Astronomer Dan Marrone has just spent the last 30 hours traveling 5,000 miles from the Smithsonian Astrophysical Observatory in Cambridge, Mass., to the Observatory’s newly built Receiver Lab Telescope on Cerro Sairecabur peak in the frigid Atacama Desert in the mountains of northern Chile. Now, his truck is stuck in the snow, its wheels spinning helplessly, only a mile from his destination.

As he gets out to dig free, the wind suddenly picks up and begins piling snow under and around the vehicle, entombing it with frightening speed. Marrone hurries to his companion’s truck, and they quickly drive away, returning the next day to clear the snow and reach the telescope.

Why would an astronomer subject himself to such risks? For the chance to get in on the bottom floor of an entirely new field of astronomy.

“Personally, I’m excited to be part of the first group to do terahertz astronomy from the ground,” Marrone says of this realm of astrophysics. It previously has been conducted only intermittently by scientists with access to balloons or aircraft.

Terahertz
Terahertz astronomy is the science of receiving and analyzing waves of radiation from space emitted by molecules at frequencies of more than 1 trillion hertz. Such signals have frequencies roughly 10,000 times faster than signals broadcast by an FM radio station.

The universe is filled with clouds of gas composed of different types of molecules, each of which radiates energy at a different frequency. By measuring the strength of terahertz signals coming from a cosmic cloud at the known frequency for, say, carbon monoxide, terahertz astronomers can determine the amount of carbon monoxide in the cloud. They can also measure where inside the cloud these molecules are congregating and how fast they are moving.

Terahertz astronomy holds great potential for new discoveries in interstellar chemistry and star formation.

Location is key
But doing terahertz astronomy from the ground is like stargazing from inside a limousine with darkly tinted windows—even the brightest star is tough to see. Just a tiny amount of water vapor in the air can block incoming terahertz radiation.

When asked how it is possible to overcome this obstacle, project leader Ray Blundell of the Smithsonian Astrophysical Observatory replies with the real estate agent’s mantra: “It’s all about location.” The search for dry, clear skies led Blundell and his team to a site 18,000 feet up in the Atacama Desert of northern Chile. “We’re operating the highest telescope in the world,” he says.

The Receiver Lab Telescope site is so arid that if all the water vapor in a narrow column stretching straight up from the ground to the edge of space were condensed, it would form a film only about 1/100th of an inch thick. “That’s about 100 times less humidity than found over Washington, D.C., on a crisp fall day,” Blundell says.

Like rolling down the tinted glass of the limo, moving to such a high and dry location opens a window onto the terahertz universe.

Detector design
In addition to finding the right location, Blundell and his colleagues at the Receiver Lab Telescope had to build the sensitive receivers that detect terahertz radiation.

The detector, a superconducting thin film of niobium nitride cooled to 4 degrees above absolute zero, is only a few atomic layers thick. It acts like a tiny, really fast thermometer and can measure the smallest temperature change induced by the incoming radiation in less than a billionth of a second.

“The Receiver Lab Telescope is the only telescope using this type of terahertz detector,” Blundell says. “It is state of the art.”

Even the antenna dish, which collects cosmic radiation as it flows Earthward like falling rain, exhibits groundbreaking technology. “The antenna has the most accurate surface of any radio telescope in the world because, at these high frequencies, we need a very good surface,” Marrone says. “It was made from a single piece of aluminum and cut by the largest commercially available diamond-turning lathe in the world.”

Orion Nebula
The Receiver Lab Telescope began recording data in November 2002, probing wisps of gas floating between stars and clouds of dust hidden throughout the Milky Way.

One of the telescope’s first targets was the famous Orion Nebula. There, hot young stars blast the gas and stars around them with powerful winds and intense ultraviolet radiation. Peering into this seething realm, astronomers are analyzing signals from carbon monoxide molecules to map the distribution of the warm molecular gas within the nebula.

Astronomers using the Receiver Lab Telescope also can detect ionized nitrogen atoms within the gases floating between the stars of our galaxy. Data from these relatively warm gases, carbon monoxide and nitrogen, combined with radio observations from other telescopes that see cold gas a few tens of degrees above absolute zero, give astronomers a more complete picture of the complex chemistry inside these clouds.

Finally, the Receiver Lab Telescope soon will be used to map a three-atom form of the heavy hydrogen molecule that is stored in a deep freeze within dusty cosmic clouds. “We want to look inside these cold clouds because complex molecules form there,” Marrone says.

By studying the molecular composition of everything from the nebulae from which new stars are born to the clouds that hold the seeds of life in the universe, Smithsonian astronomers using the terahertz-frequency Receiver Lab Telescope are gradually helping to forge a new understanding of the cosmos.