space Congress II 29/31
Interstellar Mission Look Back As our first interstellar spacecraft leaves the solar system, a look back would provide us with an unusual view of our home stellar system, a view from the outside This view back would be a glimpse of what a truly interstellar mission of the distant future would encounter in approaching a target star The astrosphere can be characterized by its image in ENAs and the hydrogen wall in Lyman-α The stellar wind can be probed by detecting the neutral stellar wind, penetrating the surrounding interstellar medium, before the starship would enter the astrosphere filled with the stellar wind. Houston, October 2002 World Space Congress II 30/31
Look Back Sun - the brightest star Apparent magnitude m = -13.7 at 400 AU Sirius m = -1.46 Only in 7 thousand years at 113,000 AU (0.55 pc or 1.79 l.y.) the sun will surrender its supremacy as the brightest star in the sky The Sun as Seen from Pluto By Chesley Bonestell, 1961 From: R. Miller and F. C. Durant III, The Art of Chesley Bonestell, Collins & Brown, 2001. Houston, October 2002 World Space Congress II 31/31