# Can radio telescopes such as arecibo image the subsurface of asteroids or planets?

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This was done with the moon: Radio Dishes Peer Beneath Moon's Surface

It is important to understand how this process works. The method described in your article is known as Bistatic Radar. In effect, a transmitter sends out a signal (generally a radio telescope in the microwave region) which hits the surface of some body and bounces off to be received back on Earth by a second, separate radio telescope. Now, because a microwave's wavelength is so long ($sim 0.1-100:cm$) the microwaves don't bounce off from the exact surface, but instead are able to penetrate slightly into the subsurface before being reflected. This means the receiver gets a reflection of the subsurface of the body.

In the case you linked, they used the Arecibo Telescope to provide the transmitted signal to the Moon and the Green Bank Telescope to receive the signal. A similar process could be done to other bodies besides our Moon. However, you run into the problem that the farther away an object is, the harder it will be to detect the returning signal.

I believe that most objects within our solar system (and certainly all objects outside it) are outside the distance where this method will work. I was able to find an instance where someone used this method for an asteroid that passed by the Earth at a distance 11 times that of the Moon. I'm not sure what the limit is to how far away something must be before this method no longer works, but I imagine it wouldn't work even for Mars unless we seriously upgrade our technology and power.

Of course your question seems to presuppose the transmitter and receiver are both on Earth. If your transmitter is much closer, say a satellite orbiting that planet, then it is certainly feasible. Here is a source which discusses the concept of using Bistatic Radar probing via a satellite, which also goes into the history a bit on what has been done before.

Yes, but not Earth-based ones.

SHARAD (Shallow Radar) is an instrument on the Mars Reconnaissance Orbiter which performs subsurface imaging of Mars.

Here is an image from Wikipedia of deposit layers on Mars's north pole, taken by SHARAD:

## Interview with Dr. Ann Virkki of Arecibo Observatory — Astronomy News with The Cosmic Companion May 5, 2020

This week on Astronomy News with The Cosmic Companion, we welcome a very special video guest as Dr. Ann Virkki, head of planetary radar studies for Arecibo Observatory, joins us on the show. She is an astronomer who recently made the news with her discovery of an unusual “face mask” on the asteroid 1998 OR2. Join us as we talk about asteroids, and the dangers our planet faces from near-Earth Objects.

In this week’s episode of Astronomy News with The Cosmic Companion, we also learn about the unfortunate fate of Comet Atlas, which recently shattered as it approached the Sun, quashing dreams of what could have been a magnificent celestial spectacle. Next, we will learn how temperatures seen on worlds orbiting alien stars are, often, lower than theories predict, and we will discuss a new model that could, potentially, explain these strange findings. We also take a look at how a new range of instruments, both on Earth and in space, could help us search for life around white dwarfs — the corpses of dead stars that were once the size of the Sun.

Watch the video version of this episode:

Or, listen to the podcast version of the show (also available from all major podcast providers)

Comet Atlas, once a contender to become the greatest comet in a generation, has shattered into dozens of pieces. The massive iceberg in space heated, as all comets do, as it approached the Sun. Astronomers believe uneven heating of the comet may have caused fissures develop through parts of the nucleus. These cracks would have spread through the icy bodies, cleaving the comet into fragments large and small. The Hubble Space Telescope took a pair of images showing the comet flying apart into dozens of pieces, many of which were as large as typical houses.

Astronomers studying planets around other stars have noticed something unusual. Temperatures on massive worlds orbiting near their parent stars seemed to be lower than theories predict. Most of these worlds are tidally locked to their stars, with one face always pointing to their local star, much as the face on the Moon always points to Earth.

A group of researchers from Cornell University have now developed a new mathematical model, suggesting the side of these worlds facing their suns experience temperatures hundreds or even thousands of degrees warmer than astronomers had previously measured. On massive worlds caught in this gravitational trap, chemical reactions could be far different on each side of the planet, potentially resulting in significant differences in chemistry, geology, climate, and even the potential for life.

A new generation of telescopes, both on and above the Earth, could soon make it possible to search for signs of extraterrestrial life on exoplanets surrounding white dwarf stars. These tiny stellar remnants are the remains of dead stars, collapsed to the size of the Earth.

As stars like our Sun die, they go through periods of shrinking and swelling, as they heat up and cool over time. This process can destroy planets near the star, and it is unlikely, however possible, that lifeforms on another world might survive the experience.

If even primitive life survived such an experience, or rose from a once-dead planet, astronomers may soon be able to see telltale traces of certain gases in the atmospheres of these distant worlds.

Join us on May 19, as we talk with Thea Kozakis from the Carl Sagan Institute at Cornell University. She is a pioneering researcher on studying the atmospheres of distant worlds for signs of extraterrestrial life.

D id you like this episode? Join us onThe Cosmic Companion Network for our podcast, weekly video series, informative newsletter, news briefings on Amazon Alexa and more!

## Iconic #Arecibo Radio Telescope Collapses!

This is a follow-up story to recent developments at Puerto Rico’s Arecibo Radio Observatory.

At 7:53:50 AM EST, December 1st, the observatory’s radio transceiver superstructure, suspended above the 305-meter spherical radio dish, collapsed and fell to the dish below (watch video at the foot of this article). The NSF news release reporting the event follows:

The instrument platform of the 305-meter telescope at Arecibo Observatory in Puerto Rico fell at approximately 7:55 a.m. Atlantic Standard Time Dec. 1, resulting in damage to the dish and surrounding facilities.

No injuries were reported as a result of the collapse. The U.S. National Science Foundation ordered the area around the telescope to be cleared of unauthorized personnel since the failure of a cable Nov. 6. Local authorities will keep the area cordoned off as engineers work to assess the stability of the observatory’s other structures.

Top priorities are maintaining safety at the site, conducting a complete damage assessment as quickly as possible, and taking action to contain and mitigate any environmental damage caused by the structure or its materials. While the telescope was a key part of the facility, the observatory has other scientific and educational infrastructure that NSF will work with stakeholders to bring back online.

“We are saddened by this situation but thankful that no one was hurt,” said NSF Director Sethuraman Panchanathan. “When engineers advised NSF that the structure was unstable and presented a danger to work teams and Arecibo staff, we took their warnings seriously and continued to emphasize the importance of safety for everyone involved. Our focus is now on assessing the damage, finding ways to restore operations at other parts of the observatory, and working to continue supporting the scientific community, and the people of Puerto Rico.”

The investigation into the platform’s fall is ongoing. Initial findings indicate that the top section of all three of the 305-meter telescope’s support towers broke off. As the 900-ton instrument platform fell, the telescope’s support cables also dropped.

Preliminary assessments indicate the observatory’s learning center sustained significant damage from falling cables.

Engineers arrived on-site today. Working with the University of Central Florida, which manages the observatory, NSF expects to have environmental assessment workers on-site as early as tomorrow. Workers at the observatory will take appropriate safety precautions as a full assessment of the site’s safety is underway.

“We knew this was a possibility, but it is still heartbreaking to see,” says Elizabeth Klonoff, UCF’s vice president for research. “Safety of personnel is our number one priority. We already have engineers on site to help assess the damage and determine the stability and safety of the remaining structure. We will continue to work with the NSF and other stakeholders to find ways to support the science mission at Arecibo.”

NSF intends to continue to authorize UCF to pay Arecibo staff and take actions to continue research work at the observatory, such as repairing the 12-meter telescope used for radio astronomy research and the roof of the LIDAR facility, a valuable geospace research tool. These repairs were funded through supplemental congressional appropriations aimed at addressing damage from Hurricane Maria.

Once safety on site is established, other work at the observatory will be carried out as conditions permit.

NSF will continue to release details as they are confirmed. Additional information, including engineers’ assessments of the structure, can be found in NSF’s Nov. 19 news release.

Background

Although the platform’s fall was unplanned, NSF, UCF and other stakeholders, including engineering firms contracted by UCF, had been monitoring developments at the 305-meter telescope that indicated an increased risk of a collapse.

In August, one of the 305-meter telescope’s cables unexpectedly detached. The remaining cables were expected to bear the load without issue as engineers worked on plans to address the damage. However, a second cable broke Nov. 6. Engineers subsequently found the second snapped at about 60% of what should have been its minimum breaking strength, indicating that other cables may be weaker than expected, and advised that the structure could not be safely repaired.

Both cables were attached to the same support tower. If the tower lost another cable, the engineer of record noted, an unexpected collapse would be the likely result. Since NSF’s Nov. 19 announcement that it would plan for decommissioning of the 305-meter telescope, surveillance drones found additional exterior wire breaks on two cables attached to the same tower. One showed between 11-14 broken exterior wires as of Nov. 30 while another showed about eight. Each cable is made up of approximately 160 wires.

-NSF-

Significant in the press release is the following:

NSF intends to continue to authorize UCF to pay Arecibo staff and take actions to continue research work at the observatory, such as repairing the 12-meter telescope used for radio astronomy research and the roof of the LIDAR facility, a valuable geospace research tool. These repairs were funded through supplemental congressional appropriations aimed at addressing damage from Hurricane Maria.

This tragic end to the unique and iconic scientific landmark closes out the current chapter of the 57-year-old radio observatory, made famous by the 1997 feature-length Hollywood Production Contact.

It is important to note that a “Public-Private” partnership was entered into in 2018 to manage the observatory between the University of Central Florida, The National Science Foundation and “Yang Enterprises“, a for-profit government contractor with Democratic Senator from Florida Bill Nelson having secured $20M in federal repair money to repair damages to the observatory sustained during Hurricane Maria’s 2017 rampage across the island. The question begs asking, how is it possible that the facility has prevailed during many hurricanes since 1963 as well as surviving two upgrades, the first in 1974 and another in 1997, with the later adding much additional mass and corresponding engineering upgrades to the support cables, towers and infrastructure. Now, how is it that, having received$20M in public repair money in 2018, the main radio transceiver superstructure support cables suffer a general failure, causing catastrophic collapse and rendering any future discussions of “repair/upgrade” as moot? Where did the $20M go? Was it ever used? Was more money needed and who made the determination of how much was enough? These are legitimate questions that need to be asked. Rather than lament over how this tragedy could have been prevented (and it could have been prevented if the observatory’s benefactors were determined to do so), let’s focus on what it has accomplished over the intervening 57 years since its commissioning and look forward to new possibilities as expressed here: Here is the view of The Arecibo Observatory. A sad day for science, for Puerto Rico, and for the entire world. We will not rest until we #RebuildAreciboObservatory. Now we will fight faster and stronger. We can’t lose our Observatory forever. @SaveTheAO @NAICobservatory pic.twitter.com/AvCPO2bmbm &mdash Wilbert Andrés Ruperto (@ruperto1023) December 1, 2020 1. The observatory was instrumental in the Exploration of Black Holes begun by this Year’s Nobel Prize Winners in Physics. 2. Completed in 1963 and stewarded by U.S. National Science Foundation since the 1970s, Arecibo Observatory has contributed to many important scientific discoveries, including the demonstration of gravitational waves from a binary pulsar, the first discovery of an extrasolar planet, composition of the ionosphere, and the characterization of the properties and orbits of a number of potentially hazardous asteroids. 3. The only Radar Equipped (planetary radar system) instrument in its class, allowing for detailed [radar] studies of the surfaces of Mercury, Venus, Titan, the mapping in real time of various asteroids of recent note as well as other solar system objects. Along with other accomplishments, details of all these studies can be found at the observatory’s web portal here. Arecibo observed near-Earth asteroid (505657) 2014 SR339 using its planetary radar system on February 9, 2018. Radar images reveal 2014 SR339 to have a lumpy, elongated shape at least 1.5 km long and a rotation consistent with the 8.7 hour period determined from optical lightcurves (B.D. Warner). 4. Arecibo and Cassini collaborate to resolve a bright radar reflection anomaly on Saturn’s large moon Titan. 5. Arecibo was a member node in the VLBI (Very Large Baseline Interferometer) 6. Extensive observations and studies of PULSARS conducted at Arecibo. 7. Arecibo Observatory Helps Test Einstein’s Theory of Relativity for Heavy Objects 8. To celebrate and commemorate the first of these upgrades, a 450 Kw (kilowatt) signal was beamed at a frequency of 2.4 Ghz towards the core of the great globular star cluster M-13 in Hercules, 25,000 Light years distant on 16 November 1974. The message contained basic information about humanity, our biology, our location in the galaxy and our home planet. The transmission, with a duration of 3 minutes, was meant more as a demonstration of human technical prowess rather than a real attempt to begin a conversation with extraterrestrials. The “Legacy Discoveries” page lists many notable items with a few of the more noteworthy below: 1. Arecibo discovered two extremely strange pulsars 2. VLBI observations of quasar 3C273 revealed a brightness temperature greater than 1013 K. 3. Arecibo discovered the first ever repeating Fast Radio Burst (FRB). ### The Physics Involved We’ve received inquiries as to why the support system experienced a general failure “all at once”. Regarding the general failure of the transceiver superstructure support cables falling all at once and why one of the support masts also failed and why didn’t the remaining 2 masts sustain the falling transceiver? The support masts were never designed to support the falling transceiver and once that began, a whole new set of dynamics came into play. To suspend the structure is one thing but to stop it once it begins falling is a whole new problem. Newton’s second law informs us that the force required to stop it, once in motion, is not the same as suspending it above the dish this force would be tremendous. Once it started to fall, the other cables, already compromised, would quickly fail, bringing with them the section of one of the masts and thus, we have the tragedy witnessed. This is why “they all seemed to fail at once“. ### Related Stories Imagination is more important than knowledge An index of all articles can be found here. If you enjoyed this article, please consider supporting us with a modest donation or through a subscription on our Patreon Page Membership at Astronomy for Change is Free! ## Hurricane Maria On Sept. 20, 2017, Hurricane Maria ravaged the island of Puerto Rico, damaging the Arecibo Observatory. The Category 4 storm killed hundreds of people and caused widespread power outages that lasted for months. Power to the observatory was restored on Dec. 9, 2017. The most significant damage was to the 96-foot (29 meters) "line feed" antenna, which was suspended above the radio dish. It broke off during the hurricane and punctured the dish below when it fell. A federal spending bill passed in February 2018 to provide relief to Puerto Rico allocated$16.3 million to repairing the Arecibo Observatory.

"Emergency repairs that needed immediate attention, such as patching roofs and repairing electrical feeds, have been underway since May after the site received hurricane-relief funding," the University of Central Florida said in a statement issued in August 2018. "Additional repairs that will require more time and expertise will be completed as soon as possible."

## Corroding cables

In the 1960s, engineers built Arecibo’s mammoth radio dish in one of Puerto Rico’s natural sinkholes. An overhead, triangular equipment platform helps aim the telescope at different parts of the cosmos. That platform is packed with receivers, line feeds, and a complex reflector system that precisely focuses radio waves—and it’s where James Bond fought Alec Trevelyan in 1995’s GoldenEye.

Although it might look small relative to the dish, the suspended structure is truly massive—a small house could easily fit inside the dome housing the reflector system.

The platform is held up by 18 thick, steel cables strapped to three concrete towers, the tallest of which measures 365 feet. In addition to the four primary cables on each tower, two auxiliaries per tower were installed in the 1990s to help stabilize the structure and bear additional weight.

Observatory staff regularly inspect the towers, cables, and platform, looking for any signs of weakening or corrosion caused by the salty, tropical air.

“There’s nothing worse, corrosion-wise, than salt fog,” says Dennis Egan, an engineer at the Green Bank Observatory in West Virginia. “It’s better to be underwater.”

Those inspections have turned up some evidence of popped strands in the cables, a problem that Nolan suspects could have been exacerbated by Hurricane Maria and a recent swarm of sizable earthquakes. But they found no indications of widespread weakening or impending failure. In a Q&A posted on Facebook, Arecibo’s director, Francisco Córdova, said the rupture was unexpected and indicative of structural degradation.

The observatory “is 50 years old, and there’s never been a situation where suddenly a whole bunch of different strands are breaking,” says Drake, who famously sent a message into outer space from the observatory in 1974. “I would not want to be on that thing now. There’s no escape. You’re just stuck.”

If tower four fails, the platform could either crash through the dish or make a pendulous swing into a nearby cliff. Without the platform’s weight keeping the towers balanced, it’s possible all three could topple into the surrounding jungle.

If engineers can stabilize the structure, they could then repair or replace some of the aging cables. Two new cables are already on order, Córdova said on Facebook, due to arrive at the observatory in December.

But to replace the cables, workers will need to go up onto the platform. “They’ve got to do something to verify that the existing cables are correct and undamaged, in a way that doesn’t endanger people on the structure,” Drake says.

## The problem of artificial intelligence

Another concern was over artificial intelligence. Here the concern was not so much existential. By this, I mean the speakers were not fearful that some computer was going to wake up into consciousness and decide that the human race needed to be enslaved. Instead, the danger was more subtle but no less potent. Susan Halpern, also one of our greatest non-fiction writers, gave an insightful talk that focused on the artificial aspect of artificial intelligence. Walking us through numerous examples of how "brittle" machine learning algorithms at the heart of modern AI systems are, Halpern was able to pinpoint how these systems are not intelligent at all but carry all the biases of their makers (often unconscious ones). For example, facial recognition algorithms can have a hard time differentiating the faces of women of color, most likely because the "training data sets" the algorithms were taught were not representative of these human beings. But because these machines supposedly rely on data and "data don't lie," these systems get deployed into everything from making decisions about justice to making decisions about who gets insurance. And these are decisions that can have profound effects on people's lives.

Then there was the general trend of AI being deployed in the service of both surveillance capitalism and the surveillance state. In the former, your behavior is always being watched and used against you in terms of swaying your purchasing decisions in the latter, you are always being watched by those in power. Yikes!

## A NEW HOPE

“Arecibo is not dead we have a lot of stored data that we are still processing and from which we can learn a lot through big data techniques. Some of it has been on magnetic tape for decades,” says Pinilla, “and the research complex will remain active, as we have a new 12-metre radio telescope that has not been used but is operational.” In light of the news that NSF funding for the reconstruction has been cancelled, Pinilla makes it clear: “We need to think quickly and deeply, to begin work on rebuilding not the original radio telescope, but a better one.”

The new 12-meter radio telescope seen from the instrument platform. Top left, Noemí Pinilla. Credit: Noemi Pinilla

At this moment, public support is key, as it has been at other times when the observatory’s budget has been cut. This facility has proven its scientific worth for more than half a century and so far there is no commitment to fund its reconstruction. With the slogan “Save Arecibo” it is possible to access the campaign to support the rebuilding, which among many actions is also gathering petitions to send to the White House. Right now, saving Arecibo may seem of little importance, but it would be in all of our interests because one day in the future a new radio telescope might just save us.

## 12 times the Arecibo telescope helped us figure out the universe

The Arecibo telescope in 2019. Image credits: University of Central Florida.

The collapse of the Arecibo radio telescope in Puerto Rico sent ripples through the world of science. Many mourned the loss of a telescope that helped us learn so many important things about the universe, with local researchers even tearing up during interviews.

The telescope, which was built in 1963, was still doing science — here are just some of the important discoveries made thanks to the Arecibo telescope.

### Discovering the first-ever exoplanet

Artist’s impression of pulsar PSR B1257+12 and the planets orbiting it. The one in the foreground is planet “C”. Image credits: NASA/R Hurt.

In 1990, researchers working at Arecibo discovered a millisecond pulsar, a kind of neutron star, with a rotation period of 6.22 milliseconds (9,650 rpm).

In 1992, subsequent measurements found the first-ever extrasolar planets: two planets orbiting the pulsar. Two years later, more refined methods found one more planet orbiting the pulsar.

### The origins of water ice on Mercury

Mercury, the nearest planet to the Sun, isn’t the first place you’d expect to find water (or anything) frozen. But in 1991, astronomers at the Arecibo Observatory discovered “extremely reflective” material radiating from Mercury’s surface — which many interpreted as evidence of ice.

Images from Messenger confirmed the Arecibo findings. Image credits: NASA.

In 2017, data from the Messenger spacecraft around Mercury confirmed the existence of pockets of ice on Mercury, in cratered areas which are permanently shaded. Mercury doesn’t have an atmosphere, which means the heat doesn’t diffuse, so you can have scorching hot temperatures in close proximity to sub-freezing temperatures.

### The Arecibo message

The Arecibo telescope was heavily involved in the SETI project, looking for potential signals from alien civilizations. In 1974, humanity even sent an interstellar radio message carrying basic information about humanity and the Earth (the message was aimed at the globular star cluster M13). It was meant to serve as a demonstration of human technological achievement, a way to show we can send out interstellar messages, rather than a real attempt to start a conversation.

A demonstration of the message with color added to highlight its separate parts. The binary transmission sent carried no color information.

The message was designed by Frank Drake, with the help of Carl Sagan and other astronomers and broadcast at Arecibo. Among others, it carried information about the numbers from 1 to 10 (white), the atomic numbers of chemical elements that make up DNA (purple), the dimension of an average human (blue/white), the graphic figure of a human (red), a graphic of the solar system (yellow), and a graphic of the Arecibo radio telescope (purple, white, and blue).

### Discovering the first binary pulsar

Pulsars are highly magnetized rotating compact stars that emit beams of electromagnetic radiation out of their magnetic poles. Sometimes, pulsars have companions, like a white dwarf or a neutron star, in which case it’s called a binary pulsar. The first pulsar was discovered in 1967, but it was Russell A. Hulse and Joseph H. Taylor in 1974 that discovered the first binary pulsar.

Pulsars, like the Crab Nebula depicted in this composite optical/X-ray image, have been extensively studied by the Arecibo telescope. Jocelyn Bell discovered the first pulsar in 1967. Image credits: NASA.

The two researchers at the Arecibo telescope discovered the binary pulsar using gravitational physics, paving the way for the discovery of the fabled gravitational waves. Their work was rewarded with a Nobel Prize.

### Finding a dark matter galaxy

In 2006, astronomers discovered a mysterious cloud of hydrogen 50 million light-years from Earth. They called it VIRGOHI 21. Much to the surprise of astronomers, VIRGOHI 21 turned out to be a dark matter galaxy that didn’t emit any visible light (which is why a radio telescope turned out to be so useful).

Animation of Arecibo data. VIRGOHI 21 is the structure in the center. Source and full story here.

While there is still some controversy about what this galaxy really is (or if it even is a galaxy at all), data from Arecibo allowed its discovery and analysis, taking us one step closer to understanding one of the more exotic astronomical phenomena out there.

### Understanding the ‘Weird!’ signal

In 2017, a weird signal (formally named ‘Weird!’ by astronomers) was reported. As if often happens, people’s imagination immediately went to aliens, but it turns out this wasn’t the case. Astronomers suspected a signal from a dim red dwarf, but this also didn’t turn out to be true.

Using data from the Arecibo telescope, researchers found that the signal was much more prosaic: it was interference from a nearby satellite.

### Studying an asteroid close to Earth

The Bennu asteroid was intercepted by the NASA OSIRIS-REx spacecraft, which closed in and captured an image from a distance of 600 metres (2,000 ft) from Bennu’s surface. But before NASA could get up close and personal with the asteroid, Bennu was studied extensively with the Arecibo telescope, helping to better prepare the mission.

In 2000, researchers at Arecibo also captured the first images of near-Earth asteroids.

Mosaic image of Bennu consisting of 12 images collected by OSIRIS-REx from a range of 24 km (15 mi).

### Radio maps of Venus and Titan

The first radar maps of Venus were done with the Arecibo telescope in the late 1970s, showing some of the Venusian relief and geology, and showing that its surface is less than one billion years old.

The maps were constantly refined and finessed. Many features, including mountain ranges, volcanic domes, and craters can be seen.

This is a radar image of the planet Venus made by transmitting a signal at 13 cm wavelength from Arecibo. Credit: Donald Campbell, Jean-Luc Margot, Lynn Carter, and Bruce Campbell

Titan, the largest moon of Saturn, is a weird place. It’s an icy world whose surface is completely obscured by a golden hazy atmosphere — and, as we’ve learned thanks to the Arecibo telescope, it has liquid hydrocarbon lakes on its surface.

As it is often the case, observations by Arecibo inspired future missions that analyzed things in greater detail. Here, the Cassini mission surveyed Titan and snapped the beautiful image below.

Radar images from NASA’s Cassini spacecraft reveal many lakes on Titan’s surface, some filled with liquid, and some appearing as empty depressions. Image credits: NASA/JPL/USGS.

### Neutron stars can be very large, but forming black holes is difficult

Neutron stars and black holes are the two most massive objects known in the universe. But they’re not always what they seem to be. In fact, neutron stars can be considerably more massive than previously believed, and it is more difficult to form black holes, according to 2008 research from Arecibo.

“The matter at the center of the neutron stars is the densest in the universe. It is one to two orders of magnitude denser than matter in the atomic nucleus. It is so dense we don’t know what it is made out of,” said Paulo Freire, an astronomer from the observatory, who presented the research. “For that reason, we have at present no idea of how large or how massive neutron stars can be.”

### The most metal-poor galaxy in the known universe

In astronomy, metalicity is the abundance of elements present in an object that are heavier than hydrogen or helium. Galaxies with low metallicity are of special importance for astronomers as they could provide crucial insights about chemical evolution of stars and astrophysical processes occurring in the early universe.

In 2016, astronomers found the galaxy with the lowest known metallicity, which could offer a glimpse into the early days of the universe, and also mark a paradigm shift in the search for metal-poor galaxies.

### Distant galaxies could hold ingredients for life

In 2008, astronomers from the Arecibo Observatory detected the molecules methenamine and hydrogen cyanide — two ingredients that build life-forming amino acids — in a galaxy some 250 million light-years away.

The fact that they could be observed at such a huge distance suggests that the compounds are highly abundant in the galaxy. It’s remarkable that we can make any observations about a galaxy this far away, let alone that we can tell that it has potentially life-forming molecules.

### Solving the mystery of vanshing pulsars

Pulsars are often considered the orderly ticking clocks of the universe. A 2017 survey carried out at Arecibo contradicted that view, finding that sometimes, pulsars undergo a “vanishing act”.

“These disappearing pulsars may far outnumber normal pulsars,” said Dr. Victoria Kaspi of McGill University in Canada and the principal investigator on the PALFA project. “In fact, they may redefine what we think of as normal.”

In addition to all these discoveries (and many which we’ve missed), Arecibo was also an iomportant part of NANOGrav, the orth American Nanohertz Observatory for Gravitational Waves (NANOGrav), a consortium of astronomers who aim to detect gravitational waves via regular observations of an ensemble of pulsars. The NANOGrav group posted this statement:

“The NANOGrav Collaboration is greatly saddened by the impact of the planned decommissioning of the 305-m Arecibo telescope on the staff and scientists who have worked so hard for so many years to ensure its success. We will miss the telescope itself, as one of our own. Many of our scientific careers began with the training we received and camaraderie we enjoyed at Arecibo, for which we will be forever grateful. We also stand in solidarity with our fellow citizens in Puerto Rico for whom Arecibo has been an inspiration and source of pride for so many years. We urge the National Science Foundation to identify uses for the site and staff, as soon as practicable, that benefit from Arecibo’s unique characteristics and promote its continued inspirational role in STEM fields.”

## Detection of Radio Energy from Space

It is important to understand that radio waves cannot be &ldquoheard&rdquo: they are not the sound waves you hear coming out of the radio receiver in your home or car. Like light, radio waves are a form of electromagnetic radiation, but unlike light, we cannot detect them with our senses&mdashwe must rely on electronic equipment to pick them up. In commercial radio broadcasting, we encode sound information (music or a newscaster&rsquos voice) into radio waves. These must be decoded at the other end and then turned back into sound by speakers or headphones.

The radio waves we receive from space do not, of course, have music or other program information encoded in them. If cosmic radio signals were translated into sound, they would sound like the static you hear when scanning between stations. Nevertheless, there is information in the radio waves we receive&mdashinformation that can tell us about the chemistry and physical conditions of the sources of the waves.

Radio waves are reflected by conducting surfaces, just as light is reflected from a shiny metallic surface, and according to the same laws of optics. A radio-reflecting telescope consists of a concave metal reflector (called a dish), analogous to a telescope mirror. The radio waves collected by the dish are reflected to a focus, where they can then be directed to a receiver and analyzed. Because humans are such visual creatures, radio astronomers often construct a pictorial representation of the radio sources they observe. Figure (PageIndex<2>) shows such a radio image of a distant galaxy, where radio telescopes reveal vast jets and complicated regions of radio emissions that are completely invisible in photographs taken with light.

Figure (PageIndex<2>) Radio Image. This image has been constructed of radio observations at the Very Large Array of a galaxy called Cygnus A. Colors have been added to help the eye sort out regions of different radio intensities. Red regions are the most intense, blue the least. The visible galaxy would be a small dot in the center of the image. The radio image reveals jets of expelled material (more than 160,000 light-years long) on either side of the galaxy. (credit: NRAO/AUI)

Radio astronomy is a young field compared with visible-light astronomy, but it has experienced tremendous growth in recent decades. The world&rsquos largest radio reflectors that can be pointed to any direction in the sky have apertures of 100 meters. One of these has been built at the US National Radio Astronomy Observatory in West Virginia (Figure (PageIndex<3>)). Table (PageIndex<1>) lists some of the major radio telescopes of the world.

Figure (PageIndex<3>) Robert C. Byrd Green Bank Telescope. This fully steerable radio telescope in West Virginia went into operation in August 2000. Its dish is about 100 meters across. (credit: modification of work by &ldquob3nscott&rdquo/Flickr)

Table (PageIndex<1>): Major Radio Observatories of the World
Observatory Location Description Website
Five-hundred-meter Aperture Spherical radio Telescope (FAST) Guizhou, China 500-m fixed dish fast.bao.ac.cn/en/
Arecibo Observatory Arecibo, Puerto Rico 305-m fixed dish www.naic.edu
Green Bank Telescope(GBT) Green Bank, WV 110 × 100-m steerable dish www.science.nrao.edu/facilities/gbt
Effelsberg 100-m Telescope Bonn, Germany 100-m steerable dish www.mpifr-bonn.mpg.de/en/effelsberg
Lovell Telescope Manchester, England 76-m steerable dish www.jb.man.ac.uk/aboutus/lovell
Canberra Deep Space Communication Complex (CDSCC) Tidbinbilla, Australia 70-m steerable dish www.cdscc.nasa.gov
Goldstone Deep Space Communications Complex (GDSCC) Barstow, CA 70-m steerable dish www.gdscc.nasa.gov
Parkes Observatory Parkes, Australia 64-m steerable dish www.parkes.atnf.csiro.au
Square Kilometre Array(SKA) South Africa and Western Australia Thousands of dishes, km2collecting area, partial array in 2020 www.skatelescope.org
Atacama Large Millimeter/submillimeter Array (ALMA) Atacama desert, Northern Chile 66 7-m and 12-m dishes www.almaobservatory.org
Very Large Array (VLA) Socorro, New Mexico 27-element array of 25-m dishes (36-km baseline) www.science.nrao.edu/facilities/vla
Westerbork Synthesis Radio Telescope (WSRT) Westerbork, the Netherlands 12-element array of 25-m dishes (1.6-km baseline) www.astron.nl/radio-observatory/public/public-0
Very Long Baseline Array (VLBA) Ten US sites, HI to the Virgin Islands 10-element array of 25-m dishes (9000 km baseline) www.science.nrao.edu/facilities/vlba
Australia Telescope Compact Array (ATCA) Several sites in Australia 8-element array (seven 22-m dishes plus Parkes 64 m) www.narrabri.atnf.csiro.au
Multi-Element Radio Linked Interferometer Network (MERLIN) Cambridge, England, and other British sites Network of seven dishes (the largest is 32 m) www.e-merlin.ac.uk
Millimeter-wave Telescopes
IRAM Granada, Spain 30-m steerable mm-wave dish www.iram-institute.org
James Clerk Maxwell Telescope (JCMT) Mauna Kea, HI 15-m steerable mm-wave dish www.eaobservatory.org/jcmt
Nobeyama Radio Observatory (NRO) Minamimaki, Japan 6-element array of 10-m wave dishes www.nro.nao.ac.jp/en
Hat Creek Radio Observatory (HCRO) Cassel, CA 6-element array of 5-m wave dishes www.sri.com/research-development/specialized-facilities/hat-creek-radio-observatory

## NASA Tracking Huge Asteroid with Radar for Tuesday Encounter

A quarter-mile wide asteroid called 2005 YU55 will slip close by Earth Tuesday (Nov. 8) while astronomers around the world watch through telescopes. But some scientists are using a different way to scan the space rock: radar.

The huge Arecibo radio telescope in Puerto Rico and a NASA antenna in California are bombarding asteroid 2005 YU55 with radar signals to get a rare and close look at a huge space rock. The asteroid, which is about 1,300 feet (400 meters) wide, is the first giant space rock in 25 years to make a close pass by Earth with enough warning that astronomers could prepare to observe it in advance.

Asteroid 2005 YU55 will fly inside the orbit of the moon, coming within 201,700 miles (324,600 kilometers) on Tuesday at 6:28 p.m. EST (2328 GMT), when it makes its closest approach. The asteroid poses no threat of impacting Earth during the close encounter, NASA astronomers have said.

But asteroid 2005 YU55 isn't the only space rock in NASA's radar sights. Of the more than 8,400 objects passing through Earth's neighborhood as the planet cuts its way through the solar system, more than 1,000 objects have orbits classifying them as potentially hazardous.

Pinpointing these near-Earth objects, or NEOs, requires more precision than optical telescopes can provide. Astronomers turn to radar to accurately predict how close to our planet an object will pass. [Photos: Flyby of Giant Asteroid 2005 YU55]

While most astronomers depend on emissions either originating from or reflected by the asteroid, some rely on signals sent from our planet.

Astronomers ping the body &mdash usually a comet or an asteroid &mdash and measure how long it takes for the radio signal to return, then use that information to calculate the distance. The method is extremely precise, locating the comet or asteroid within about 30 feet (10 meters), a narrow window on an astronomical ruler.

Radar can also measure how fast an object is traveling toward Earth with an accuracy of up to 1 millimeter per second. Knowing the location and distance allows scientists to compute its orbit and determine whether or not it may collide with the Earth.

Radar can also map the details of the exterior of an NEO. When 2005 YU55 passes Earth Tuesday, radar will be used to study its surface features.

"If there is a crater on the surface of the object only a few meters in diameter, we're hoping to see it," NASA's Near-Earth Object Office manager Don Yeomans told SPACE.com. "There's no way on Earth you could see that via optical."

Signal origin: Earth

For pinging asteroids, radar signals can be sent from two places on our planet: the Arecibo Observatory in Puerto Rico, and Goldstone Deep Space Communications Complex in southern California.

With a diameter of 1,000 feet (nearly 305 meters), Arecibo boasts the larger telescope and is capable of capturing more in-depth images. It can also peer further out into space. But its vast size makes it sedentary, locked into only a particular patch of the sky.

The smaller, more mobile telescope at Goldstone can cover up to 80 percent of the sky, catching objects missed by its southern partner. This also gives it more time on a single target.

Most of the time, the two compliment each other. Used together, they can provide a vast amount of data about the comets and asteroids that pass near Earth.

"Arecibo is used for about thirty different near-Earth asteroids each year," Yeomans said. Goldstone is used only slightly less.

Not surprisingly, the moon was the first target pinged by radar, in the mid 1940s. Venus was the next choice, followed quickly by two asteroids, the terrestrial planets, and the rings of Saturn between 1960 and 1975. [Photos: Asteroids in Deep Space]

Since then, objects as distant as the Galilean moons of Jupiter and Saturn's moon, Titan, have been examined by radar from the surface of the Earth.

Various space missions have also been outfitted with radar to study the planets up close.

But over the last few years, the use of radar has picked up steam, particularly for near-Earth objects.

"It became more prevalent in the '90s," Yeomans said. "And even more prevalent today."

Despite the benefits of radar for space observations, the method does have its drawbacks.

It can only monitor the surface features, not their composition. It can't be used to discover new objects they must be found optically, then zoomed in on with radar.

But for objects that most likely won't ever have their own mission, radar can garner spacecraft-quality data. It can chart their paths, which can then be calculated into the future.

If a comet or asteroid is considered likely to crash into Earth, radar can be used for an in-depth study to determine what countermeasures can be taken.

Almost 280 NEOs have been studied by radar, broadening our understanding of them. Radar reveals rotation, speed, shapes, and occasionally turns up surprise asteroid companions.

The snapshots of the early solar system provided by radar help us understand what things were like when our planet was just beginning. That such a method could help Earth avoid a catastrophic ending is an added bonus.