Toward observation of planetary atmosphere by planetary space telescope
There is no doubt that the use of planetary orbiters or landers is the most effective way because it enables direct study of atmosphere or plasma on planets. Meanwhile, observation by ground telescopes is also important because it allows us to continually observe multiple objectives with lower risk. Observation from the ground, however, suffers from strong interference by the Earth’s atmosphere and aerosol, so it is not always a satisfactory method for observation of planetary atmosphere or plasma, which requires stable monitoring-observation.
If we could deploy a space telescope used only for planetary observation in earth orbit, its performance would be exceptional even if its aperture were small. First, continual monitoring without interference by weather is possible. We can acquire stable images with spatial resolution close to the diffraction limit. Even with a 30cm aperture, for example, it can provide continuously high-resolution images covering the range from near-ultraviolet to visible light equivalent to the Subaru Telescope at its best. Secondly, no absorption by the atmosphere makes it possible to accurately measure the ultraviolet and near-infrared regions such as Jupiter’s aurora and water vapor that plays an important role in the planetary atmosphere. Thirdly, with no atmospheric scattering, we can obtain high-contrast images of the outgoing atmosphere from planets or the night-side of inner planets.
As stated above, observation by space telescope is significant and very advantageous. Multiple proposals for planetary telescopes have been submitted in the U.S. and Europe, but unfortunately none has been realized yet. A group including the author is proposing a planetary space telescope TOPS (Telescope Observatory for Planets on small-Satellite) with 30cm aperture, covering the range from ultraviolet (121nm wavelength) to visible light and near-infrared (1,100nm wavelength) as shown in Fig. 3. By making use of the latest commercial technologies, it will have unique, outstanding functions such as spectral imaging covering over 400 wavelengths and shape-changeable occulting mask (by masking the light day-side of planets, we can catch the faint light of the night-side or atmosphere in the exosphere).
One of the important targets of TOPS is stereoscopic observation of Jupiter’s clouds and atmospheric composition including its lightning-discharge emissions. It was already pointed out that thunder on Jupiter must occur in places where cumulonimbus clouds exist. Nonetheless, there has been no simultaneous observation of such clouds and lightning on Jupiter. Using the functions of the so-called meteorological satellite for Jupiter with advantages of the space telescope’s high-resolution and continuity, it is expected that dynamic, never-before-observed aspects of atmospheric activities will be captured. The capabilities of such observation must also be put to work on lightning-discharge and cloud observation on Venus’s night-side.
Observation by a space telescope like TOPS will provide prospects for a number of planetary orbiter probes planned around the world, including PLANET-C, BepiColombo, Solar Sail and the Cosmic Vision Project now under consideration by Japan and Europe for realization in 20 years later. In addition, if TOPS and other probes can operate simultaneously, an ideal planetary-observation program could be realized taking advantage of complementary functions.