There are general-purpose telescopes and missions that astronomers can use to study specific objects. And there are survey missions that look at the entire sky, which astronomers can use to answer questions about the Universe. We’ve talked about surveys in the past, but the landscape is changing fast so it’s time for an update.
PODCAST: Ep. 118: Sky Surveys (Astronomy Cast)
The VLA FIRST Survey (Space Telescope Science Institute)
The NRAO VLA Sky Survey (NRAO)
The Drift-Scan Technique (Astrosurf)
Stellar Parallax (Las Cumbres Observatory)
Proper motion (Swinburne University)
The Gaia Sausage: The Major Collision that Changed the Milky Way Galaxy (Carnegie Mellon University)
CASTLES Survey (Center for Astrophysics | Harvard & Smithsonian)
Gravitational Lensing (Hubblesite)
Einstein Ring Spotted By Hubble (NASA Goddard)
GOODS: The Great Observatories Origins Deep Survey (Space Telescope Science Institute)
Spitzer Space Telescope (Caltech)
Chandra X-ray Observatory (NASA)
Is Dark Energy Bad for Astronomy? (Sky & Telescope)
MeerKAT radio telescope (SARAO)
LIGO Laboratory (Caltech)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, Episode 640: Science surveys, news projects and results. Welcome to Astronomy Cast, our weekly fact-based journey through the cosmos, where we help you understand not only what we know but how we know what we know. I’m Fraser Cain, publisher of Universe Today. I’ve been a space and astronomy journalist for over 20 years. With me is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela. How are you doing?
Dr. Gay: I am doing well. I feel like I need to start saying, “I’ve been a podcaster for 15 years,” or something.
Fraser: Sure. Yeah. Yeah, you have.
Dr. Gay: Yeah. I don’t know.
Dr. Gay: Yeah. I have. Well, 2005. So, 17 years. Yeah.
Fraser: There you go. Yeah. I’d only been doing Universe Today for five years when we started up the podcast – when we started up Astronomy Cast. And now I can say, “Oh, I’ve been a science journalist for 23 years.” Yeah. It’s pretty funny.
All right. So, there are general purpose telescopes and missions that astronomers can use to study specific objects. And there are the survey missions that look at the entire sky which astronomers can use to answer questions about the universe. We’ve talked about surveys in the past, but the landscape is changing quickly. So, it’s time for an update.
When we thought about surveys – and I haven’t looked, but it was several hundred episodes ago when we talked about surveys.
Dr. Gay: Yeah.
Fraser: And I’ve always been a gigantic fan of survey missions in general because it allows you to go after the fact, go back through all of this data, and try to answer specific questions. But at the time there weren’t a lot. There was the Sloan Digital Sky Survey.
Dr. Gay: There was first an NVSS on the radio side of things.
Fraser: Right. Yeah. And not a lot and not at high detail and not at different wavelengths. It’s a totally different world now, even today, not to mention what’s coming soon. So, let’s sort of come back around and talk about sort of some really exciting survey missions that have launched or telescopes that have been deployed and then what comes next.
Dr. Gay: So, I think we have to start by acknowledging the majesty that is Gaia. I’ve said it before, I’m gonna say it again, I will keep saying it until I die, Gaia –
Fraser: All hail Gaia.
Dr. Gay: Yeah. Yeah. It is one of the best designed missions I have ever seen in terms of they are using what’s called drift scanning so that this funky telescope is rotating so that the stars drift across – and galaxies and other objects – drift across the detector at the same rate that they’re reading out the detector. And this allows them to basically move the stars from one detector to the next detector to the next detector, and they’re doing different preprocessing between each of these different things, so they know which data to focus on.
The only flaw in the system is it is taking data at a significantly higher rate than it can send it back to earth. So, it has to use onboard algorithms to sort out just what the important stuff is that we get to get our curious little hands on once the data gets sent back.
Fraser: And this is like your classic example of a survey mission.
Dr. Gay: Yes.
Fraser: This space telescope is serving one-and-a-half-plus billion stars, more than a percentage of the entire Milky Way galaxy, detecting the position, location, distance, movement of these various – of various versions. Some of these stars it’s detecting all of that data, and in some cases, it’s only doing some in terms of motion and so on. So, a fraction of it. But in general, it is surveying hundreds of millions of stars for all of that really interesting information.
And what have we gotten out of Gaia that’s changed astronomy?
Dr. Gay: Well, what it’s able to do, and it is the first mission to be able to do this, is it can look at a field of stars and see how one of the stars gradually moves compared to the others over time, and against background quasars, which are totally not going to move. And this allows it to measure the movement of stars in the plain of the sky that is triggered both by proper motion, so the actual motion of the objects, and by parallax, which is the motion that is apparent due to the earth’s motion.
You can figure out what parallax is by holding up your thumb, blocking out an object with one eye, and then switching and seeing how the thing you blocked out switches to the other eye.
Fraser: Wait. Wait. You know what? Everyone just stop. Everybody, I know you’re listening to this as a podcast, take your arm, hold it at arm’s length, stick up your thumb, and then go back and forth with your eyes, opening one, then the other, and watch how your thumb jumps back and forth from the background.
Dr. Gay: Yeah.
Fraser: That’s what it’s doing.
Dr. Gay: Yes.
Fraser: Seriously, if you’re not gonna do this right now, why are you listening to Astronomy Cast. Do this right now. No matter how hilarious you look. If you’re on a bus, you’re in the office, you’re taking care of your kids, I don’t care. I want you to practice the parallax method right now.
Dr. Gay: And once you do it with your thumb all the way out, bring your thumb closer and repeat it with your thumb closer and notice how much more that thing you’re blocking out appears to move when your thumb is closer.
Dr. Gay: And that change in the angle that things appear to jump allows us to calculate how far away stars are. So, it’s getting us the absolute distance, the proper motion in the plain of the sky.
Fraser: If you imagine that this swirling – the Milky Way as a swirling vortex of stars going around with various stars going faster than others and they’re interacting with their gravity and some are shifting farther out into the galaxy and some are moving closer in. Gaia is teasing out all that information all together and giving us this map of our surrounding.
Dr. Gay: And that’s not all. I feel like we should do an infomercial for this satellite.
Fraser: Yeah. Exactly. Now how much would you pay? Yeah.
Dr. Gay: So, once they’ve gotten all of this detail about its position in the plain of the sky, they also very carefully measure its colors. So, this allows you to start to get information on what – if it’s a star, what stage in its evolution is it? It can get you some very basic compositional information. And then there’s still more.
Fraser: There’s still more.
Dr. Gay: Because they can do spectroscopy with this, which will get you the in and out of the plain of the sky motion of the object by looking at the red shifts.
So, you have the motion in the plain of the sky, you have the motion in the in and out of the sky direction. That gives you the full three-dimensional vectorial motion of an object. You have color information that allows you to start to get at the most rudimentary – what stage of evolution and what is the composition of this object, that also you can get from the spectra. And this allows us to start to find families of stars that our galaxy ate all at once that form amazing tidal streams. And my favorite discovery so far is pretty much anything related to the Gaia-Enceladus-Sausage galaxy.
Fraser: Yeah. Which is one of the most recent galaxies that the Milky Way consumed.
Dr. Gay: It’s not one of the most recent, it’s one of the largest.
Fraser: Right. But you can roll back the history of the Milky Way. You can unravel the Milky Way’s history just by seeing how these stars are moving and how they all came together. And you can see clumps of stars that are moving together because they were once a star cluster or were the remnants of some dwarf galaxy. And if you act now, we’ll throw in tens of thousands of exoplanets.
Dr. Gay: Yeah. They can actually see the motions in the plain of the sky caused by exoplanets. We’ve only previously really been able to do this with the in and out motion that we can see through Doppler shifting. They can see the planets that are going around in a direction that doesn’t cause Doppler shifting that we can see. It’s amazing.
Fraser: Yeah. They’re making little circles. They’re making little, tiny circles in the sky because their planets are yanking them around from our perspective.
And I think Gaia is just the greatest example that we have right now of the survey mission because – imagine you’re an astronomer and you wanna ask a question. You’re like, “I wonder how many white dwarfs are within 1,000 lightyears of us and are moving to the left in the sky.” You just dig up the Gaia data, you filter it for white dwarfs, you pull out all the white dwarfs that are within the distance that you’re looking for, you measure their proper motion. Imagine if you had to do it the other way where you look at a star and you go, “Is that star a white dwarf?”
Dr. Gay: Oh, I know.
Fraser: “No. Okay. What about this star? Oh, yeah. That is a white dwarf. Okay. Great. How far away is it? Oh, it’s too far. Okay. Nevermind. Next star.” It would be your whole life.
But in this case, you just run a database filter, pull the data out, dump it out, start analyzing it because instead of you analyzing a single object, you’ve got all the objects simultaneously analyzed for you to then sift through that data.
Dr. Gay: And it starts to become surveys all the way down where you can use surveys like the Sloan Digital Sky Survey, which has been around as long as we’ve been around doing this show, the Gaia Survey. All these other surveys you can start to use them to do database queries to say, “I want all the galaxy clusters.” You’re not gonna do that one with Gaia, but you can do that one with Sloan.
And this is leading to another kinda survey where you’re not necessarily using a purpose-built telescope, but you are getting extraordinarily large amounts of time on telescopes to compete for how their time gets used. And this is where you start to see things like the CASTLES Survey. The CASTLES Survey is out there. They have found a number of large galaxy clusters using other surveys. And now with CASTLES, they’re going through taking extremely deep images of cluster after cluster after cluster looking for gravitationally lensed background objects. And this is how we’re suddenly starting to see these papers, they’re like, “The new farthest star. The new farthest galaxy.”
Well, it turns out that sociologically speaking, as soon as you say the furthest, largest, smallest, tiniest, nearest, any of those cool adjectives, the Top 10, people are more likely to click on it. But scientifically, when we can find these furthest things, it allows us – because light takes time to travel, it allows us to look back in time and start to see, “Well, what were the stars acting like? How quickly do galaxies form in different ways?”
And each of these amazing, different galaxies where they’re finding these lenses, it’s a new chance to look back in time and do all the science that we’ve been waiting for JWST to use. And we still can’t use JWST for science. So, we’re gonna figure out how to do it other ways. We are.
Fraser: So, same kinda thing. Show me all the database – show me all the clusters that are doing – that are gravitational lensing. Show me all the Einstein rings. Right?
Dr. Gay: Yeah.
Fraser: It’s crazy.
Dr. Gay: And there’s lots of these different surveys where folks are figuring out, “Okay. We’re gonna combine things in different ways.” And one of the first of the big surveys to do this was the GOODS Survey. And I always have to look at this acronym, because it’s really cool, but I forget it every time.
This is the Great Observatories Origins Deep Survey. And it’s a survey that’s now complete, but they used the Hubble, Spitzer, and Chandra together to look at a series of galaxies so that we could understand across a large swath of temperatures because each of these different wavelengths reflects different temperature phenomena. What’s going on with galaxy formation, star formation, the flow of material into angrily eating black holes. All black holes are angry. That’s my headcanon and I’m sticking to it.
Fraser: Sure. Yeah. Yeah.
Dr. Gay: And so, GOODS was out there getting dedicated time using some ground-based telescopes to pull it all together because you need that wide field image, and here they were using a lot of the Kitt Peak observatory telescopes, then zoom in on stuff in detail with the great observatories.
And quite often when you see these image releases that are like, “And the funky colored blobs are from Chandra. And the purply-pinky details are from Spitzer. And the stuff that looks like what you expect the universe to look like is from Hubble.” That’s often from the GOODS Survey where they used specialized time on these general use telescopes to study one specific problem in a great amount of detail.
Fraser: These was a press release that just came out today, I don’t know if you even saw it? I haven’t even had a chance to really deeply read it.
Dr. Gay: I haven’t yet.
Fraser: But astronomers that use the Hubble Space Telescope and released survey data on 25 Hot Jupiters. They’ve characterized 25 different Hot Jupiters and provide all the information that astronomers might need on those 25 planets. And that is just the beginning. There are some missions, I know I’m not allowed to talk about them, but there are some missions coming that we’ll talk about when they launch that are gonna be characterizing exoplanet atmospheres. All of them, right?
Dr. Gay: All of them.
Dr. Gay: That we know of.
Fraser: That we know of. Of course. And then you’ll be able to do the same thing. “Show me all the planets that have a rich carbon dioxide atmosphere that are within the habitable zone of a G-star. Filter the database.”
Dr. Gay: What I really love about how this is happening is – once upon a time there was the Simon White Rocky Colb debate where they argued over whether or not it is better for the profession of astronomy to have purpose-built systems like WMAP that just solve one fundamental suite of problems involving the cosmic background radiation, or is it better to have general observatories, like Hubble, that you build with the flexibility to take on new problems over time.
And what we’re finding is you need the purpose-built systems, like Gaia. You do not get more specialized of a system than Gaia. And then you make these families of discoveries, and you go to the flexible creatively designed systems like the Hubbles, the Spitzers, the Chandras, and you say, “Okay. We now have 30 of this specific kind of thing that was found. And now let’s get the time to follow up.”
And then you’re always gonna find that one freak in the system, that one galaxy that makes zero sense, and then you just sit on it forever. And it’s not really a survey, you’re just looking at one object, but sometimes you can learn amazing things. And so, we need every kind of telescope to truly understand our universe.
Fraser: Have you got anymore interesting new survey missions to think about?
Dr. Gay: So, what I’m loving is watching all the southern hemisphere telescopes with their enormously high-resolution spectrographs. So, we have HARPS is the old one, ESPRESSO is the new imaging system coming on, and they’re finding planets, they’re following up on planets, and they’re showing that we can do a lot of stuff from the ground that I would’ve said 10 years ago we could only do from space.
And it’s really amazing to think we are now at the point with finding and following up on planets where we were finding and following up on quasars 30 years ago. And if we are able to progress consistently, hopefully within our lifetimes we’ll be to the point where it’s no big deal to point a telescope at a planet and say, “Yeah. This one probably has continents and clouds based on how the light is reflecting off of it.” Just these are the things I dream of, what we’re going to learn about planets, which when we were kids, we didn’t know we’d be able to do someday.
Fraser: Yeah. And I’m not saying there’s gonna be a planet called Ariel launching in 2028 whose only job is to characterize exoplanet atmospheres, but if such a mission was to exist, that’s what it would do.
But there was a recent release again from ALMA, maybe it was about a year ago, where they showed you 25 protoplanetary systems and just all lined up in this nice little grid. And so, you could just like visually go like, “Oh, so that’s what 25 newly forming planetary systems look like. Interesting. Some look like little spirals and others look like record players.”
Dr. Gay: MeerKAT and ALMA, they’re showing us the earliest stages in planetary formation. And MeerKAT is a pathfinder telescope or pathfinder array. It’s defining the technology that the square kilometer array is going to be using. And while I am always reticent to talk about things that have to be launched into space, before they are successfully functioning in space…
Fraser: Oh, interesting. You’re okay with talking about stuff built on the ground.
Dr. Gay: Ground-based. Yes.
Fraser: That can be troubleshot.
Dr. Gay: Troubleshot. And unless it’s the TMT, they pretty much can rescue anything. And so, we have – hopefully by the end of the year, Vera Rubin Observatory with their LSST, which is a survey abbreviation, they’re going to start surveying the sky. In optical, we have the square kilometer array is going to be coming online in the next decade. And it’s going to be working in all the wavelengths of the radio, going longer than we’re used to thinking.
Fraser: But I just wanna stop for one second. You mentioned Vera Rubin, and I’m glad you brought it up because now I’m just gonna go whole-hog now. It’s an eight-meter class telescope. The same-ish size telescope. I mean the Kecks are 10, but the Subaru telescope, the Gemini telescope, each of the elements of the very large telescope, essentially one of the biggest telescopes in the world, that is going to be observing the entire night sky at an incredible depth and resolution every few days from the southern hemisphere. The northern hemisphere is out of luck, but the southern hemisphere will get it.
You’re gonna get this just – like it’s just gonna take all of the survey knowledge that we have to this date and just provide, by orders of magnitude, fainter objects, more changes, to the point that you’re watching every single supernova that appears in the sky, the locations of all the asteroids as they move, the comets, the location of Kuiper belt objects, Planet Nine, you name it, that we will have this snapshot existing night after night after night. This has never happened before.
Dr. Gay: No.
Fraser: And it’s gonna change everything.
Dr. Gay: And the data pipelines that they’re trying to build for this – because we do not have nearly enough humans, bandwidth…
Fraser: Hard drives.
Dr. Gay: Yeah. To deal with the flood of data that’s going to come off of this telescope. And saying that it’s an eight-meter class telescope fails to acknowledge that they are building some of the most sensitive camera systems ever constructed. This is not your grandma’s eight-meter telescope. This is the best hardware attached to it. It’s like going from trying to take pictures on your iPhone to putting a really good DSLR, that E50 from Canon, if I got the right letters. Don’t at me. It’s a significant camera improvement as well. And it’s that improvement in the cameras that is really going to allow them to do the kinds of science that hasn’t been done.
Fraser: So, the camera on Vera Rubin is 3,200 megapixels.
Dr. Gay: Yeah. Yeah. Yeah.
Fraser: Right? 32?
Dr. Gay: Yeah.
Fraser: Like my DSLR, I’ve got a good DSLR, it’s in the 20-megapixel range. 3,200-megapixel camera. We’re just sort of nearing the end the of the show, do you think that the survey telescope will become the dominant form of observatory moving into the future?
Dr. Gay: No. No. I think we’re always going to need to have the single-purpose observatories. Because a survey telescope like LLST, the Vera Rubin Observatory, it can’t sit on a variable star all night long because it’s gonna get the whole sky done. There’s all these different time series things where you need to plop yourself down in one place and watch. There’s also targets of opportunity.
So, the next time LIGO and Virgo spot what is probably two neutron stars colliding, we need to be able to get on target and follow that. And that again requires a discretionary use of a telescope, which you can’t do with a survey telescope.
So, because astronomy is the science of things that vary with time, come and go as they please, and require you to sometimes just sit on them for days at a time, we need the freedom to say, “Right now this telescope is dedicated to this thing. And this other telescope is looking for all of the things we need to dedicate this telescope to.”
Fraser: But survey missions have earned a prominent seat at the astronomy table.
Dr. Gay: They have. It’s not an either/or, it’s an and. We require both to understand the universe.
Fraser: Amazing. Well, thank you, Pamela.
Dr. Gay: Thank you. And thank you to everyone out there following us on Patreon. I know times are tough and we’re starting to see it and people dropping out of our Patreon as inflation goes up. To those of you still here, thank you. You keep us going. You allow us to pay Beth, Nancy, Rich, Allie, our whole team that keeps us going.
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Kim Barron, Vitaly, Paul Esposito, Arthur Latz-Hall, Frank Stuart, Ganesh Swaminathan, Bob Zatzke, Karthik Venkatraman, Michelle Cullen, Moose and Deer, Ruben McCarthy, Geoff MacDonald, Iggy Hammick, Leigh Harborne, Rebekkah and Buzz Parsec. Thank you all so much for being part of our Patreon community, even in these inflationary times. And if you have it in you to join us, join us at patreon.com/astronomycast.
Fraser: Awesome. And we’ll see you all next week. Thanks everyone.
Dr. Gay: See you. Bye-bye.
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