Measuring Earth's energy budget from space is an essential ingredient for understanding and predicting Earth's climate. We have demonstrated the use of vertically aligned carbon nanotubes (VACNTs) as photon absorbers in broadband radiometers own on the Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) 3U CubeSat. VACNT forests are some of the blackest materials known and have an extremely at spectral response over a wide wavelength range. The radiation measurements are made at both shortwave, solar-reflected wavelengths and in the thermal infrared. RAVAN also includes two gallium phase-change cells that are used to monitor the stability of RAVAN's radiometer sensors. RAVAN was launched November 11, 2016, into a nearly 600-km sun-synchronous orbit and collected data over the course of 20 months, successfully demonstrating its two key technologies. A 3-axis controlled CubeSat bus allows for routine solar and deep-space attitude maneuvers, which are essential for calibrating Earth irradiance measurements. Funded by the NASA Earth Science Technology Office, RAVAN is a pathfinder to demonstrate technologies for the measurement of Earth's radiation budget that have the potential to lower costs and enable new measurement concepts. In this paper we report specifically on the VACNT growth, post-growth modification, and pre-launch testing. We also describe the novel door mechanism that houses the gallium black bodies.
Two thematic drivers are motivating the science community towards constellations of small satellites, the revelation that many next generation system science questions are uniquely addressed with sufficient numbers of simultaneous space based measurements, and the realization that space is historically expensive, and in an environment of constrained costs, we must innovate to ―do more with less‖. We present analysis that answers many of the key questions surrounding constellations of scientific satellites, including research that resulted from the GEOScan community based effort originally intended as hosted payloads on Iridium NEXT. We present analysis that answers the question how many satellites does global system science require? Perhaps serendipitously, the analyses show that many of the key science questions independently converge towards similar results, i.e. that approximately 60+ satellites are needed for transformative, as opposed to incremental capability in system science. The current challenge is how to effectively transition products from design to mass production for space based instruments and vehicles. Ideally, the lesson learned from past designs and builds of various space products should pave the way toward a better manufacturing plan that utilizes just a fraction of the prototype‘s cost. Using the commercial products industry implementations of mass customization as an example, we will discuss about the benefits of standardization in design requirements for space instruments and vehicles. For example, the instruments (payloads) are designed to have standardized elements, components, or modules that interchangeably work together within a linkage system. We conclude with a discussion on implementation plans and the new paradigms for community and international cooperation enabled by small satellite constellations.
Detecting energetic particles is a useful approach in studying space plasmas. Of specific interest are energetic neutral atoms (ENA) because their trajectories are unaffected by electric or magnetic fields. Imaging the ENA flux allows for the mapping of remote plasmas. In order to detect such particles, solid-state detectors are advantageous due to their lightweight and low power. However in the sensing environment the photon flux is usually several orders of magnitude higher than the ENA flux. Thus, in order to detect the energetic particles the photon flux must be blocked. Therefore, thin metal or carbon film filters that allow the transmission of ENAs while attenuating the photon signal are used. Here we report tests of low-density mats of carbon nanotubes (CNTs) as a filter medium. For a given mass per unit area (the parameter which sets the particle transmission energy threshold), CNTs are expected to absorb photons significantly better than thin films. The CNTs were grown by a water assisted chemical vapor deposition technique and pulled from their substrate to generate a CNT sheet covering an aperture. In order to test the performance of the CNT sheet as a filter, the transmissions of light and alpha particles were measured. We were able to achieve filter performance that resulted in alpha particle energy loss of only 5 keV with an optical density of 0.5.
Hand-held instruments capable of spectroscopic identification of chemical warfare agents (CWA) would find extensive
use in the field. Because CWA can be toxic at very low concentrations compared to typical background levels of
commonly-used compounds (flame retardants, pesticides) that are chemically similar, spectroscopic measurements have
the potential to reduce false alarms by distinguishing between dangerous and benign compounds. Unfortunately, most
true spectroscopic instruments (infrared spectrometers, mass spectrometers, and gas chromatograph-mass spectrometers)
are bench-top instruments. Surface-acoustic wave (SAW) sensors are commercially available in hand-held form, but rely
on a handful of functionalized surfaces to achieve specificity. Here, we consider the potential for a hand-held device
based on surface enhanced Raman scattering (SERS) using templated nanowires as enhancing substrates. We examine
the magnitude of enhancement generated by the nanowires and the specificity achieved in measurements of a range of
CWA simulants. We predict the ultimate sensitivity of a device based on a nanowire-based SERS core to be 1-2 orders
of magnitude greater than a comparable SAW system, with a detection limit of approximately 0.01 mg m-3.
We describe the fabrication of an Orotron driven by a sheet beam of electrons. The sheet beam is generated by a carbon
nanotube field emission electron gun, which is less than 2 mm in total thickness. The orotron cavity is 2 cm long and 1
cm wide, and houses a microfabricated Smith-Purcell grating which generates the THz radiation. The sheet beam is 5
μm thick and 6 mm wide, and it travels within 15 μm of the top surface of the Smith-Purcell grating for the length of the
cavity. The Orotron is discretely tunable, which means that there are a number of cavity resonances that can be driven
by changing the energy of the beam such that for the period of the Smith-Purcell grating the cavity is driven on one of
the resonances. For this work, a target frequency of 0.5 THz, corresponding to a beam energy of 3 keV, was used.
Observations of the Earth are extremely challenging; its large angular extent floods scientific instruments with high flux
within and adjacent to the desired field of view. This bright light diffracts from instrument structures, rattles around and
invariably contaminates measurements. Astrophysical observations also are impacted by stray light that obscures very
dim objects and degrades signal to noise in spectroscopic measurements. Stray light is controlled by utilizing low
reflectance structural surface treatments and by using baffles and stops to limit this background noise. In 2007 GSFC
researchers discovered that Multiwalled Carbon Nanotubes (MWCNTs) are exceptionally good absorbers, with potential
to provide order-of-magnitude improvement over current surface treatments and a resulting factor of 10,000 reduction in
stray light when applied to an entire optical train. Development of this technology will provide numerous benefits
including: a.) simplification of instrument stray light controls to achieve equivalent performance, b.) increasing
observational efficiencies by recovering currently unusable scenes in high contrast regions, and c.) enabling low-noise
observations that are beyond current capabilities. Our objective was to develop and apply MWCNTs to instrument
components to realize these benefits. We have addressed the technical challenges to advance the technology by tuning
the MWCNT geometry using a variety of methods to provide a factor of 10 improvement over current surface treatments
used in space flight hardware. Techniques are being developed to apply the optimized geometry to typical instrument
components such as spiders, baffles and tubes. Application of the nanostructures to alternate materials (or by contact
transfer) is also being investigated. In addition, candidate geometries have been tested and optimized for robustness to
survive integration, testing, launch and operations associated with space flight hardware. The benefits of this technology
extend to space science where observations of extremely dim objects require suppression of stray light.
We describe a design concept for a flat (or conformal) thin-plate laser phased-array aperture. The aperture consists of a
substrate supporting a grid of single-mode optical waveguides fabricated from a linear electro-optic material. The
waveguides are coupled to a single laser source or detector. An arrangement of electrodes provides for two-dimensional
beam steering by controlling the phase of the light entering the grid. The electrodes can also be modulated to
simultaneously provide atmospheric turbulence modulation for long-range free-space optical communication. An
approach for fabrication is also outlined.
We describe progress towards an Oroton-based sub-millimeter-wave source with a design frequency of 500 GHz. Key
features of the devices are a microfabricated, carbon nanotube field-emission-based electron gun which creates a sheet-beam
at the required current density without the need for beam compression, and a microfabricated Smith-Purcell
grating, and a uniform Z-direction magnetic field confinement.
We report the results of scanning micro-Raman spectroscopy obtained on Au-Ag nanowires for a variety of chemical
warfare agent simulants. Rough silver segments embedded in gold nanowires showed enhancement of 105 - 107 and
allowed unique identification of 3 of 4 chemical agent simulants tested. These results suggest a promising method for
detection of compounds significant for security applications, leading to sensors that are compact and selective.
We describe a scanning Orotron Terahertz radiation (THz) source. The operational principle is as follows: a
sheet beam of electrons passes near a corrugated metal surface (Smith-Purcell grating) contained in a resonant
cavity. The periodic forces on the electrons drive the cavity on its resonances in the THz regime. We describe
theoretical predictions for the sheet beam parameters required and the likely performance of the device. We
also describe experimental progress towards sheet-beam generation using field-emitted electrons from a carbon-nanotube
array. We describe the carbon nanotube growth process and demonstrate sheet-beam current densities
which exceed the predicted turn-on current density of the Orotron cavity.
Surface enhanced Raman spectroscopy (SERS) has promise as an optical sensor for the detection of chemical and biological agents, in particular when combined with front-end processing for sample preparation prior to analysis. In this paper, we report preliminary results from a SERS analysis of Bacillus cereus T strain (BcT), which was prepared for sensor analysis via a microfluidics-based sample processor. In the microfluidics hardware, low and high molecular weight analytes from a sonicated spore sample were separated via mass-dependent diffusion into two independent microchannels. SERS analysis of the sample outputs revealed a significant separation of the low molecular spore biomarker, dipicolinic acid, from the high molecular weight protein and nucleic acid background. In addition to the processing study, measurements were performed on gold core-shell nanospheres, which are considered a potential SERS substrate for the microfluidic system. Finally, field-induced aggregation of silver nanoparticles, an alternative to chemical aggregation, was shown to be an effective approach for the production of highly enhancing SERS substrates.
We report results of scanning micro-Raman spectroscopy obtained on isolated nanowires and networks of nanowires with different geometries and surface morphologies. We measured a strong, relatively homogeneous, surface enhancement of the Raman response from nanowires with a rough surface morphology, and detected a more sporadic enhanced response detected from smooth nanowires. These results provide the first steps towards the development of selective sensors for hazardous bio- and chemical-agent detection that rely on a combination of electronic conductance measurements and Raman spectroscopic measurements from metallic nanowire networks.