An astrophysicist comes in from the dark

 kw: book reviews, nonfiction, memoirs, science, astrophysics, widowhood, exoplanets

Dr. Sara Seager, an astrophysicist, designed something called Starshade, which NASA may build to allow a spaceborne telescope see a planet next to a star. This illustration by NASA shows the shape of the shade, which would actually be a great distance from the orbiting telescope, tens of thousands of kilometers away.

She fell in love with the night sky as a young child, as many of us do. She never fell out. During graduate school she became obsessed with exoplanets, planets around other stars. She was into them before they were cool. Her fervent desire is to discover the first planet that is enough like Earth that "someone" could be living there.

Her journey to become an astronomer, and her journey through life, are told in The Smallest Lights in the Universe, a memoir and scientific discovery journal all in one. She didn't have it easy, getting into her chosen field, but the trials were minor compared to what life handed her. Nearing age forty, with two very young sons, she was widowed. I surmise that the title of the book has a secondary meaning, of the way she thought of herself, being ground to powder by an uncaring universe.

Dr. Seager is incredibly talented, but was overwhelmed, and at a time of desperate need she had the great good fortune to meet a set of friends, all young widows like herself, whom she calls the Widows. From her telling, they sound like the best support group ever. Their help didn't make her transit through grief easy, they just made it possible. She also gathered others who could help care for the boys, backstop her slender housekeeping skills, and help her navigate the million little terrors of "the real world". I won't mention any names, because in the introduction she states that names were changed to protect them. Those she has to name, such as her (very supportive) advisors and mentors, of course, are named. I wish I'd had a few of those in grad school!

Her husband died of cancer, a cancer related to one that I survived 21 years ago. I was lucky to survive. I remember going into surgery, wondering how my wife, in her 50s, and our teenage son, would do, because I gave myself a slim chance of survival. Had I lost that battle, I like to think my widow might have found help at the level this book describes. It's no sure thing.

Finding happiness amidst enormous grief is harder than finding a planet "out there." When the author began studying the first exoplanets, only a handful had been found, and most astronomers were very skeptical of the work. It took a few years, and several more discoveries, for the majority of them to be convinced that exoplanets were indeed being located. In a few more years, as technology advanced and better tools were created, finding these tiny orbs got easier, and now a few thousand have been located.

A key element of their discovery was the Kepler telescope. I spent many a pleasant hour perusing light curves via the Zooniverse Project, along with thousands of other "citizen scientists", looking for that subtle dip that indicates a planet has crossed in front of its star, from our perspective. A not-so-subtle dip usually indicates a companion star (AKA an eclipsing binary), or sometimes, a "hot Jupiter", a big planet, not enough like Earth to be of interest except for statistical purposes. The Kepler instrument was intended to find planets not too much bigger than Earth, far enough from their stars to be possible sites of life. Among the discovered planets, there is at least one with my handle attached, along with at least a dozen other citizen scientists who also flagged it as a possible planet.

Dr. Seager works at a level that enables people like me to get in on the fun. She is a kind of mother hen to hundreds of planets. She also led the work to get a satellite built called ASTERIA. It was a proof-of-concept for a family of small satellites that just might locate a sister Earth, or a few of them. Planets, not just the right size and the right temperature, but with the right atmospheric chemistry to make them good candidates for having a biosphere.

Starshade hasn't been built yet, though models up to half size have been. The final shade would have to unfurl in space, "growing" into shape with an accuracy in the thousandth-of-an-inch range. In the 1970s I worked on a team that was building a radio telescope that had to have similar accuracy, but was much smaller; its diameter is 10 meters (34 feet, and it is still in use). Starshade's full diameter will be 34 meters (111 feet), and the central disk is 20 meters across.

I haven't done the math yet, so I'll do some on the fly to see what the parameters need to be for Starshade to work.

• The shade's radius is 17 meters. It needs to block the glaring light of a star so that a nearby planet becomes visible. 
• The sunflower-petal design is needed to cancel out the diffraction rings that would form if a simple circle were used.
• A parsec is 3.26 light-years. The term is from "parallax-arc-second", and represents the distance to a star which has a parallax of one arc second as viewed from Earth. That is 31 trillion km.
• The distance to the nearest star, Proxima Centauri, is 4.25 light years, or 1.3 parsecs.
• From Proxima Centauri, the maximum elongation of Earth from the Sun is 1/1.3 or 0.77 arc seconds.
• The tangent of one arc second is about 1/206,000. The tangent of 0.77 arc seconds is about 1/270,000.
• 270,000 x 17m = 4,590 km.

That's the absolute minimum distance between Starshade and the telescope it serves. The system would barely be able to detect a planet at that distance that was the same distance from its star as we are from ours (a unit called one Astronomical Unit, or 1 AU), and only for a couple of days in the year. There are more than 100 stars within about eight parsecs from Earth. Most of them are smaller than the Sun, and thus dimmer, and thus their "Goldilocks zone" of earthlike temperature is smaller. A factor of 16 makes sense to me, to look for planets at least 0.5 AU from their stars, up to 8 parsecs away.

• 16 x 4,590 ≈ 73,400 km.

That's almost one-fifth of the way to the Moon! It is just a little less than the diameter of the synchronous orbit.

Hmm. Perhaps the two craft, the one carrying the Starshade (and able to move it about and orient it properly) and the one carrying the telescope (ditto), should be in synchronous orbit, maybe at a 120° spacing so the Earth doesn't intrude. That would limit their bailiwick to a narrow band of sky near the celestial equator. But it would make communications with them easier. I sure hope it gets built! In the current political climate, NASA gets no respect, and the chances are pretty slim…for the time being.

I'll leave it to you to read the book to learn of the other quite literally one-in-a-billion chance that life handed Sara Seager, to bring her out of the night of the soul.