Part 2: The History and Future of Planetary Radar

To reach the U.S. National Science Foundation’s Green Bank Observatory, you take the road less traveled, winding through scenic and remote regions of the Allegheny Mountains and the Monongahela National Forest of West Virginia. About an hour away, you’ll start to lose cell phone service. The Green Bank Observatory – a collection of radio telescopes that search the heavens for faint radio signals from black holes, pulsars, neutron stars or gravitational waves — sits near the heart of the United States National Radio Quiet Zone, a unique area the encompasses an area of approximately 13,000 square miles, spanning the border between Virginia and West Virginia.

Here in the NRQZ, human-generated radio transmissions are limited to shield the radio telescopes from Earth-based radio signals called RFI (Radio Frequency Interference), which are high-frequency electromagnetic waves that emanate from electronic devices such as computers, cell phones, microwave ovens, and even digital cameras. Even the weakest RFI signals can drown out the faint radio waves coming from the cosmos.

“You can only use basic, old-style film cameras here within 2 miles of the Green Bank Telescope,” said Paul Vosteen, Media Specialist at Green Bank Observatory who provided a tour of the facilities. Vosteen recounted a time he took a group out to see the gigantic (and very photogenic) 100-meter Green Bank Telescope (GBT) and unwittingly, a member of the group started snapping photos with a digital camera. While he quickly got the photographer stopped, Vosteen later coyly checked in with technicians who had been running diagnostics on the GBT that day. They were scratching their heads about a strange spike in signals earlier that morning. Turns out, it was the exact moment the photographer used their digital camera. 

“The slightest electronic signal can cause interference,” Vosteen explained. “We can only use diesel vehicles here on the premises because gasoline engines have spark plugs. Everything that sparks produces radio waves.” Diesel engines, on the other hand, ignite by compression.

To keep the amount of interference on-site in check, the observatory’s control room and the nearby Science Center’s exhibit hall are completely surrounded by copper Faraday cages, wire-mesh devices built into the walls to block electromagnetic signals. Even windows are covered with a thin wire mesh, and the heavy door to the control room opens and closes like an entrance to a high-security bank vault.

Green Bank is home to six large radio telescopes ranging in size from 14 meters to 100 meters in diameter. The 20-meter and the 40-foot telescopes are full-time educational telescopes used by students around the country.

The observatory also contains many relics of radio astronomy history. There’s an exact replica of the dipole array antenna Karl Jansky used when he discovered quite by accident that radio waves were emanating from the center of the Milky Way. That was the beginning of radio astronomy as we know it today. There’s also the actual parabolic dish radio telescope (the world’s first) built by Grote Reber in 1937 to follow up on Jansky’s detection. Then there’s the 85-foot Howard E. Tatel telescope that Frank Drake used in 1960 to perform the world’s first search for extraterrestrial intelligence with Project Ozma.

GBT – “Great Big Thing”

At 485 feet (148 meters) tall, the Robert C. Byrd Green Bank Telescope (GBT – sometimes called ‘Great Big Thing’ by locals) is the tallest and most eye-catching dish at the observatory, and the largest steerable radio telescope in the world. The maneuverability of its large 100-meter dish allows it to quickly track objects across its field of view, and see 85% of the sky.

While the GBT has been in operation since 2000, as we discussed in an article last week, a new upgrade for the telescope is under development. ngRADAR is a next-generation radar system that will allow the GBT to track and map asteroids with unprecedented resolution, making GBT the most advanced planetary radar system in the world. It will also be able to study comets, moons and planets in our Solar System. When finished it will not only help astronomers study the composition of other planetary bodies, but also help defend against potential large meteor strikes on Earth by mapping the precise trajectories of asteroids that cross Earth’s orbit.

Astronomers study the Universe by capturing light from stars, planets, and galaxies. But they can also study nearby objects by shining radio light on them and analyzing the signals that echo back. This is called planetary radar, and the process can reveal incredibly detailed information about our planetary neighbors.

“When astronomers are studying light that is being made by a star, or galaxy, they’re trying to figure out its properties,” said Patrick Taylor, the project director for ngRADAR and the radar division head for the National Radio Astronomy Observatory, in our article last week. “But with radar, we already know what the properties of the signals are, and we leverage that to figure out the properties of whatever we bounced the signals off of. That allows us to characterize planetary bodies – like their shape, speed, and trajectory. That’s especially important for hazardous objects that might stray too close to Earth.”

Previously, the workhorse for planetary radar was the 1,000-foot-diameter (305 meters) Arecibo Observatory which collapsed in 2020, as well as the Goldstone 70-meter dish in California, which is primarily used for communicating with spacecraft as part of NASA’s Deep Space Network. Taylor said that the idea for ngRADAR has been discussed for years — even before Arecibo’s demise — but with the loss of Arecibo, the upgrade is even more important.

Radar signals transmitted by the ngRADAR at the GBT will reflect off astronomical objects, and those reflected signals will be received by the Very Long Baseline Array (VLBA), a network of ten observing stations located across the United States.

“The idea is for GBT is to do the transmitting almost constantly and the VLBA — either all ten of those or any subset of those telescopes — doing the receiving,” said Taylor. “This new system will allow us to characterize the surfaces of many different objects in a different frequency or wavelength that hasn’t been used before.”

Radio Frequencies

All light travels through space in waves – think of how ripples move across a pond. Each ripple has a peak and a trough, which is called a cycle. An object emitting radio waves produces many cycles in a very short period. During each cycle, the wave moves a short distance, which is called its wavelength. Radio waves have the longest wavelengths in the electromagnetic spectrum. They range from sub-millimeter lengths to over 100 kilometers.

For radio waves of all wavelengths, the number of cycles per second is called a frequency, with one cycle per second being one hertz. That means one thousand cycles per second is a kilohertz and a billion cycles per second is a gigahertz. Radio astronomers are interested in objects in a wide range of frequencies, but mostly from between 3 kilohertz and about 900 gigahertz.

“Arecibo worked at 2.38 gigahertz, the Goldstone 70-meter primarily works at 8.56 gigahertz,” said Taylor. “For ngRADAR, we are looking at even higher frequencies, at 13.7 gigahertz, something that really hasn’t been used for planetary radar before. This is a way to offer something new and different, while the capabilities of the two instruments – GBT and Goldstone – also would complement each other.”

But more importantly, since Goldstone is now “the only planetary radar game in town,” as Taylor described it, that means planetary radar in the US has a single point failure. The antennas of Goldstone Deep Space Communications Complex are busy 24 hours a day communicating with spacecraft around the Solar System.

“If Goldstone is down for whatever reason or if it’s not available because of its work with the DSN,” said Taylor, “having a radar transmitter on the GBT gives us more flexibility and redundancy.”

Taylor said there are several applications for the future of radar, from not only advancing our knowledge of objects in the Solar System and characterizing asteroids and comets, but also aiding in future robotic and crewed spaceflight.

The GBT worked with the Goldstone telescope to help confirm the success of NASA’s Double Asteroid Redirection Test (DART) mission in 2022, the first test to see if humans could successfully alter the trajectory of an asteroid. In a two-week campaign, the radio telescopes were able to track how the orbit of Dimorphos, the asteroid that was hit by DART, changed after the impact.

But the main goal ngRADAR is for is planetary defense.

“That will be one of the highest priority uses for the radar system, where we can track and characterize near earth-asteroids and comets to evaluate any hazard they might present to Earth in the future. Radar delivers very precise data that allows you to predict where these small bodies will be in the future. We can determine its size, how it rotates, what it might be made of, is it just a round ball, or does it look like a potato, or does it have a moon that you also must worry about.”

Building ngRADAR

As we discussed last week, a scaled-down prototype of ngRADAR at the GBT produced some of the highest resolution planetary radar images ever captured from Earth. Not only will the new full-scale system need to be built, but several changes will need to be made to the GBT. 

“This will be a pretty intensive infrastructure project,” Taylor explained. “We’ll have to build the transmitter and mount it onto the GBT. With the size and weight of the system, as well as the cooling systems that will be needed, extra structures will be needed to support all that.”

Taylor said the timeline for completion would depend on approval and funding, but a reasonable goal is that in the next five years – perhaps by 2029-2030 – ngRADAR could be up and running.

But Taylor feels that ngRADAR will allow the GBT to come full circle.

“Some of the first science done with GBT was receiving radar signals when it was first inaugurated,” he said. “It’s been a receiver for radar for over 20 years but now we are trying to take the next step and have it be a transmitter as well.”

Read part 1 of this series, Next Generation Radar Will Map Threatening Asteroids.

The Green Bank Observatory and the National Radio Astronomy Observatory are facilities of the U.S. National Science Foundation (NSF) operated under cooperative agreement with Associated Universities, Incorporated.  The opinions expressed in this article do not necessarily represent those of the NSF, and the implementation of any radar system on these facilities would be subject to NSF review and approval.