If an alien civilization wanted to study planet Earth, how might they do it? They could use powerful telescopes to measure the physical characteristics of our planet, or they could listen for signals from our TV and radio broadcasts. These are things we are doing in our search for alien civilizations. But a really advanced alien civilization might try something a bit more ambitious, such as an actual mission to Earth. One way to do this would be to build a probe within an asteroid, and send it on a journey across the stars. The asteroid could shelter the probe during it’s long trip through interstellar space. Once it arrived in our solar system, the probe could gather detailed information about Earth and the solar system. It might even try to communicate with humans by beaming a radio signal in Earth’s direction. Such an alien probe would look a lot like the recently discovered asteroid Oumuamua, which is why the Breakthrough Listen project wants to study it.

Oumuamua was discovered in October by the Pan-STARRS 1 telescope. Unlike any other asteroid, Oumuamua has an interstellar orbit. It is moving through our solar system so quickly that could not have originated in our solar system. Based on its trajectory, it came to our solar system from the general direction of the star Vega. Coincidentally, Vega is star aliens first communicated with humans from in Carl Sagan’s novel Contact. In addition to being the first confirmed interstellar object to enter our solar system, it also has a highly unusual cigar shape, with a length about 10 times longer than its width. Add to this the fact that Oumuamua made a relatively close approach to Earth, within 15 million miles of our planet, and it begins to look a bit alien.

Odds are this asteroid is just a chance visitor to our system. We’ve known that some asteroids can escape the solar system through close flybys of large planets like Jupiter, so it makes sense that asteroids from other star systems could travel between the stars. Such interstellar visitors might be rare, but they don’t require aliens to send them on their way. But Breakthrough Listen is interested in finding evidence of alien civilizations, no matter how long the odds. So when Oumuamua was discovered, it made for a promising target.

Oumuamua is currently about 2 astronomical units away from Earth. About twice as far as the Earth is from the Sun. That’s still much closer than many of the probes we’ve sent into space, such as Cassini and New Horizons. If it is an alien probe sending radio transmissions we should be capable of detecting them. So Breakthrough Listen will use the Green Bank Telescope to look for any evidence of alien technology, searching across four radio bands, from 1 to 12 GHz, for a total time of about 10 hours. If an alien probe wants to be detected, that’s a good frequency range to search.

Just to be clear, the odds of Breakthrough Listen finding anything are really slim. Studies so far haven’t found anything that would imply an artificial origin. But even if Breakthrough Listen doesn’t find anything, their observations will add to those we already have, and help us better understand asteroids that are rare, but natural, alien visitors.

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When we send a spacecraft out into the solar system, we have to be able to communicate with it. New instructions have to be transmitted to the spacecraft, and data from its observations need to be received. The signals a spacecraft sends are faint by our standards, and you need a strong directed radio signal for the spacecraft to be able to detect it. To achieve this you need large radio antennas to send and receive messages. Since the Earth rotates, you need a global network to communicate with satellites throughout the solar system. It’s known as the Deep Space Network (DSN).

The locations of the DSN provide full sky coverage.

The Deep Sky Network is a collection of radio antenna dishes located in Madrid (Spain), Canberra (Australia) and California in the U.S. Since they are spaced fairly evenly across the globe, any satellite in the solar system can be in contact with at least one station on Earth. The only time they can’t be contacted is when the Sun is near the line of sight between Earth and the spacecraft, or if the spacecraft is on the far side of a planet. Each station has one large 70-meter dish, and several smaller dishes. While other radio antennas can be used to communicate with spacecraft, the DSN is the primary means of spacecraft communication.

One of the biggest challenges facing DSN is the fact that we have an increasing number of spacecraft in our solar system. Often a spacecraft will remain active long after its original mission timeline, and the number of active missions in 2020 is expected to be double the number of 2005. Some of the most ambitious missions, such as the Pluto flyby of New Horizons also require significant use of the largest dishes. As a result, the Deep Space Network is becoming increasingly taxed. It also faces funding challenges. It’s much easier to generate support for the latest “cool” science mission than it is to get funding for the less-than-sexy support infrastructure for such a mission. Without strong support of DSN, our ability to explore the system may soon be limited not by our ability to launch spacecraft, but by our ability to communicate with them once they’re there.

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Just when you think you’ve got a mystery solved, new data revives the mystery again. It’s a common story in science, and this time its about fast radio bursts. 

Fast radio bursts (FRBs), as you might recall are short, intense, bursts of radio waves. They have indications of being distant in origin, but similar bursts known as perytons were found to be due to local radio noise. Because of their short duration they are difficult to study, or even to verify their origin in space. Recently there was observation of a particularly strong FRB that seemed to be from a distant galaxy. The evidence to support this idea came from two radio telescopes. The first observed the FRB, while the second observed a radio afterglow in the same general region. From theses two observations the source could be triangulated as a distant galaxy.

But new evidence indicates that the radio afterglow isn’t from the FRB, but rather from the material surrounding a supermassive black hole in the distant galaxy, completely independent from the fast radio burst. So the origin of these radio bursts is still a mystery after all.

If that wasn’t bad enough, there’s also evidence that FRBs can repeat. So far these bursts seemed to be one-time events, which would imply they are caused by catastrophic events such as colliding neutron stars or a neutron star collapsing into a black hole. If they repeat that would indicate a transitory origin, such as flares from highly magnetized neutron stars. It’s also possible that FRBs can have multiple causes, with some repeating and some not.

At this point what’s clear is that we don’t have a good understanding of FRBs after all.

Paper: P. K. G. Williams and E. Berger. Cosmological Origin for FRB 150418? Not So Fast. arXiv:1602.08434 [astro-ph.CO] (2016)
Paper: L. G. Spitler, et al. A repeating fast radio burst. Nature doi:10.1038/nature17168 (2016)

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They aren’t local, they aren’t aliens, and they might help us understand dark matter. 

Fast radio bursts (FRBs) are blasts of radio energy that last for only a fraction of a second but are extraordinarily bright. Because of their short duration, and the fact that the same FRB never repeats, they’ve been very difficult to study. They were first discovered in 2007, and for a while they were observed only at the Parkes radio telescope in Australia. Because of this, and the fact that FRBs were so incredibly bright, there was a great deal of speculation that they could be due to local radio interference rather than some new astronomical event. In fact a similar short-burst phenomenon known as perytons were found to be due to stray signals from an unshielded microwave.

Fast radio bursts are somewhat different from perytons, and have two distinct features that imply they are distant in origin. The first is that rather than being a simple burst with a range of frequencies happening at once, the frequencies are spread out, with higher frequencies arriving first and lower ones later. This whistler effect is characteristic of a pulse that has traveled through the interstellar medium. It occurs because when an electromagnetic pulse interacts with charged ions, different frequencies are slowed by different amounts, with the lower frequencies slowed down more. So you get a dispersion effect. Stray bursts or chirps from terrestrial sources generally don’t have the same dispersion because they don’t travel through plasma and they don’t travel far. The second is that the bursts seen by a single radio detector rather than a range of nearby detectors. This implies it comes from a particular point in the sky rather than somewhere near the telescope.

Unfortunately radio telescopes are not good at determining an FRB’s location in the sky. This makes it difficult to determine their cause. This led to a great deal of speculation about their origin, including the idea that they might be due to some alien civilization. Combined with the fact that perytons were caused by a microwave, FRBs began to take on a fringe science status.

But recently the Parkes observatory detected a fast radio burst, then two hours later the Australia Telescope Compact Array in New South Wales saw a fading radio glow in the same region of the sky. Using the two observations to triangulate its position in the sky, a team of astronomers narrowed the source down to an elliptical galaxy 6 billion light years away. To verify this source the team compared the dispersion effect of the FRB signal with the amount of ionized material between us and this particular galaxy as estimated by the WMAP probe. The amount of frequency dispersion agreed with the estimated amount of ionized material, indicating that it did indeed originate from the distant galaxy.

Now that we know they are real astrophysical events, the next step is to confirm their cause. The leading idea is that they are due to neutron stars, either through neutron star collisions or perhaps when a neutron star collapses into a black hole. They might also help us solve the mystery of dark matter. In order to better understand dark matter we need to know exactly how much faint regular matter there is between galaxies. Since the dispersion measure of FRBs gives us an excellent measure of the amount of material between us and a particular distant galaxy, they can be used to help map the distribution of faint matter throughout the universe.

There’s still much to learn about fast radio bursts, but this recent work brings them clearly back out of the fringe.

Paper: E. F. Keane, et al. The host galaxy of a fast radio burst. Nature 530, 453–456 (25 February 2016).

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The Wow! signal is one of the great mysteries of radio astronomy.  It was detected in 1977 by the Big Ear radio telescope, and is so named because of its powerful signal. It’s origin is unclear, but one possible explanation is that is was an intentional signal from an alien intelligence.

Big Ear was a drift telescope, and used the rotation of the Earth to scan the sky for radio signals. It was designed to run for long periods autonomously as a way to scan the heavens. The Wow! signal came from a specific region of the sky, and emitted a strong signal at 21 cm wavelengths, which is an emission light produced by atomic hydrogen. It was observed for 72 seconds, which is how long it would take a specific point in the sky to drift across the range of the telescope.

One interesting aspect of the signal is that it doesn’t clearly originate from a known object. The area of the sky where the signal originated doesn’t have anything that would produce a strong hydrogen line. But new work suggests that back in 1977 there was something there, possibly two somethings.

Between the end of July 27 and mid August of 1977, two comets known as 266P/Christensen and P/2008 Y2 (Gibbs) were in the vicinity of the Wow! signal location. Comets are known to emit gas and dust, including monotomic hydrogen. So there may have been a hydrogen cloud in the region during that time. The Wow! signal was detected on August 15, 1977.

While this is not the definitive answer, it would explain some of the strange aspects about the signal, such as why later observations of the region didn’t detect any signal. Since the comets had moved on, any hydrogen cloud would have dispersed and any signal from it would have faded.

So there’s no need for aliens after all.

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A fast radio burst (FRB) is a short burst of intense radio energy originating from outside our galaxy. We aren’t sure what causes FRBs, though the likely candidate is a white dwarf or neutron star falling into a black hole. They only last a few milliseconds, which makes them a challenge to study, but their brief duration may also allow us to test the limits of general relativity.

If relativity is wrong, then different wavelengths from an FRB should arrive at different times. Credit: Purple Mountain Observatory, Chinese Academy of Sciences

The foundational idea of general relativity is known as the principle of equivalence. On a basic level it states that two objects of different masses should fall at the same rate under the influence of gravity. The principle is necessary to equate the apparent force of gravity with a curvature of spacetime. So far all tests of the equivalence principle have confirmed it to the limits of observation, but there’s an interesting catch. Since relativity also states that there is a connection between mass and energy, the equivalence principle should also hold for two objects of different energy. Specifically, two beams of light with different wavelengths (and therefore different energies) should be affected by gravity in the same way.

We know that the path of light is changed by the curvature of space (an effect known as gravitational lensing), but the curvature also affects the travel time of light from its source to us (known as the Shapiro time delay). According to relativity, the amount of curvature and the time delay shouldn’t depend upon the wavelength of light. This means we can in principle use FRBs to test this idea.

Since FRBs only last milliseconds, they provide a sharp pulse of light at a range of frequencies. If relativity is correct, then the pulse we observe won’t be affected by gravity. If the equivalence principle is wrong, then shorter wavelengths of radio waves from the burst could arrive at a different time than longer wavelengths. We already see different wavelengths arrive at different times due to the interaction between the radio waves and the interstellar plasma in our galaxy, but we know from other observations how much that shift should be. The key is to test whether there is an additional shift not accounted for by standard physics.

Relativity is an extremely well-tested scientific theory, so I wouldn’t count on FRBs showing an energy-based effect, but it’s great that we could have yet another way to test our model. It’s a win-win, since we’ll either confirm our theory yet again, or we’ll discover something new to explore.

Paper: Y. F. Huang & J. J. Geng. Collision between Neutron Stars and Asteroids as a Mechanism for Fast Radio Bursts. arXiv:1512.06519 [astro-ph.HE] arxiv.org/abs/1512.06519

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While general relativity is an extremely well tested theory, we’re always looking for more precise ways to test it further. Either relativity will pass the test yet again or we’ll find evidence of something beyond relativity. But tests of general relativity are hard to come by, particularly for the most subtle effect of the theory, gravitational waves.

The best tests we have of gravitational waves is through binary pulsars. The first indirect evidence came from the Hulse-Taylor system, which consists of a pulsar orbiting a neutron star. Most of the systems we can use to test GR contain a pulsar orbiting another object such as a neutron star or white dwarf. But a system known as J0737-3039 consists of two pulsars. One pulses at a rate of 23 milliseconds, while the other at about 2.8 seconds. Their orbital period is only about 2.4 hours, so they are quite close to each other.

The regular radio pulses of a pulsar allow us to determine their motion with great accuracy, and since we can measure the motion of both pulsars we can test relativity with greater precision. While Hulse-Taylor system confirms relativity to within 0.2% of the theory, J0737-3039 allows for confirmation to within 0.04%. So far, general relativity still wins.

Paper: Michael Kramer. Experimental tests of general relativity in binary systems. 28th Texas Symposium on Relativistic Astrophysics, (2015)

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This is an image of a black hole. Actually, the black hole is in the center and can’t be directly seen. What we see are two large jets streaming away from the black hole. When matter accretes around a black hole, some of it is captured, but some of it is pushed away into long jets. In the case of this image, the jets stream out for about 1.5 million light years. It’s known as Hercules A, and it’s one of the brightest radio galaxies.

The 3C 348 galaxy in the visible range.

The purple lobes in this image show radio emissions, not visible light. It was taken using the Very Large Array (VLA). Since the jets are made of plasma, they are bright at radio wavelengths, but not very bright at optical wavelengths. The supermassive black hole driving Hercules A is at the center of a rather bland elliptical galaxy known as 3C 348. The image above combines an image of the galaxy in the visible spectrum with the radio image of the lobes.

Since the VLA is an array of radio antennas, it can produce detailed radio images. We can see, for example how the jets stream out in a narrow beam at nearly the speed of light, eventually slowing and interacting to create wide turbulent lobes. By studying images such as this we can better understand how high energy plasma interacts in space.

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Different wavelengths of light interact with matter in different ways. Our atmosphere, for example, is largely transparent to visible wavelengths, but absorbs much of the ultraviolet. Dust surrounding the center of our galaxy absorbs visible light, but is more transparent to radio waves. As a result it is useful to observe objects at a range of wavelengths. Sometimes this light is reflected from natural sources or emitted by the objects themselves, but sometimes we actively shine light on an object to get an image. 

A radio map of Venus. Credit: B. Campbell, Smithsonian, et al., NRAO/AUI/NSF, Arecibo

One way to do this is through radio waves. The radio telescope at Arecibo not only detects radio waves, it also has a large transmitter capable of sending radio signals into space. While it was once used to send a message to potential aliens, the transmitter’s main use it to reflect radio signals off solar system bodies. The timing of these signals can be used to measure the distance to planets more precisely, but with high resolution imaging we can also make detailed maps. Recently maps were made of the Moon (seen above) and Venus by beaming a radio signal from Arecibo and observing its reflection with the 100-meter radio telescope at Green Bank.

Reflected radio images such as these are useful because radio waves are not only transparent to the atmospheres of Earth and Venus, but they are also largely transparent to fine dust. For Venus this means we can get a high resolution image of the surface obscured by thick atmosphere. For the Moon this means we can get a map of any surface features that may be obscured by dust on the Moon’s surface.

What’s amazing about these images is how high their resolution is. While they look like regular telescope images, they are actually radio images. The shadows you see are radio shadows from the transmitted beam, similar to the way a rough surface can cast shadows from the beam of a flashlight. The resolution of these images are a good demonstration of just how sophisticated radio astronomy can be.

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In astronomy, we often lament the rise of light pollution. As populations rise and our use of artificial lighting becomes greater, we lose the dark skies of our ancestors. We can see this effect through the increasing difficulty in observing a night sky filled with stars. While we can’t see radio waves, radio astronomy also suffers from an increase of light pollution.

The mobile radio van to detect stray radio signals.

While the radio quiet zone is often portrayed as a region where modern society can’t encroach, but that’s actually not the case. Folks in the region have internet just like everyone else. They can stream Netflix, stalk Facebook and all the rest of modern society just like everyone else. They just don’t have wifi or cell phones. Except for the “always in your pocket” access to the web, things aren’t that different than any rural area in the US. Even then, there are folks who have wifi in their home and such. Though it’s discouraged, the radio quiet zone can’t control what people do in their own home. It only regulates things like radio stations and mobile phone towers.

Most of the efforts within the region is making sure that things like faulty wiring and old transformers don’t fill the air with loud stray radio signals. These can be addressed by proper equipment maintenance and sometimes a bit of shielding. But we also live in a world that is increasingly radio loud. Bluetooth and wifi are now used not only in laptops and cell phones, but in exercise monitors, smart watches and even batteries in smoke alarms. As the “internet of things” increasingly connects objects to each other, keeping the radio quiet zone truly quiet will become an increasing challenge.

All of these radio loud devices pose no harm to us personally, any more than an electric lamp does. They can even make our lives more convenient. But that convenience comes at a cost to radio astronomy. Modern radio telescopes are so sensitive that even snapping a picture with a digital camera near the telescope can flood the detector with radio light. Hence the need for a radio quiet zone.

It’s the only way radio telescopes can have truly dark skies.

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If a typical mobile phone was placed on the surface of Mars, it would be the brightest radio object in the night sky. It’s not that mobile phones are generating massive amounts of radio energy, but rather that astronomical radio objects are extraordinarily faint. That’s why we need super-sensitive radio telescopes, and why some radio telescopes need to be protected against stray radio signals.

One solution is to create a radio quiet zone, where the use of radio and microwave technology is severely limited. In the United States, a National Radio Quiet Zone was established in 1958 to protect radio telescopes at Green Bank observatory in West Virginia. It’s a 34,000 square kilometer region spanning parts of Virginia and West Virginia. In the Zone there’s no cell coverage, no wifi, and limited internet. Near the telescopes themselves even digital cameras are too radio loud to be used.

It’s also where I happen to be heading for the rest of this week. I won’t have access to the web while I’m away, and I don’t have a backlog of posts, so this site will also be entering a kind of radio silence. Posts will start back up again once I’m back on Monday, but until then enjoy the break from my usual broadcast.

See you all when I’m back.

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One of the more powerful techniques of radio astronomy is the use of interferometry to combine the signals of several radio antennas into a single virtual telescope. Through interferometry we can make radio images with resolutions greater than that of the Hubble telescope. But how does interferometry work?

At a basic level, interferometry is simply the combining of signals from two different sources. If the two signals are similar they will combine to make a stronger signal, and if they aren’t they will tend to cancel out. Where this becomes useful for astronomy is that if two signals are out of sync you can shift them (correlate them) so that they are in sync. When the signal is strongest you know they are lined up.

A schematic of an interferometer system based on Thompson et al.

When two radio antennas are aimed in the same direction, they receive the same basic signal, but the signals are out of sync because it takes a bit longer to reach one antenna than the other. That difference depends on the direction of the antennas and their spacing apart from each other. By correlating the two signals, you can determine location of the signal in the sky very precisely. It’s the precision that you need to create a high resolution image.

Two antennas only give you one point in the sky, but with dozens of antennas (such as the array at ALMA) you can get lots of points, one for each paring of antennas. But even that only gets you a discrete set of image points. If the Earth were fixed in relation to the sky, then our radio image would look like a pointillist painting.

But the Earth rotates with respect to the sky, so as time goes by the relative positions of the antennas shift with respect to an astronomical signal. As you make observations over time, the gaps between antennas are filled to create a more solid image. This isn’t easy to do. It takes lots of observations and lots of computing power to combine the images in the right way. At ALMA, for example, it takes a custom built supercomputer that spends all its time correlating signals.

But the results are nothing short of spectacular.

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