The presence and distribution of volatile elements on the moon, on asteroids, in comets, and on other airless bodies is of great significance to our understanding of the origin and evolution of the solar system and Earth's biosphere. An optical instrument placed on a suborbital reusable launch vehicle (sRLV) can investigate the distribution of water, hydroyxl, methane, carbon dioxide, carbon monoxide, and other volatiles on these bright bodies in our solar system.
There are varied approaches to conducting optical-spectral imaging of solar system objects. In the mid-IR (∼3–5μm) range, for instance, two options are a multispectral imager (see Figure 1) and an imager such as the commercially available Telops Hyper-Cam midwave imaging spectrometer. A multispectral imager acquires 2D images one wavelength at a time that can be stitched together in post-processing to create a 3D image with spectral information in the third dimension. Alternatively, an imaging spectrometer obtains a combined spatial/spectral image, and the second spatial dimension is built up over time by precisely scanning the spectrometer's aperture in the second direction. Both approaches require cryogenic detectors for mid-IR operation and possibly entail the cooling of optics, including telescope mirrors, baffles, and any gratings. We have found that the spatial resolution possible from a diffraction-limited, mid-IR telescope on an sRLV can be close to the resolution achieved from large-diameter ground and airborne instruments, which are often seeing in a limited fashion at mid-IR wavelengths. As an example, a ∼20cm telescope has a diffraction limit of ∼20μrad (about 500 pixels across a full moon) at 3μm.
Figure 1. Conceptual design of a mid-IR multispectral imager. The aperture front would point out the port. The filter can be inserted by hand or be part of a filter wheel.
Science objectives and the resulting measurement requirements will likely create challenges for observations from an sRLV. Using a pressurized cabin is attractive for manual operation and design simplicity because a crewed cabin allows manual control of the instrument, such as gross pointing or easy filter changes, significantly reducing its complexity while increasing flexibility. However, a warm pressurized cabin also requires purging naturally occurring spectrally obscuring gases such as carbon dioxide, water vapor, and methane, and possibly challenges the cooling of instrument components. Operating on an external mount would enable evacuation of the environment around the instrument, removing the need for purging, although it would require remote instrument operation. Whether internally or externally mounted, the instrument would need to access a viewport of the correct optical qualities, such as sapphire or zinc selenide (ZnSe) for the mid-IR, magnesium fluoride for UV observations, and ZnSe for long-wave IR measurements.
The expected few minutes at altitude should be sufficiently long to observe multiple bright objects. A complete spectral image or multiple filter images can typically be acquired in seconds. The observation duration might be extended if measurements are made during ascent and descent, although we do not feel that would be practical under extreme acceleration. Mid-IR wavelengths become fully accessible in the upper stratosphere. However, UV observations require a near-vacuum because of the well-mixed oxygen and nitrogen in the atmosphere. Pointing accuracy and stability may favor a multispectral, filter-wheel approach over a scanning spectrometer. Typical sRLVs have a pointing accuracy of one to several degrees and a drift rate of ∼0.5 deg./sec.,1 so individual frames would need to be taken in a fraction of a second. Multispectral imaging is well suited for this, taking an image one wavelength at time. The secondary mirrors of scanning spectrometers could potentially be used to compensate for spacecraft motion and obtain a contiguous spatial and spectral image.
These instruments cannot generally be rack-mounted and may require special considerations if there is potential for significant vibration during observations. Although an imaging spectrometer tends to be larger than a camera, a compact imaging grating spectrometer was recently developed by the Jet Propulsion Laboratory.2 Thus, the volume of either an imaging spectrometer or a multispectral camera would likely be dominated by its telescope,3 whose aperture would ideally abut or be very close to the view port.
Commercial sRLVs are potentially useful platforms for conducting spectral imaging of solar system bodies because of the availability of spectral windows that do not exist from the ground or from airborne vehicles operating in the lower stratosphere. An operator would enable significant flexibility in the design and operation of the instrument, although remotely operated optical instruments have flown in space and others are commercially available. Possible challenging aspects of conducting these measurements from sRLVs include the availability of an optically suitable view port, spacecraft pointing control and accuracy, and the presence of spectrally absorbing atmospheric gases if mounted inside a cabin Thus, we believe successful optical measurements from commercial sRLVs may require specific accommodation by the spacecraft provider as well as flexibility by the instrument provider to work within the platform limitations.
Platform capabilities will directly affect the instrument design and implementation. Thus, our first step to advance this effort will be to better quantify the current and future capabilities of commercially available sRLVs.
Johns Hopkins University
Applied Physics Laboratory,
Charles Hibbitts obtained a PhD from the University of Hawaii investigating the compositions of the Galilean icy satellites using IR reflectance spectroscopy. He holds undergraduate degrees from Cornell University in physics and from the University of New Mexico in geology.
2. P. Mouroulis, R. O. Green, T. G. Chrien, Design of pushbroom imaging spectrometers for optimum recovery of spectroscopic and spatial information, Appl. Opt. 39, no. 13pp. 2210-2220, 2000.
3. Conard, Design and fabrication of the New Horizons Long-Range Reconnaissance Imager, Proc. SPIE
5906, pp. 59061D, 2005. doi:10.1117/12.616632