As space exploration advances, NASA’s Jet Propulsion Laboratory (JPL), with support from Northrop Grumman, has started running a gamut of tests on new exoplanet imaging technology known as the Starshade.
Researchers believe it is only a matter of time before we discover a planet orbiting a star outside of our solar system (an exoplanet) that could be considered a replica of Earth, but to discover those planets, we need to image them first.
“Astronomers have been thinking about how one might image exoplanets for decades, at least since the early 1960s, and probably before that,” says Kasdin, but only three types of technology can be used to image exoplanets: “Interferometric, which is actually how NASA thought it was going to go early on; Coronography (also known as general field high-contrast imaging), which dates back to the 1920s and runs into the problem of imaging a dim object next to a bright star; and Occulter, which is the idea of putting up a screen to block a nearby star.”
The Starshade uses occulter technology and a flower-shaped design to create what could be considered a reverse aperture. The Starshade sits 50,000 km in the sky in front of a high-power telescope. The technology then acts like a miniature eclipse, blocking a major portion of light expelled from distant stars, making exoplanets more visible. The light diffraction can be fine-tuned with the petals of the Starshade. “You can control diffraction by varying the transmission of the disk, but it has to be really, really big. The problem is nobody knows how to make that [happen], that’s why it never went anywhere,” explains Kasdin. “That’s the only way to control the diffraction, just putting up a circular disk doesn’t work.”
Designing with Accuracy from Scratch
According to David Webb, project mechanical engineer, the Starshade needs to be designed with incredible accuracy and must deploy within the same tolerances. “The shape of the petal has to be accurate to an order of 50 microns, which is similar to the width of a human hair,” he says. “That’s what we did at JPL a couple of years ago.” Recently, Webb and his team placed the petals with extreme accuracy.
NASA’s JPL puts new designs through technology readiness levels. “What we’ve done is identify areas that we need to spend time on and develop new technology for,” Webb continues. To this point, the team has shown that the technology can deploy the Starshade petals to the right position with accuracy. “We got really good results, on the order of 0.005” repeatability. Now that we’re looking at all the other areas, we need an inner disk that can actually perform with the Starshade.
“Because this is something that hasn’t really been done before, we’re not really tweaking a design, it’s about designing it so that is meets the requirements. We’re building a proof-of-concept this summer with a team of interns from varying schools,” he explains. “That will prove that we can build the truss that we need for the Starshade. In coming years, we’ll be testing to not only see if that truss works, but also to check if we will get the accuracy that we need.”
Though this concept has been around since the 1960s, this design is brand new. “It wasn’t so much as tweaking the design, but rather designing it to meet the needed tolerances,” Webb says. “Also, thinking about manufacturing it in a way that can meet those tolerances.”
A major challenge with such tight tolerances in space is the drastically fluctuating temperatures. “One of the things about thermal stability is that you have this really large structure that’s getting really hot and cold, depending on where it’s positioned with respect to our sun, things like aluminum and steel expand and contract,” Webb explains. “One of the things that secures this design is that it’s made of carbon-fiber, which has a very low coefficient of thermal expansion. With temperature variation, it barely changes in size.”
All About the Money
Like most space-related projects, the Starshade isn’t cheap, though it has the potential to be cheaper than its competition. “Once you know that it works, the next question is, is it affordable? That’s a two-part question. Does the government have the money and willingness to pay for a mission that uses that technology,” says Kasdin. It’s about whether or not the Starshade is “the highest priority science that the government wants to pay for, which is based on input from the astronomy community. That’s where we are right now, cost does cause a bottleneck, and the [decadal] survey determines the most ideal way to use it.”
Once each decade, NASA and its partners ask the National Research Council to look out ten or more years into the future and prioritize research areas, observations, and notional missions to make those observations.
According to Kasdin, NASA is trying to figure out what the next mission is going to be after the James Webb Space Telescope, which is scheduled to launch in 2018. “Any of the possible scenarios could happen in the next 10 years, and we’ll know which one of those things is going to happen within the next three years,” he explains.
Exoplanet imaging is vital to the continuation of space exploration and not only understanding distant planets, but also our own. Though there are a few options to continue imaging distant planets, the Starshade team feels that the flower-like technology will be a front-runner when NASA starts picking future budgets.
“My guess is that the first version of an occulter will be built with a telescope that’s about the same size as Hubble,” Kasdin says. The feasibility of the Starshade is high, and the team has its eyes set on being the next project launching to deep space.
Filed Under: Aerospace + defense