The NASA TPF is a proposed mission to directly image extrasolar planets and, in one design, record their thermal emission spectra to search for biogenic indicators. If selected, TPF would take the form of either a coronagraph operating at visible wavelengths or a large-baseline interferometer operating in the infrared and would be placed either in an Earth-trailing heliocentric orbit, or at the Earth-Sun L2 position.
Preliminary designs for a visible light coronograph (TPF-C) use a single telescope with an effective diameter of 6.5m-8m, operating at room temperature. A corono-graph works by blocking the direct and diffracted light of a bright object so that faint nearby objects and structures can be seen. Such instruments were originally developed to study the corona about the Sun, hence the name. The diffraction pattern of a telescope with a simple circular aperture is the well-known Airy pattern, consisting of a bright central spot surrounded by concentric, circular rings of ever decreasing intensity. To observe a faint planet close to a star requires that the first several bright rings of starlight are suppressed without blocking out the light of a planet. By using masks to simulate a telescope with a different effective entrance aperture shape, the diffraction pattern may be modified so that the starlight is much dimmer closer to the center at some rotation angles, and brighter in others. Hence, by then rotating the telescope about the line of sight to the star, the degree of obscuration of a planet orbiting that star will be periodically modulated and thus the planet may be detected. To achieve the sensitivity needed will require the use of active optics in order to correct for wavefront distortions arising from optical imperfections.
Preliminary IR TPF (TPF-I) concepts would use multiple, smaller telescopes (3-4 m mirrors) configured as an interferometer and spread out either over a large boom (of less than 40 m in length) or alternatively operated on separate spacecraft over distances of a few hundred meters (Figure 8.11). At thermal-IR wavelengths an Earth-like planet at a distance of 1 AU from a Sun-type star would appear a million times fainter than the star, and hence discriminating its thermal emission from that of the star is by no means an easy task. However, at visible wavelengths the light reflected by such a planet would appear a billion times fainter than the star, and thus it is theoretically easier to detect a terrestrial planet in the thermal-infrared. In order to discriminate between the radiation from the star and the planet it is proposed to use the technique of nulling interferometry outlined in Figure 8.12. The interferometer system is centered on the star and the beams from two telescopes separated by distance D are combined with a carefully controlled A/2 path difference, maintained by an active delay line system, such that the starlight interferes destructively and is almost completely canceled at the detector. The light from a nearby planet however, offset by an angle O, introduces an extra path difference of D sin O « DO. If DO = A/2, then the planet's light will interfere constructively and so can be detected. Several telescopes (3-5) would need to be used at a number of separations and rotations in order to scan the different separation and rotation angles of planets in the system and these telescopes would need to be precisely aligned with respect to each other. In addition the telescopes would need to be extremely cold in order to
detect the faint, cold planetary emission, although the requirements on the telescopes' optical quality are less than for the visible coronograph system, and the mirror sizes are less. A major advantage of such a system would be that once planets have been detected, the thermal-IR radiation emitted by them could be integrated for a very long time in order to build up a thermal emission spectrum of the planet, from which the atmospheric composition may be determined together with the temperature of the atmosphere and surface. The IR designs that are planned are tailored to detect warm terrestrial planets and will thus not be able to detect the emission of cold, distant giant planets, which emit more in the far-infrared. However, the really exciting thing about TPF (and Darwin outlined in Section 8.7.5) is that it will be optimized to detect the major absorption bands of CH4, 03, C02, and H20 from 6 ^m to 15 ^m. The atmosphere of the Earth is peculiar in that it has a high abundance of molecular oxygen 02, a highly reactive gas not found freely in such high concentrations, as far as we know, anywhere else in the universe. The oxygen is photolyzed to create a significant abundance of ozone (03) in the stratosphere, which has a clear absorption band at 9.6 ^m, easily detectable from space. What makes our atmosphere even more peculiar is that there are also significant levels of methane, a highly combustible gas existing alongside the molecular oxygen. The reason for Earth's peculiar atmospheric
Lieht from Star
Liaht from planet at angular separation 8
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