The giant radio telescope at the Arecibo Observatory was destroyed twice: the first time deliberately to save the planet, the second time as a result of natural disasters—making it harder to save Earth in the future.
Although the first “destruction” was a special effect in the 1995 James Bond movie GoldenEye, the world’s most powerful telescope for studying near-Earth asteroids was, in real life, damaged beyond repair by multiple storms before finally collapsing in December 2020. For more than 50 years, the Arecibo Telescope (often referred to simply as “Arecibo” when there’s no confusion with the observatory as a whole) was the largest in the world: a radio dish 305 meters (1,000 feet) in diameter set into a natural karst depression in Puerto Rico. It played a part in studies of Earth’s ionosphere, attempts to detect signs of—and send messages to—potential alien civilizations, the characterization of pulsars, and many astronomical discoveries.
The greatest loss posed by Arecibo’s collapse, however, may be the ability to identify potentially hazardous asteroids and comets using radar to track their orbits and other properties. Our solar system contains more than 2,000 known near-Earth objects (NEOs). The vast majority don’t pose a danger to us for the next century or more, but at the same time, the danger from an asteroid impact is large enough that we want as much warning as possible—just ask the dinosaurs. Radar astronomy is a key part of the field known as planetary defense for that reason.
“Asteroid or comet impacts are maybe the one natural disaster that you can prevent.”
“Asteroid or comet impacts are maybe the one natural disaster that you can prevent,” said Patrick Taylor, who heads the radar astronomy division of the National Radio Astronomy Observatory. Though we can’t prevent largely erratic disasters caused by earthquakes, volcanoes, or hurricanes, extraterrestrial rocks follow predictable—and possibly changeable—orbits around the Sun. “If you know about it soon enough, you can think about a mission to it,” Taylor said.
Without Arecibo, the entire world currently has only a single operational radar observatory, and it’s part-time: the transmitter and receiver that make up the Goldstone Solar System Radar at the Goldstone Deep Space Communications Complex in California, part of NASA’s spacecraft communication network. Researchers have big proposals for future radar observatories, most notably the Next Generation Radar (ngRADAR) project at the Green Bank Telescope (GBT) in West Virginia. Meanwhile, the upcoming Vera C. Rubin Observatory in Chile and NASA’s NEO Surveyor spacecraft (which operate in visible and infrared light, respectively) will discover hundreds of NEOs every year, all of which will need follow-up radar observations.
The catastrophic loss of Arecibo highlights how much we needed it for planetary defense and how problematic it is to rely on any single observatory for this essential work. Next-generation radar proposals involve multiple telescopes for sending and receiving signals, as well as exploiting modern technology to make it possible to carry on when components fail.
“We’re civilized people, we can have a pretty darn good planetary defense program to spot the objects well in advance,” said Amy Mainzer of the University of California, Los Angeles, who is one of the scientific leads on NEO Surveyor. She argued that we have the technology to do that right now: “There’s nothing here that’s incredibly unusual or requires development of some unobtainium-based material.”
“So Many Dog Bones”
Most telescopes only receive light, whether in optical (visible), infrared, radio, or other wavelengths. Radar actively sends radio light to space, bouncing it off planets, the Moon, comets, or asteroids. As with radar in aviation and meteorology, comparing the radio waves received to those sent allows researchers to measure the position and speed of objects precisely, along with surface details.
“Every other technique in astronomy depends on reflected sunlight or emitted radiation,” said Marina Brozovic, a radar astronomer at NASA’s Jet Propulsion Laboratory. “We bring our own flashlight, and the echo comes back carrying lots of valuable information from super precise measurement of where that object is in space to physical characteristics [like] how fast it’s spinning.”
The time between sending and receiving radio waves reveals the distance, whereas shifts in wavelength provide velocity data. Polarized radio waves—imagine a type of corkscrew motion for the beams—yield information about the surface of the target object, such as its roughness and how metallic it is. And, of course, if the object is large or close enough, radar can produce an image of it, which is especially useful for NEOs, which are too small and fast moving for optical imaging.
“Once an optical or infrared telescope says, ‘Hey, here’s an asteroid!’ if you give me 3 minutes on it with radar, I can nail down its orbit,” said radar astronomer Edgard Rivera-Valentín. Currently at the Johns Hopkins Applied Physics Laboratory, Rivera-Valentín spent 4 years at Arecibo before its collapse. “Optical telescopes don’t get enough data to constrain the orbit without radar, so a lot of the times, we actually lose the asteroid. You don’t want to lose a potentially hazardous object!”
Arecibo pioneered the use of radar on asteroids by measuring the orbit of the large asteroid Eros in 1975 and provided the first direct radar image of Castalia in 1989. Both of these NEOs had been identified using other methods (Eros was discovered in 1898), but radar provided detailed information unavailable other ways. The image of Castalia, for instance, showed it to be a “contact binary”: a peanut-shaped body consisting of two asteroids stuck together or, perhaps, a single asteroid in the process of being pulled apart.
“With radar, [NEOs] are not just a little dot moving in space,” Rivera-Valentín said. “We can tell the entire shape of it: Is it actually spheroidal? Is it a diamond shape? Is it a dog bone? There’s so many dog bones. This one looks like a ghost. This one looks like a skull.”
Arecibo radar image of the near-Earth asteroid 2015 TB145. Despite its macabre appearance, this asteroid poses no immediate threat to us. Credit: NAIC-Arecibo/NSF
Radar also has allowed astronomers to identify asteroid moons: smaller bodies separated from and orbiting the main asteroid. By measuring the orbit of these moons, researchers can use basic physics to obtain asteroid mass, which then reveals density and clues about composition.
“One out of six asteroids could be contact binaries, and you can’t really tell those apart from ellipsoidal asteroids just from optical observations,” said Anne Virkki of the University of Helsinki, who formerly headed a radar astronomy research group at Arecibo. She emphasized the importance of getting the shape of asteroids to send spacecraft to study them scientifically or to attempt to redirect them away from Earth. “When you have better shape models, then you can get also gravitational models. It’s very different for a spacecraft to orbit something that’s spherical, or if it looks like this peanut thing.”
From the Cold War to the 21st Century
Amazing as it is, radar astronomy does come with some drawbacks.
One drawback is attenuation, or reduction in signal strength. Light spreads out as it travels through space, resulting in greater attenuation the farther away the source is. Attenuation literally gets radar going and coming: The beam arriving at the target is diminished by the square of the distance; then its return to Earth sees the signal dropping off by the square of the distance again. Ultimately, this means an object twice as far away will have a radar signal 1/16 as strong.
The ngRADAR proposal is designed to mitigate the attenuation problem. The GBT is the world’s largest fully steerable telescope, which means it can be pointed at targets of interest. Beyond updating the telescope’s amplifiers, however, ngRADAR will use the Very Long Baseline Array (VLBA) to receive the returning radar signal. The array consists of ten 25-meter radio telescopes distributed from the Virgin Islands to Hawaii, which act together as a single giant observatory. Using the VLBA will increase the sensitivity of radar observations, mitigating many of the issues with signal drop-off and enabling astronomers to get size, shape, and rotation data from more distant asteroids than before.
Another drawback to radar astronomy is power—a radar observatory requires a lot more power than a normal telescope because it sends signals rather than just receiving them. That problem is compounded by observatories relying on Cold War era technology.
“Arecibo used and Goldstone still use something called klystrons, which are big vacuum tube amplifiers,” Taylor said. To get enough power to do radar astronomy, he added, the klystrons need to be huge: 2 meters (6 feet) tall and weighing hundreds of kilograms (thousands of pounds). To make matters worse, “they’re known to fail catastrophically. You can often lose fifty or even a hundred percent of your capability if they fail,” he explained.
Next-generation radar proposals involve solid-state amplifiers called monolithic microwave integrated circuits (MMICs, pronounced “mimics”), which produce less power than klystrons but are much smaller and more robust to failure. In Brozovic’s analogy, MMICs are like multiple LEDs in your metaphorical flashlight instead of a single halogen bulb: less bright but advantageous in other ways.
“Instead of having one or two big components, you have thousands of [amplifiers] built into a larger array,” Taylor said. “If something breaks, you can pull that out, keep working, replace it, and be back up to speed again.”
Last Telescope Standing
Until ngRADAR or similar projects begin operation, a single observatory is carrying the entire burden of radio astronomy, the lone hero in the breach of planetary defense.
The ngRADAR system, illustrated here, will transmit radar signals from the Green Bank Telescope (top right) and bounce those signals off the Moon (middle right). The reflected signal will be received by antennas of the Very Long Baseline Array in various locations across the continental United States, Hawaii, and the Virgin Islands (bottom right). The combination of antennas acts as a giant, high-resolution radar imaging system. Credit: Sophia Dagnello, NRAO/AUI/NSF
“Goldstone was always complementary to Arecibo,” said Brozovic, who has been performing radar observations there since 2007. “Arecibo was about 15 times more sensitive than Goldstone is. However, we are a fully steerable antenna that covers about eighty percent of the sky. We observe at Goldstone about 50 near-Earth asteroids every year.”
In other words, astronomers can point the telescope at the objects of interest, whereas Arecibo was set into the ground and required its targets to pass overhead. Goldstone demonstrated its responsiveness in 2022, when the Double Asteroid Redirection Test (DART) mission deliberately slammed into the asteroid Dimorphos.
The Goldstone Solar System Radar, part of a 70-meter antenna at the Goldstone Deep Space Communications Complex in California’s Mojave Desert, is currently the only operational radar observatory in the world. Scientists say it spots about 50 near-Earth objects every year. Credit: NASA/JPL-Caltech
“The radar observations gave us a rough estimate of the [orbit] change less than a day after the impact,” said Cristina Thomas of Northern Arizona University, who was one of the scientific leads on DART. “It was really phenomenal! You know where Dimorphos is supposed to be, and you see it in a different place in the radar observations.”
Goldstone, like Arecibo, can see only the Northern Hemisphere sky, although potentially hazardous asteroids could come from any direction. For that reason, researchers have begun using the Australia-based Canberra Deep Space Communication Complex, which is identical to the Goldstone dish and also primarily serves NASA spacecraft communications.
However, those two won’t be able to keep up with new NEO discoveries, not least because they’re old. Originally built in 1966, Goldstone will be shut down entirely in 2026 for needed upgrades—leaving Earth completely without a radar observatory.
“You want to have a backup system,” Rivera-Valentín said, adding that the problem isn’t lack of awareness but lack of funding.
The U.S. Congress passed the George E. Brown, Jr. Near-Earth Object Survey Act in 2006, legally obligating NASA to identify every asteroid larger than 100 meters.
“That was supposed to be completed like a decade ago, but it didn’t have the funding and the facilities to make it happen,” Taylor said. “That’s where Rubin and NEO Surveyor are going to come in, but if you don’t follow up [the NEOs] and secure their orbits so that you know where they are in the future, then you just have to rediscover them.”
Mutual Aid for Planetary Survival
When it begins scientific operations in early 2025, the 8.5-meter Vera C. Rubin Observatory will scan a huge swath of the sky, looking for any changes from night to night. Researchers estimate the data could include over a million new asteroid detections within the first 6 months, likely doubling the number of known NEOs, as well as possible hazardous asteroids and comets. NEO Surveyor, slated to launch in 2027, will add even more.
First light for the Vera C. Rubin Observatory in Cerro Pachón, Chile, is scheduled for August 2024. The observatory is expected to identify millions of new asteroids. Credit: Olivier Bonin/SLAC National Accelerator Laboratory
The need for next-generation radar observatories, in other words, could not be greater. At the same time, convincing governments to invest has proven challenging. Scientists in the United States have struggled to keep Arecibo and GBT operational, even with the support of a congressional mandate under the George E. Brown Act. China has proposed building its own radar system that uses arrays of emitters as well as receivers, which, as Taylor noted, might cost as much as building much of the U.S. radio observatory program from scratch.
Pooling resources to make an international planetary defense network would make sense, but that scale of cooperation remains rare in the space industry. In some ways, such dogged independence is as much a Cold War relic as the klystrons: Arecibo was originally designed to assist with ballistic missile deterrence by the U.S. Department of Defense, with scientific applications following later. ngRADAR is being developed in collaboration with defense contractor Raytheon.
“These kinds of telescopes are expensive. They need the political will to get them built, which is the hardest part of getting projects started.”
“Europe could have its own radar, but the funding is very tricky, because these kinds of telescopes are expensive,” Virkki said. “They need the political will to get them built, which is the hardest part of getting projects started.”
Similarly, Congress balked at rebuilding Arecibo, which is why ngRADAR supporters are taking a different tack in pursuing development.
ngRADAR is “not creating brand new facilities, which makes it attractive to the people who pay for it,” Taylor said. “It gives the Green Bank Telescope and the VLBA another use, potentially bringing in other stakeholders who would be interested in keeping the facilities going.”
Like GoldenEye’s James Bond, the original radar astronomy program was a Cold War relic. Detecting potentially dangerous asteroids is not a job for a single telescope, as the loss of Arecibo demonstrated. The next generations of radar observatories, ngRADAR and beyond, will expand Earth’s planetary defense to give us hopefully enough warning to preserve the world.
—Matthew R. Francis (@DrMRFrancis), Science Writer
Citation: Francis, M. R. (2024), Saving the planet with radar astronomy, Eos, 105, https://doi.org/10.1029/2024EO240303. Published on 19 July 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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