Planetary Radio: Space Exploration, Astronomy and Science - Planetary Radio Extra: Shining a Million Watt Flashlight on an Asteroid

Episode Date: June 28, 2016

PlanRad’s celebration of Asteroid Day (June 30th) continues as we call UCLA grad student Adam Greenberg at the Arecibo Observatory in Puerto Rico.Learn more about your ad choices. Visit megaphone.fm/adchoicesSee omnystudio.com/listener for privacy information.See omnystudio.com/listener for privacy information.

Adam, thanks for taking time to talk to us on this Planetary Radio special edition. First of all, tell us where you are right now. Well, Matt, I am staying at Arecibo Observatory, which is in Puerto Rico. I'm staying in the Visiting Scientist Quarters, which is just north of the observatory. And it's basically a hotel in the middle of the rainforest. And it's basically a hotel in the middle of the rainforest. And it's absolutely gorgeous. You basically step outside in the morning and you're surrounded by nature

and then you go and do science. If you ever get a chance to visit, which I'm sure you will, you really must. One of the nice things that they do for you is the VSQ where you stay has a very powerful air conditioning unit. And so if you want to, you can just stay indoors all day at a balmy 15 degrees Celsius and just literally chill out.

Sounds good to me. Let's get this out of the way up front, how incredibly envious I am and how much I want to put a check mark next to Arecibo on my bucket list. Is this your first time there? No. I was here last year observing two objects, 2010 NY65 and 1566 Icarus. And this year I'm back observing 2010 NY65 again. Both times have been absolutely incredible experiences for me. We had Dante Loretta. He's the principal investigator for the OSIRIS-REx asteroid sample return spacecraft. He was on the show just last week,

and he talked about how vital the radar observations of asteroid Bennu, that's his target, have been for his mission. And he's not alone in worrying about losing this tool, at least at Arecibo, as your boss, UCLA professor Jean-Luc Margot, when he suggested that I give you a call, it was obviously out of, partly at least out of his concern, that there are people actively considering the shutdown of that whole facility, radar and everything else that it does. Yeah, radar characterization

of asteroids is hugely important for a variety of reasons. The probably two most fundamental ones are that radar can give you shape information about the surface of an asteroid in a way that other observational methods simply can't. And shape information helps inform a variety of things about asteroids, primarily the potential impact risk. Larger asteroids are obviously a bigger deal. And also shape informs you on how their orbits will develop over time and can even give you information about their past histories. The other reason radar is very important is because it gives you highly accurate information about the asteroids position relative to the Earth.

And this can be used to refine orbit determination. Normally orbit determination is done with mainly optical measurements, and these have relatively large error bars relative to radar, and they also only give you information in the plane of the sky. Radar on the other hand, is highly accurate. It has ranging, radar ranging is spatially accurate on the order of meters. And also, it gives you information orthogonal to the plane of the sky. And this is very important for orbital fitting.

What does that mean, orthogonal? So optical observations give you information within the plane of the sky, and radar gives you information out of the plane of the sky. In other words, in terms of distance from the observer, and also relative velocity to the observer out of the plane of the sky. And these two measurements greatly affect orbit determination. A single radar observation of an object can reduce the uncertainties on orbit determination and where the object will be in future years by an order of magnitude. Wow. All right. So there are other big dishes around the world that do these kinds of radar observations. What would be so bad about shutting down Arecibo?

Well, from a science perspective, Arecibo is the largest single-dish telescope in the world by a factor of over four. The Arecibo dish, main collecting dish, is 305 meters in diameter. The next biggest is 70 meters. That's the Goldstone Observatory. Just given the way that the diameter corresponds to sensitivity, a factor of four difference in diameter corresponds to a factor of 20 increase in sensitivity. So Arecibo is the most sensitive radio telescope in the world by a factor of 20. This in turn corresponds to a radar range of double. If we

get rid of Arecibo just from the scientific side of things, we'll be having our total range of radar capabilities. And this is hugely important for the reasons that I said before. If nothing else, for planetary defense, because if you have your range of radar, that's a much smaller volume that you can sense in space and be able to see those asteroids with radar and accurately refine their orbits. We're not going to see the one with our name on it coming toward us unless we've got the tools to find it. Absolutely. And Arecibo images so many and performs ranging measurements on so many asteroids every year. Removing it from mankind's capabilities is going to be a huge hit to both our science and our ability to potentially protect the planet from a cataclysmic event. Arecibo is also useful for a variety of other reasons. The dish itself is home to a variety of species.

Shutting down the facility could potentially endanger these species. Shoving down the facility could potentially endanger these species. Arecibo, as the institution, performs a variety of jobs for the local community. It brings in tourism. It hires a lot of people. It also performs a lot of public outreach and education for locals and people who come to visit. I've read about some of that, but I had no idea that there was a role in biological diversity in the dish itself. Yeah, absolutely. You have had a pretty good view of this. You were telling me just before we started talking that you've made that somewhat harrowing trip out to the focus high above the dish. Yes, yes. Just yesterday morning, I was able to go up into the dome. Francisco, the director of the facility, gave me permission to go up there. It is a mind-blowing experience. Standing up there really gives you perspective on the kinds of science tools that people can build. Arecibo is, I think, 60 years old, maybe even older. It really stands as a testament to our ability to do science. The dish may be old, but it has some pretty cool electronics. Yeah. From the radar perspective, which is what I know most about, there is an S-band transmitter, which is capable of achieving up to a megawatt in power, thanks to two 500 kilowatt Klystrons. The dish itself is constantly being tuned and refined to make sure that the focus stays constant.

When I went up there, they send you up there with a sort of a tour guide, I guess. One of the mechanical engineers, Jaime, who takes care of the detector and the transmitter. And he was telling me about all of the incredible engineering feats that they perform in order to make sure that the telescope is able to do science. They have advanced techniques for reshaping the inside surface of the dome, the secondary mirror to make sure that it is always as efficient as possible. Throughout the day, the cables that hold up the dome will contract and expand depending on the heat. And they have to take that into account when they're doing their observations. All these things

they take into account. And the end result is that we're able to do incredible science and get incredible radar images with this observatory. Wow. I want you to say another word about the radar technology itself, since there are a lot of us who still think of radar basically as the device that puts little dots on an air traffic controller's screen, how do you use basically the same technology to come up with an image of something so far away? The basic concept of how radar works is you send a thin pulse, a plane wave into space that's in principle very thin. That plane wave will travel through space and wash over the surface of an asteroid. And as it washes over that surface, it will bounce back. Parts of the asteroid that are closer to the observer will get reflections before parts that are farther away.

While the original pulse that you send out may be very tightly constrained in time, once it returns to you, it will be spread out in time. And the characteristics of how it's spread out in time give you dimensional information about the range extent of the object. At the same time, the object is sitting in space and it's spinning. Everything in space, for the most part, spins. And so not only do the reflections get spread out in time, but they also get spread out in frequency, because objects that spin have

different points on their surface that are moving at different speeds, and when light bounces off of those points on the surface that are moving at different speeds, it will come back to us at different frequencies. And so you can actually analyze this spread in frequency, and that gives you information along an orthogonal axis in frequency space. And so the end result is you have information spread out in the range dimension, and you also have information spread out in the frequency dimension. And those two dimensions constitute a radar image. And those two dimensions constitute a radar image.

How tough is it to turn this stream of radio waves coming back into an image? There is a lot of math that goes into it. I thought so. I simplified things a little bit. In principle, if you could send out a very thin pulse of energy at an object, then you could perform the analysis that I just described. In actuality, you have power restrictions. In order to actually be able to see the reflected signal, you need to send out a lot of energy all at once. Because of the fact that radar reflectance drops as r to the fourth, in other words, the distance to the fourth power, this is a very steep drop off, you need to send out a lot of energy in order to be able to get a detectable signal.

But at the same time, you also need to send out that energy very quickly, because you want your wave to be much thinner in the range dimension than the object is. Otherwise you won't have any resolution. Turns out that that's just not possible. The power requirements to do that are completely unfeasible. Luckily, there is a very clever method called range compression, which allows you to effectively get the amount of energy that you would have in a very long radio pulse, very long in time, that is,

and at the same time get the spatial resolution that you would have from a very short radio pulse. The basic idea is you encode a binary signal into the signal that you send out and you take the signal that comes back and you correlate the signal that comes back with the original signal that you sent out and the result of that correlation gives you information on the range of the asteroid. And then you do this multiple times in a row, do a Fourier transform on the resulting range information, and that will give you the frequency information.

So the range information is the y-axis of a typical radar image that you might see, and the frequency information is the x-axis. And those two things combined give you enough information to get an idea of the shape and the size and the rotation rate of the object. Sounds like a pretty good trick. Let me go back to that art of the fourth. We talk about the inverse square law periodically on this show. I assume that this is because the signal's got to go there and come back. Yes, absolutely. The way you can think about it is when the signal is sent out, it represents a shell of radius r that is spreading out from the Earth.

The radius of that shell is increasing like r squared. And then this shell will intersect the object, and a small section of that shell will actually hit the object and bounce off of the object. That represents a new shell in space that needs to expand back out to us. So the first shell has power in a specific area dropping like one over r squared and then that specific area hits the object and then that area needs to reflect back to us and that area drops like one over r squared until it finally gets back to your receiver. And so the result is one over R squared squared, which is one over R to the fourth.

How about other challenges? I mean, one that occurred to me is, is it a problem that you have to deal with? Light doesn't travel instantaneously to a different place. It's got to get there and back. Is that a problem? Because I'm assuming that once the signal comes back to you, you know, the dish has moved on with the rest of the Earth. Yes, these are all calculations that need to be done before you are able to do radar imaging. In fact, one of the complications of any sort of radar analysis is you already need

to have a basic idea of where the object is and where it's going before you can do any sort of radar analysis is you already need to have a basic idea of where the object is and where it's going before you can do any sort of radar analysis on it. The process of doing this is called generating an ephemeris. It means that you basically never do discovery with radar. You only do follow-up and orbit refinement. But as I mentioned before, the process of doing this refinement is well worth the extra work because of the great improvement it gives to our knowledge. Yeah, again, this is something Dante Loretta talked about because we know so much about Bennu. It just, in some ways, made his mission possible, or at least a great deal more practical. And we talk to amateur astronomers now and then who mostly do the follow-up, doing the same kind of work on a small scale that you're doing at Arecibo.

Professor Margot also mentioned there is a database of radar-observed asteroids. Is this something that UCLA is leading, or is this more of an internationally supported tool? There are a few databases that exist in the world. The one that Professor Margot is working on is, at least in my opinion, one of the more advanced ones. What he is doing is, and this is hosted at UCLA, this is entirely under the auspices of Jean-Luc Margot, is creating a database that can automatically collate all of the radar information that we have on near-Earth objects. This database can be found at RadarAstronomy.org.

The long-term goals of the database is to automatically take in any new radar information that we have of objects, automatically take in the new radar information that we have of objects, automatically take in the radar measurements, and then possibly in the long term, use those measurements to automatically generate shape information and have this be easily accessible to anybody, anywhere in the world, especially scientists and students, who can then use these data to do even better science. And we will put that URL up on the page at planetary.org slash radio, where people will

be able to see all sorts of other links and information about this work that's underway. I read that some of this work that you are doing is going to also help others refine our understanding of general relativity. How is that working? The basic idea of the experiment is that it is in principle possible to probe the near gravitational field of the sun by looking at the orbits of objects that go relatively close to the sun, look at how those orbits change over time. And how those orbits change over time, if they get close to the sun, is affected by how space is curved near the sun. And near the sun, thanks to general relativity, the curvature of space is complicated. And the basic idea is that you can,

To say the least. and see how you can tweak those parameters to make the orbits that you fit match the measurements that you took. And this takes many measurements over long periods of time, and it requires radar measurements. If it's done correctly, which we are trying to do at UCLA, then you can actually put constraints on general relativistic parameters in ways that have not been done before. Adam, you're a grad student, a fairly fortunate one. I bet you feel that way. Absolutely. Do you hope to keep doing this kind of work? Yeah. I have enjoyed the work that I've been doing. When I first started, I was not anywhere near planetary science.

I used to study white dwarfs and, after that, cosmology. I sort of drifted into planetary science thanks to some talks I had with Jean-Luc. I've really been enjoying the work that I've been doing. I hope to either continue with a postdoc or possibly work for one of the new emerging companies that will be doing asteroid-specific mining. Oh, yes. We know some of those guys. We'll put in a good word for you. Thank you. Thank you, Adam. This has been a blast. What's on the agenda for today? Are you doing some

observations? Well, actually, today is a maintenance day for the observatory. And so I will be driving around the island and visiting the sites. I think I might drive to Old San Juan and check out some of the forts that overlook the ocean. All right. We're just going back to making me envious, so I think it's time to bring this to a close. Thank you so much for what you're doing and for taking a few minutes to share this information about the value of radar observations of asteroids from Arecibo. And best of luck with that work. Thank you, Matt. And by the way, I have to say that I love your podcast. It's one of the things that got me through some

of the long drives that I've done on this island. That means a lot to me. Thank you very much, Adam. I really appreciate that. We've been talking to Adam Greenberg. He's a graduate student at UCLA. His boss is Jean-Luc Margot. They are together with the rest of Jean-Luc's team conducting these observations of asteroids that reveal information, data about these asteroids that really would be impossible to get, at least to this degree of accuracy and precision, in any other way. If you're concerned about that and losing a tool like Arecibo, well, you might want to drop a line to the National Science Foundation. That is it for this special edition of Planetary Radio Extra. Of course, we hope that you'll listen to the regular show.

It comes out every Tuesday morning at planetary.org slash radio, on SoundCloud, in iTunes, and all kinds of other places online. And you can catch us on about 150 radio stations as well. This is Matt Kaplan. Thanks very much for listening.