Excitement is building over the total solar eclipse that will occur in the US on August 21, 2017. This is a rare opportunity to witness a visually stunning celestial event. Many of my astronomer colleagues will be traveling to places along the path of the eclipse, which runs from Oregon to the Carolinas. My wife and I will watch from Nashville.
For most of history, total solar eclipses—when the Moon blocks out the light from the Sun— were rare occasions when astronomers could study the Sun's atmosphere. This atmosphere— called the 'corona,' from the Latin word for crown—is much fainter than the Sun itself. That means the corona is impossible to see unless the Moon blocks out the Sun's glare, which is about a million times brighter than the corona.
We are lucky because the apparent size of the Moon on the sky is almost exactly the same size as the Sun. Thus, the Moon is almost perfectly able to block out the Sun when the alignment is just right. But this only occurs occasionally, and over small areas of the Earth. However, in 1931, the coronagraph was invented. This instrument allows astronomers to block the light from the Sun in order to study the corona at any time. Coronagraphs are inserted into a telescope to block the light from the Sun, but not from the corona, making an artificial eclipse. Recently, improved coronagraphs have allowed us to block the light from other stars in order to see the significantly fainter planets orbiting those stars. We call planets outside of our solar system 'extrasolar planets,' or more simply 'exoplanets'.
I have previously written about WFIRST (Summer 2015 Planetary Report). This NASA mission, planned to launch into space in the mid-2020s will have a coronagraph at least 1,000 times more powerful than any existing coronagraph. The power of a coronagraph is measured by the 'contrast ratio'—the ratio of the brightness of the central star to the brightness of the planet being studied. Current coronagraphs have a contrast ratio of about 100,000-1 million, which means that astronomers can see objects that are 100,000-1 million times dimmer (less bright) than the central star. WFIRST is being designed to achieve a contrast ratio of one billion to one! The primary difficulty in designing any coronagraph is in blocking all of the starlight. Once light has entered the telescope, it is extremely difficult to block it all with a coronagraph, partly due to a process called 'diffraction.' This is a process in which light is bent around corners or is scattered at the edges of objects. An analogy would be when you close the curtains, but there is a little gap and the light spreads as it passes through the small gap. In a telescope, this scattered light sometimes finds its way to the camera recording the telescope observations. So, achieving a contrast ratio of one billion to one requires us to keep that scattered light level very, very low.
Artist's concept of the Starshade
NASA / JPL-Caltech
ARTIST'S CONCEPT OF THE STARSHADE
An artist’s depiction of the fully-deployed Starshade spacecraft (left) next to the space telescope it supports. The two spacecraft must fly in almost perfect alignment to allow the telescope to stay in the shadow created by the Starshade.
One way to circumvent the problem caused by diffracted light within the telescope is to create an artificial eclipse in a way that is more like the eclipse we will witness in the USA this August. A Starshade—a large, free flying spacecraft that looks like a giant flower with petals—could fly between a telescope (in space) and the star being observed. The Starshade would block the star's light before it ever got to the telescope, but allow the exoplanet's light to reach the telescope. By avoiding the problem of diffracted and scattered light inside the telescope, a Starshade could reach contrast ratios of 10 billion to one or better. This 10-billion-to-one contrast ratio is the 'magic number' that would allow us to study in detail Earth-like planets in the habitable zone—where liquid water might exist—around stars that are similar to our Sun. Astronomers think this may be one of the best ways to search for the telltale signs of possible life—called biomarkers—in the atmospheres of those planets.
The Starshade would have to be large, about 25–40 m across for a telescope the size of WFIRST, and even larger for a bigger space telescope that could follow WFIRST, and would fly 30,000-50,000 km away from the telescope. Building a structure that large and doing the precision flying needed to align the telescope and Starshade are no small tasks, but some of our best minds are on it. The peculiar 'flower with petals' shape of the Starshade is to deal with the diffraction of light. A circular Starshade would cause some of the star's light to be bent (via diffraction) directly into the telescope. However, the petals are designed to diffract the light away from the telescope, allowing us to reach that magic 10 billion to 1 contrast ratio.
Why, you might ask, is the Starshade so big, and the telescope so far away? Why not build a smaller Starshade and move the telescope closer? There are two competing effects in play. The Starshade needs to be large enough to block out the light and have any diffracted light bent away from the telescope. This pushes us to a large Starshade. However, a large Starshade would also block out planets close to the star unless it is far enough away. This pushes us to a distant Starshade. For a telescope like WFIRST, the sweet spot is a Starshade of the dimensions mentioned above.
A Starshade is a long way off (possibly mid to late 2020s). Starshade technology is still actively being matured and the astronomical community would need to weigh a possible Starshade mission concept against other priorities in the coming decades. However, excitement is building over the possibility that a Starshade might one day create an artificial eclipse that would allow us to examine a Pale Blue Dot and perhaps find an answer to the question, "Are we alone?"
