討論:新世界任務

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未翻譯內容如下--Flame 歡迎泡茶 2011年5月7日 (六) 00:17 (UTC)：

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主條目：系外行星偵測法

Traditional methods of exoplanet detection all rely on indirect means of inferring the existence of orbiting bodies. These methods include:

• Astrometry – watching a star move slightly due to the gravitational influence of a nearby planet
• Observing Doppler shifts of the stars spectrum due to the star's movement
• Observing the amount of light from a star change as an extrasolar planet transits the star, preventing a portion of the light from reaching the observer.
• Pulsar timing
• Gravitational microlensing
• Observing radiation from Circumstellar disks in the infrared.

All of these methods provide convincing evidence for the existence of extrasolar planets, however none of them provide actual images of the planets.

The yellow circle represents the star, whose light is blocked by the occulter. The blue circle represents the planet, whose light passes to the observer undisturbed.

The goal of the New Worlds Mission is to block the light coming from nearby stars with an occulter. This would allow the direct observation of orbiting planets. The occulter would be a large sheet disc flown thousands of kilometers along the line of sight. The disc would likely be several tens of meters in diameter and would fit inside existing expendable launch vehicles and be deployed after launch.

One difficulty with this concept is that light incoming from the target star would diffract around the disc and constructively interfere along the central axis. Thus the starlight would still be easily visible, making planet detection impossible. This concept was first famously theorized by Siméon Poisson in order to disprove the wave theory of light, as he believed the existence of a bright spot at the center of the shadow to be nonsensical. However Dominique Arago experimentally verified the existence of the spot of Arago. Fortunately this effect can be negated by specifically shaping the occulter. By adding specially shaped petals to the outer edge of the disc, the spot of Arago will disappear, allowing the suppression of the star's light.

This technique would make planetary detection possible for stars within approximately 10 parsecs (about 32 light years) of Earth. It is estimated that there could be as many as several thousands of exoplanets within that distance.

遮星板

The starshade is a proposed sunflower-shaped coronagraph disc that was designed to block starlight that interferes with telescopic observations of other worlds.

Specifically, the starshade is a spacecraft designed by Webster Cash, an astrophysicist at the University of Colorado at Boulder's Center for Astrophysics and Space Astronomy. The proposed spacecraft is designed to work in tandem with a space telescope, such as the James Webb Telescope, or a new 4 meter telescope. The starshade is designed to fly 80,000 miles in front of a space telescope, and is to be located between the space telescope and the star whose planetary system is being observed. When opened up, the unfurled starshade resembles a flower, with different pointed sections protruding around its circumference, similar to that of a sunflower.

The Starshade is a very large coronagraph, in the form of a dedicated spacecraft, that is designed to fly in front of a space telescope. This is a light-blocking shield whose purpose is to block light of a distant star, to make it easier to observe planets orbiting that distant star. If the spacecraft passes NASA standards, the starshade could be sent to space as early as 2013. The new starshade would be located approximately 238,600 miles away from Earth and outside its orbit. The unfurled starshade could provide a light reduction from bright stars by as much as a factor of 10 billion. The light that "leaks" around the edges would be used by the telescope as it scans the target system for planets. With the reduction of the harsh light, astronomers will be able to search much farther in space than they would with no light blocking, as well as be able to analyze the chemical atmosphere of planets that are tens of trillions of miles away for the possibility of life. After launch, it is estimated that within a 2-year period the starshade could help astronomers to get a better look at upwards of 75 different planetary systems.

任務目標

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The New Worlds Mission aims to discover and analyze terrestrial extrasolar planets:

1. Detection: First, using the space telescope and 「starshade,」 or occulter, exoplanetary systems will be directly detected. Currently, Doppler techniques, which have been used to find over 300 extrasolar planets to date, have trouble detecting low-mass Earth-like planets. These techniques tell us little more than mass and orbit for the extrasolar planet. Using the newly developed technology regarding starshade design, we shall be able to detect planetary systems surrounding stars other than our Sun directly.
2. System Mapping: Following detection, system mapping would involve the direct mapping of planetary systems through the detection of the planetary light separate from the parent star. In a sufficiently high-quality image, planets would appear as individual star-like objects. A series of images of the planetary system would allow us to measure the planetary orbits, and the brightness and broadband colors of the planets would give us information about the basic nature of each planet.
3. Planet Studies: At this stage, detailed study of the individual planets will take place. With a low noise level and a modest signal, spectroscopy and photometry can be performed. Spectroscopy will allow us to perform chemical analysis of atmospheres and surfaces, which might hold clues to the existence of life elsewhere in the universe. Photometry will show variation in color and intensity as surface features rotate in and out of the field of view, allowing for the detection of oceans, continents, polar caps and clouds.
4. Planet Imaging: A quantum leap in capability is needed to achieve true planet imaging. However, techniques of interferometry show that, in principle, this is possible to achieve. Fifty to one hundred percent of a planet’s surface could theoretically be mapped, depending on the planet’s inclination.
5. Planetary Assessment: The final step in extrasolar planet studies will be the ability to study these distant worlds in the same way that Earth-observing systems study the Earth’s surface. Such a telescope will of necessity be large, to collect enough light to resolve and analyze small details on the planet’s surface. However, these kinds of studies do not lie in the foreseeable future, for it takes square kilometers of collecting area to capture the needed signal.

In addition to finding and analyzing terrestrial planets, it can also discover and analyze gas giants. The New Worlds Mission will also find moons and rings orbiting extrasolar planets. This technique will involve direct imaging of planets by blocking the starlight with a starshade. It will study the moons and rings in detail and find whether moons can also support life if gas giant planets orbit in the habitable zones of parent stars.

任務架構

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There are many possibilities for various New Worlds Missions, three of which are:

1. New Worlds Discoverer proposed to use an existing space telescope (like the soon to be launched James Webb telescope), to finding exoplanets. The size of the starshade could be optimized for the observing telescope.
2. New Worlds Observer would use two spacecraft, one that has a dedicated telescope and one with a starshade to find exoplanets. The possibility of two starshades is also a consideration. One starshade to point towards the desired target while the other moves into position for the next target. This would eliminate some of the time delay in observing different systems and allow for many more targets to be observed in the same timespan.
3. New Worlds Imager would use many spacecraft/starshades. This would allow observers to resolve the planet and obtain true planetary imaging.