11:15 AM - 11:30 AM
[PCG20-09] Optical performance evaluation of the Comet Hydrogen Imager onbord the Comet Interceptor Mission
Keywords:Planetary exploration, Comet Interceptor, EUV
The Hydrogen Imager (HI) is an ultraviolet imager on board the ESA-led long-period comet mission: Comet Interceptor. This study focuses on the optical performance of the HI, especially on the imaging resolution and the detection efficiency of the ultraviolet light, with experiments.
Comets are small bodies of ice and dust with large orbital eccentricity, and in most cases their orbits intersect the snow line. The large amounts of water that form the oceans are thought to have been brought by asteroids and comets along with organics (O'Brien et al. 2018), but how comets contributed to the origin of Earth's water is still not understood, and mapping comet ice properties to the location of comet formation is a critical step to understanding the origin of Earth's water (Mandt et al. 2024).
The ejection rate of volatiles from the nucleus increases as comet’s approaching the sun, and the water production rate can be expressed as a function of the sun-comet distance following a power law, and a relationship between this power exponent and cometary origin has been shown (Combi et al. 2005). One means of identifying the water production rate is to measure the spatial distribution of hydrogen atoms by Ly-α imaging, and Combi et al. (2005) also used data from SOHO/SWAN observing hydrogen Ly-α emission.
Model calculations are needed to connect the spatial distribution of hydrogen atoms around the comet with the water production rate. Water molecules ejected from the comet nucleus are split into H and OH, and OH into O and H through photo-dissociations, and the hydrogen produced in these two reactions spreads out at different initial speeds. It is also known that water ejection from nuclei is caused by local and transient phenomena (Kaneda et al. 1986). Therefore, the spatial distribution of hydrogen is calculated by a model of hydrogen spreading with the comet’s water production rate as a boundary condition. Conversely, this means that the water production rate of the comet can be identified from the spatial distribution of hydrogen around the comet.
Generally, in optically thin conditions, the hydrogen column density in the line-of-sight direction is converted to Ly-α luminance by g-factor. But, in the vicinity of the comet nucleus, the hydrogen column density is high and optically thick, so the luminance and hydrogen column density are not proportional to each other. Such a region is called a “multiple scattering” region, where Ly-α from the sun is resonantly scattered multiple times before reaching the observer (Suzuki 2022). According to this study, the multiple scattering region extends to ~104 km from the nucleus of a relatively active comet, and a spatial resolution that can separate them is necessary.
HI will be aboard the Comet Interceptor Mission and will make observations at 107 km from the target comet. This mission is the first detailed in-situ observation of a LPC in history, and HI's measurement of the water production rate of primitive LPCs will provide important data for identifying the origin of comets. Thus, HI must have optical performance that can view the coma extending from the nucleus to 106-107 km and resolve structures smaller than 104 km.
We must confirm that HI have an ability to resolve the optically thick 104 km region from the observation point 107 km from the comet, and UV detection performance to capture the fainter region of the coma at more than 107 km. The HI telescope has a reflecting telescope to image the cometary corona at Ly-α (121.6 nm) and obtain a two-dimensional image, followed by two glass cell filters and a BPF, and then to a detector. Therefore, in this study, we first measured the spot size from point source imaging to evaluate the imaging performance of the telescope section and compared the imaging performance before and after environmental tests to evaluate the robustness. For the ultraviolet detection performance, the reflectivity of the mirror and the transmittance of MgF2 were experimentally measured to evaluate the attenuation due to two reflections at the telescope and five MgF2 transmissions. We will present these results and evaluate and discuss the optical performance of the HI in terms of imaging performance and Ly-α detection efficiency.
Comets are small bodies of ice and dust with large orbital eccentricity, and in most cases their orbits intersect the snow line. The large amounts of water that form the oceans are thought to have been brought by asteroids and comets along with organics (O'Brien et al. 2018), but how comets contributed to the origin of Earth's water is still not understood, and mapping comet ice properties to the location of comet formation is a critical step to understanding the origin of Earth's water (Mandt et al. 2024).
The ejection rate of volatiles from the nucleus increases as comet’s approaching the sun, and the water production rate can be expressed as a function of the sun-comet distance following a power law, and a relationship between this power exponent and cometary origin has been shown (Combi et al. 2005). One means of identifying the water production rate is to measure the spatial distribution of hydrogen atoms by Ly-α imaging, and Combi et al. (2005) also used data from SOHO/SWAN observing hydrogen Ly-α emission.
Model calculations are needed to connect the spatial distribution of hydrogen atoms around the comet with the water production rate. Water molecules ejected from the comet nucleus are split into H and OH, and OH into O and H through photo-dissociations, and the hydrogen produced in these two reactions spreads out at different initial speeds. It is also known that water ejection from nuclei is caused by local and transient phenomena (Kaneda et al. 1986). Therefore, the spatial distribution of hydrogen is calculated by a model of hydrogen spreading with the comet’s water production rate as a boundary condition. Conversely, this means that the water production rate of the comet can be identified from the spatial distribution of hydrogen around the comet.
Generally, in optically thin conditions, the hydrogen column density in the line-of-sight direction is converted to Ly-α luminance by g-factor. But, in the vicinity of the comet nucleus, the hydrogen column density is high and optically thick, so the luminance and hydrogen column density are not proportional to each other. Such a region is called a “multiple scattering” region, where Ly-α from the sun is resonantly scattered multiple times before reaching the observer (Suzuki 2022). According to this study, the multiple scattering region extends to ~104 km from the nucleus of a relatively active comet, and a spatial resolution that can separate them is necessary.
HI will be aboard the Comet Interceptor Mission and will make observations at 107 km from the target comet. This mission is the first detailed in-situ observation of a LPC in history, and HI's measurement of the water production rate of primitive LPCs will provide important data for identifying the origin of comets. Thus, HI must have optical performance that can view the coma extending from the nucleus to 106-107 km and resolve structures smaller than 104 km.
We must confirm that HI have an ability to resolve the optically thick 104 km region from the observation point 107 km from the comet, and UV detection performance to capture the fainter region of the coma at more than 107 km. The HI telescope has a reflecting telescope to image the cometary corona at Ly-α (121.6 nm) and obtain a two-dimensional image, followed by two glass cell filters and a BPF, and then to a detector. Therefore, in this study, we first measured the spot size from point source imaging to evaluate the imaging performance of the telescope section and compared the imaging performance before and after environmental tests to evaluate the robustness. For the ultraviolet detection performance, the reflectivity of the mirror and the transmittance of MgF2 were experimentally measured to evaluate the attenuation due to two reflections at the telescope and five MgF2 transmissions. We will present these results and evaluate and discuss the optical performance of the HI in terms of imaging performance and Ly-α detection efficiency.