December 3, 2019


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Transcript:

[0:01] Narrator: In regions of the world without light pollution, the nights are enriched with a vast field of stars. They twinkle above in a seemingly infinite multitude.

(music)
“Twinkle, twinkle, little star
How I wonder what you are
Up above the world so high
Like a diamond in the sky…”

[0:36] Narrator: Amateur and professional astronomers spend their evenings watching the stars and galaxies, as well as planets and asteroids that look like stars because they reflect sunlight. When small asteroid grains travel through our atmosphere, they light up into Nature’s fireworks, bringing out droves of new stargazers to enjoy the spectacle of a meteor shower.

But even with so many eyes on the sky, sometimes we’re taken by surprise.

(music changes)

Just this July, an unexpected visitor knocked on our door. At its closest approach to us, an asteroid named “2019 OK” was sixty-five-thousand kilometers away – less than one-fifth the distance to the Moon. This was the largest asteroid to come this close since astronomers have been keeping track of near-Earth objects.

The asteroid is between 60 and 130 meters across – roughly the size of a football field. Scientists say that if it had hit us, it would’ve released over 30 times the energy of an atomic bomb. But we only knew about asteroid 2019 OK five hours before it cruised by. How did an asteroid this size almost slip past us?

It turned out to have an uncanny ability to exploit several limitations in asteroid detection.

First, it came toward us on a straighter path than most asteroids do. So for a few years, instead of looking like a typical asteroid traveling across the sky, it seemed to stay fixed in place, like a faint star that was growing brighter, like a supernova.

When it got close enough that we could see it moving more like an asteroid, the full Moon blocked our view for several days. Trying to watch for it then would be like looking for a firefly flitting behind an enormous spotlight.

After the Moon moved on in its slow arc across the sky, poor weather prevented two asteroid hunting teams in Hawaii from seeing the asteroid. And Catalina Sky Survey, the prolific asteroid search group described in episode one, had completely shut down for monsoon season, when the skies in Arizona are too cloudy for star gazing.

By the time the weather cleared, the Hawaiian asteroid seekers had moved on to observe different parts of the sky. Only a small, amateur-run observatory in Brazil was lucky enough to be looking in the right patch of clear sky, at the right time, to see the stealthy space rock streak past us.

Space is unimaginably vast, and our planet is small. Asteroids are even tinier. Most telescopes can’t spot asteroids until they’re close enough for us to see the sunlight reflecting off them.

In this way, searching for asteroids can be like looking for missing keys under a streetlamp. The keys may not have fallen there, but the night is so dark, and that’s where the light is.

(music)
“Twinkle twinkle, little star.”

(intro music)

[4:10] Narrator: Welcome to “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory. I’m Leslie Mullen, and this is season 2, episode 9: Hunting in the Dark for Monsters.

(music)

Vishnu Reddy, a professor at the University of Arizona in Tucson, is part of the planetary defense team that keeps track of near-Earth asteroids. In addition to studying what the various asteroids are like, he takes part in the planetary defense exercises that were featured in episode three.

Vishnu Reddy: NASA traditionally does tabletop exercises. So these are exercises where they get key federal agencies, like FEMA and others, and they go into a conference room, and they play as if there's an asteroid headed in a certain way, and it's going to impact in a certain location. How do you move people out of harm's way? So there's no real asteroid, it's a make-believe thing.

But I said, "What about we do a real exercise? Not that we're going to track an asteroid directly to impact the Earth, but we take a real asteroid and try and figure out a way to track it, and test whether we all, as scientists, play along well, since we're civilians, we're not military." NASA wanted somebody who can herd the cats, so to speak, you know, all the scientists together, to run this exercise. Since I had proposed the exercise and I was holding the mic, I was told it would be me.

[5:33] Narrator: The 20-meter asteroid 2012 TC4 seemed to be a good fit for the 2017 planetary defense exercise.

Vishnu Reddy: We had discovered it in 2012, but it was lost. We knew roughly where it would reappear in the fall of 2017, but we didn't know exactly. So it was kind of like rediscovering that asteroid.

We said, "Let's try and track this asteroid as if it was an impactor." Because based on what we knew from the previous orbit in 2012, it would come as close as a few thousand kilometers to the Earth's surface. So we thought, "This is a fantastic idea to try and track it, and exercise the whole system. How quickly can we pass on actionable information to the people doing impact modeling?"

It's like, okay. We found the asteroid is made of this material. And that would imply certain ground effects from the asteroid if were to hit the Earth. Same thing with size, if we found out, "Oh, the asteroid is actually 10 meters, or 20 meters." So it would hurt less people if it's a 10-meter object, or it will burn up harmlessly in the atmosphere. So that kind of information. Also, the communication aspect of it. How do you communicate with the public? How do you communicate with NASA leadership, and the White House?

[6:48] Narrator: Vishnu had to coordinate the observation efforts of astronomers from all around the world.

Vishnu Reddy: We ended up with something like 70 scientists, from 14 or 15 countries. It was just fascinating to watch this whole thing come together. Human beings, anytime you put a lot of them together it's challenging to push them towards the common goal, because people want to have their opinions heard. So I was pleasantly surprised that we were not as bad as I thought we would be.

[7:15] Narrator: The exercise didn’t go completely according to plan, though.

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Vishnu Reddy: We have a couple of radar facilities. These are basically giant radio telescopes or radar antennas that you can bounce microwaves off the asteroid to find out its shape, its precise location, to some extent its rotation period, and things like that. A big facility is in Puerto Rico. It's called the Arecibo Observatory, it's this giant dish in limestone set into the ground.

Basically, we had a huge hurricane, Hurricane Maria, go through Puerto Rico just before the flyby of this asteroid, and so it knocked that facility out. But luckily, we had Goldstone radar in California that was also part of the exercise. So they were able to carry out the observations for us.

As much as we humans prepare, we can't predict acts of God, so we have to have backups. Also, I was teaching a class at the time, so I took my entire class to the NASA IRTF on Mauna Kea in Hawaii to observe, and all my students were there. When we were observing the asteroid, the whole mountain lost power. Then the power came back on, but we couldn't figure out which breaker to turn on to get everything back up. So we lost that night.

[8:26] Narrator: For this year’s planetary defense exercise, astronomers decided to observe asteroid 1999 KW4. This asteroid is a binary – it has a tiny moon. Again, unexpected events made observing the asteroid a challenge.

Vishnu Reddy: Strangely enough, Goldstone radar had technical problems, so the Arecibo radar stepped in and did pretty much all the observations.

None of this stuff is easy. Technically it's challenging. So for example, this time when we were trying to track 1999 KW4, we had an incident at Arecibo. Somebody had turned on a WiFi repeater or installed a new WiFi system at somebody's house, and that was causing some weird noise that was affecting our observations.

We have to deal with these things. That's why you end up having backup systems. We have to have backups for telescopes, simply because we don't have a backup for Earth. We have one planet. If we're going to get hit, you better know what's coming at you. You better be prepared for it.

[9:28] Narrator: Vishnu is used to overcoming enormous challenges and obstacles when it comes to observing asteroids. The story of how he arrived at the University of Arizona is like something straight out of a Hollywood movie.

Vishnu Reddy: I grew up in a small village in Southern India by the ocean, not too far from what is India's version of Cape Canaveral. We launch our rockets from this location. The village is called Sriharikota. My early childhood in the '80s was kind of like what people would have experienced in the early '60s in the U.S. if they lived close to Cape Canaveral. A lot of rockets taking off, most of them not making it to orbit. So that created some spectacular fireworks. So that's always fascinated me. I wanted to do something with space.

But we didn't have the best schools, so I had to make the best of what I had. Most of my early childhood, we didn't have power, especially at night. So all we could do was look at the sky. That was pretty much the only form of entertainment, beyond you can always read a book using candles. So my introduction to astronomy happened simply because I had nothing else to do except look at the stars.

[10:39] Narrator: Vishnu’s father was a physician, but also a writer and actor in Indian movies. At first, Vishnu followed in his father’s footsteps by studying filmmaking in college. But he also continued his hobby of looking at the stars through a home-made telescope.

Vishnu Reddy: Even though I was going to film school, I was spending a lot of time in the physics department of another university in the same town because I liked astronomy so much. A lot of people thought I was a student there when I was not.

And then I realized that I need a more stable, permanent position. The movie business did not offer that. My family had a few people who were journalists, and so I ended up getting a job as a newspaper person. Then I moved to New Delhi, and I was working as a newspaperman.

Usually, your day starts around noon and then ends at two in the morning. Basically, I used to go home after two o'clock in the morning, and then observe for a few hours, and then take a nap, and then get back to work again. We had to work six days a week.

While I was a journalist, I wrote a lot about science and astronomy because I was always fascinated by it. That was a really good experience for me because there was also an astronomy club in New Delhi at the planetarium and I had a lot of interaction with the people there.

[11:52] Narrator: One day, Tom Gehrels, a professor from the University of Arizona and founder of an asteroid survey called “Spacewatch,” came to New Delhi to give a lecture at the planetarium.

(music)

Vishnu Reddy: I wanted to go and listen to his talk, but I had to work that day. So I went and made a request to my editor that I would cover this story about Tom Gehrels. And that was actually a Sunday, which the government is shut down, so it's kind of a slow news day. So he said, "Fine. I'll let you go, but you should get me a good story. Like, probably put it on page one."

I took the public transport bus and I went to this talk. And Tom was talking about religions, more in a metaphysical sense. How, when you go deep into physics or the origins of the universe, there has to be grand plan, you know, it can't be chance. Towards the end of his career, which he was at that point, I think he was kind of exploring these unexplored things, but he was a trained astronomer, he was looking at it from that perspective.

So I was a little panicking because even being in India, where religion plays a big part in everyday lives of people there, I couldn't possibly write this, because I was sent to cover an astronomer and try to bring a news story from it.

After the talk, I asked him, "I'm a journalist. I need to talk to you about a story." He said, "Yeah. Hop on the cab with me. I'm going back to my hotel. You can ask questions." So we were sitting in New Delhi traffic, and I had my little Dictaphone open, my notepad, and I was taking shorthand, asking him questions, as any journalist would do, what is the local significance of this person's visit? So I asked the question, "Why should Indians care about asteroids?" And he mentioned that "If you look at the map of India, it's surrounded by water on three sides. You have the Arabian Sea, the Indian Ocean, and the Bay of Bengal. If you have a tsunami that is generated by a big asteroid impact, you will hurt a lot of people living on the coast. He said, "It's more important for Indians to study asteroids because you have a real threat to life in India."

My follow-up question was, "Are professional astronomers doing this?" And he said, “In the U.S., yes. But in India, I think most astronomers are focused on cosmology and the large-scale structure of the universe. So there's not many people doing asteroid research."

The next question was, "Can amateur astronomers with small telescopes do this kind of work?" And he said, "Yeah, absolutely. If you have a 12 or a 14-inch telescope, this is the diameter of the mirror or the lens in the telescope, you can do this kind of work. Basically, find asteroids and help the cause of protecting the Earth and doing planetary defense."

And I got all excited and I said, "You know, I'm an amateur astronomer. I have a little telescope, and I want to do this kind of work.” He said, "You will be the one in the billion Indians to do this work, so it'll be fantastic if you can do it." So I dropped him off at his hotel, and then I was going back to catch a bus back to work to file the story, and he called me back, and he said, "You know you talked about going and finding asteroids with your telescope. That's really great, but a lot of Indians have told me the same thing and nobody has done anything about it, so if you go back and be a journalist, I wouldn't be offended by it."

[14:58] Narrator: Vishnu took it as a challenge, and made it his mission to find a new asteroid. His editor even said he could use the newspaper office computers after work, to do asteroid research.

Vishnu Reddy: I said, "Great." Simply because I lived in a 10 by 10-foot apartment in New Delhi with no heating or cooling, and I had a roommate. Similar to my childhood, there was not much electricity either, so candles would melt without you lighting them, simply because it just was so hot.

My office had air conditioning, and I was like, "If I can hang out till six in the morning, when the public transportation started running again more frequently, I could spend all my nights at work and learn about asteroids." Which is what I ended up doing for the next two years.

I read papers, found out how they find asteroids, how they measure the position. I made lots of friends with people in the U.S. through Instant Messenger, because it was daytime in the U.S. when it was night in India, so it was very convenient for me to talk to people or email them and ask them questions.

[15:57] Narrator: Vishnu also was trying to raise money to buy a telescope better suited for asteroid hunting. One of his American friends on Instant Messenger, Greg Paris, pointed Vishnu to a sale on 12-inch telescopes. Instead of four thousand dollars, they were discounted down to three thousand.

Vishnu Reddy: I reminded him I made 30 dollars a month as salary. There's no way I could buy something that was three thousand dollars. So he said, "I want to help you." He took his credit card and bought the telescope, in the understanding that I would reimburse him.

But trying to get anything into India is really tricky. So it took us two years almost to get the telescope to India because it got stuck in customs and all kinds of problems. By the time the telescope arrived in India, all the asteroids that you could find with a 12-inch telescope were discovered. Because telescopes like Spacewatch, LINEAR, Catalina Sky Survey had found all the bright asteroids by then.

So here I was, spent five years of my life, and I have this telescope in my 10 by 10-foot apartment and nowhere to put it. All the asteroids I could find were gone. So basically, I was pretty upset and depressed about the whole thing.

And then a friend on Instant Messenger, Roy Tucker, he lives in Tucson. He had a slightly bigger telescope than I did; he had a 14-inch telescope. He said, "I'm really sorry for what happened to you. If you want to use my telescope, you're welcome to use it, but you have to come to the U.S. You can come in the summertime because I don't use my telescopes in the summertime."

So I basically sold everything I could, except for the telescope, so I can afford this trip.

[17:34] Narrator: Even though it would’ve helped pay his way, Vishnu couldn’t bear to part with the telescope because he had gone through so much to get it to India. After selling his other possessions, he had just enough to make the overseas journey, but he still had to get a travel visa at the U.S. Embassy in New Delhi.

Vishnu Reddy: This is just after 9/11, and I see two lines, one person was rejecting all the visa applications. I could literally see people crying when they were leaving the windows, and so I jumped the line and went to the other line where people were a little bit more happy. So I ended up at the window and the person asked, "Why are you going to the U.S.?" I was like, "Oh, I'm going to find an asteroid." He was a little surprised because the usual answer he gets is people want to see the Statue of Liberty, or Niagara Falls, or something touristy.

He started probing me, like, "What do you do for a living?" I said, "I'm a journalist." I told him the name of my newspaper. He took that morning’s edition and he said, "Where's your name? I don't see your name in this."

So I told him, "I'm a sub-editor. I edit the stories, make the pages." Then he started asking questions about the ink, the font size, the paper, the newsprint, pretty detailed questions. And then he said, so tell me something about asteroids, and I told him. Then he started asking questions about comets: dust tails, ion tails, and pretty detailed stuff. He was trying to check and make sure that I was legit. I was very impressed. I was like, "Wow. They put really sharp people to weed people out for giving visas."

I said, "How do you know all this stuff?" He said he grew up reading the LA Times, and so he knew a lot about newspapers, and he basically was an amateur astronomer, he had a telescope at home, so he knows a lot about astronomy than a typical visa officer would. So it was my luck that I ended up with him.

[19:15] Narrator: Vishnu got a three-month visa to the U.S. to go find his asteroid. He flew to Chicago, because that was the cheapest ticket he could find. Another friend he’d met online came to the airport to get him.

Vishnu Reddy: He drove seven hours from Michigan to pick me up. He said, "I don't have much. Here's the couch. You can stay and sleep, and figure out a way to get to Tucson to go to see Roy Tucker, and find the asteroid with his telescope."

I didn't have money for a plane ticket so I took a Greyhound bus. It took about I think, with all those breakdowns and everything, three and a half days to go from Michigan to Tucson. I think I went through something like a dozen states. It went more east than west.

(music)

My version of the United States was watching Die Hard, Bruce Willis. So it was interesting to sit and talk to regular, simple people, and they were no different from the people who lived in India. The problems are the same, the concerns are the same, and the aspirations are the same. So for me, it was a really grounding experience. Human beings are the same everywhere. They want to work, they want a decent job, they want to provide for their family, have a house, good things for their kids, and do better, you know? So that was a good introduction to the United States for me.

But anyway, I ended up in Tucson, and then Roy comes to pick me up at the Greyhound bus station, and it's pouring rain. Roy turns on the radio and it says, "After 100 days with no rain, Tucson finally gets its monsoon." I looked at Roy, like “what is the monsoon?” It's like, "Yeah. We have rain for the next three months, this is the monsoon season. That's why I don't use my telescope." I was like, "Oh, no." I had to go back on the bus within five days, that's my ticket. I had to find this asteroid in five days, and I had to deal with the rain.

[20:58] Narrator: Roy had taken recent photos with his telescope when the skies were clear, so Vishnu pored over those while he waited for a break in the clouds. Another challenge to summertime asteroid hunting is the bright plane of the Milky Way, where most of the stars and other material that make up our galaxy is gathered, dominates the sky. That avenue of starlight can wash out the fainter, reflected light of passing asteroids.

Vishnu Reddy: It's almost like the worst time of the year to find asteroids. A lot of professional astronomer surveys avoid looking at the Milky Way, because the algorithms that they use to automatically find asteroids can't handle the contamination from background stars on the pictures they take. But then I realized that the Milky Way's not a uniform dense cloud of stars, it has these dark molecular clouds. These are dark clouds in the Milky Way that provide us a black background for asteroids to drift through.

[21:53] Narrator: Using this technique, Vishnu started finding asteroids whenever the monsoon clouds parted long enough for him to take photos of the Milky Way’s dark clouds. But the asteroids he saw had been discovered already, and his goal was to find a new one. He managed to extend his Greyhound bus ticket a few more days to keep searching. He worked for 20 hours a day, looking through photos taken through the telescope and reporting every asteroid he spotted. Then, on the fourth of July, his friend Roy brought him some news.

Vishnu Reddy: Roy said, "Hey, looks like you found an asteroid." We huddled in front of the computer monitor, and then he said, "Here's an email from the Minor Planet Center, which is the clearinghouse for finding asteroids."

He showed there was a bunch of numbers, and there was one number that had a parenthesis on one side. That indicates that that's a new discovery. I looked at Roy, like, "That's it?" Being Indian, I thought there would be like a ceremony and a certificate. “No,” he said, "That's all you get, because people are finding asteroids by the dozens, so you can't have ceremonies all the time. It's not like back in the 1800s."

So I was relieved, just like, "Ok, I was waiting for this email for five years now."

Since I was in Tucson, I thought it would be a good chance to see Tom Gehrels and show him that email. So Roy and I went to University of Arizona the next day, and I knocked on Tom Gehrels's door. Tom was shocked. He literally fell out of his chair and he said, "You're the journalist guy from India." He remembered me after five years, which is crazy.

I told him this whole story, and I showed him the email, and he was completely blown away. He said, "Look. You've proved your point. I think if you have this much persistence to stick to something that long, you should try and go back to school and get a PhD in planetary science.”

[23:35] Narrator: Vishnu went back to India before his visa expired, and eventually returned to the United States as a student.

Vishnu Reddy: I realized that there were a lot of people finding asteroids, and there were not many people studying what they were made of. If an asteroid is headed our way, you want to know as much information as possible, beyond the fact that it might hit us. So characterization, as they call it, is equally important as discovering an asteroid.

There were two places in the U.S. where they were doing characterization using spectroscopy, which is basically studying how the light reflected off these asteroids is changing because of the minerals or the material that is on the asteroid's surface. So kind of like fingerprinting asteroids based on their composition. One was at MIT, the other one was at the University of North Dakota. I knew with my background it would be hard to get into MIT, so I just applied to University of North Dakota.

[24:27] Narrator: North Dakota was a world away from India in many ways, but Vishnu thrived there and got both a Master’s degree and a PhD, and then went on to post-doctoral studies in Germany and Hawaii.

(music)

Then in 2011, Tom Gehrels passed away, after a long career that included the discovery of thousands of asteroids, and being the scientist in charge of a camera on NASA’s Pioneer 10 and 11 spacecraft that flew by Jupiter and Saturn. The University of Arizona eventually began looking for a new planetary scientist to continue Tom’s legacy.

Vishnu Reddy: To some extent, I thought, "This is my job. I need to take it." I was not expecting I would get it, because there's a lot of really good planetary scientists in the field.

But I guess it's destiny. It took me 18 years from that lecture that Tom Gehrels gave, to me getting his former job.

[25:20] Narrator: Now Vishnu is in a position to affect the destiny of a new generation of space enthusiasts.

Vishnu Reddy: For me, enthusiasm and energy counts a lot more than somebody with the top grade. So I always tend to pick students who are non-traditional like I was, and give them opportunity. Because life is about giving opportunities. A lot of people gave me second chances, so I have to pass that down.

The way I look at it, we have very short time in this world. Typically, people start working in their 20s, start kind of winding down late-60s. It's a very brief time to make an impact on this world. Choosing a problem or an issue that you want to dedicate yourself is very important. And that's something I tell my students, "There's so many great problems, but you have to pick something that is meaningful, something where you can actually contribute.”

So for me, asteroids is a threat to our existence on the Earth. Doing the asteroids and planetary defense, if we can answer the question, "Is there an asteroid that is out there that can impact the Earth within our lifetimes, and can we find it?" I think that's an important answer. Anytime you can make science improve human life, or life in general on the Earth, it makes it all the more worthwhile.

[26:40] Narrator: One of the biggest challenges to tracking down dark rocks in the blackness of space is spotting the meagre amounts of sunlight they reflect. Luckily, visible light isn’t the only kind of light in the universe. Asteroid hunters have other arrows in their quiver, or rather, other parts of the electromagnetic spectrum to help bag their prey.

Microwaves, for instance, are a shorter wavelength of light than what your eyes can see. Beyond heating up your food in a microwave oven, microwaves help us study asteroids. As noted earlier, radar antennae that bounce microwaves off an asteroid can give us a better picture of what an asteroid looks like, and how it behaves in its orbit around the Sun.

Here’s Amy Mainzer, an astronomer who helps us see the universe with new eyes.

Amy Mainzer: So, we see the rainbow of colors all the way from red to purple, and everything in between. But that's actually only a very tiny fraction of the kinds of light that are out there, and the different colors. There's just millions and millions of other different colors that we just simply can't perceive with our eyes, but we perceive them in other ways.

For example, ultraviolet light, we know it's there, as a human being, because we can get sunburns. That's how we interact with ultraviolet. On the opposite side, infrared light is light that’s redder than the reddest red your eyes can see, that we perceive as heat. So, when you feel the warmth of the Sun on your skin, that warm feeling comes from infrared light from the Sun that your skin is absorbing. For astronomers, we love to look at the universe in all kinds of different wavelengths of light.

[28:09] Narrator: Infrared wavelengths are longer than microwaves, and they’ve also been helpful in spotting asteroids. Amy worked on NASA’s WISE space telescope, which made a map of the entire sky in infrared.

Amy Mainzer: For WISE, the particular applications we were really interested in were to look for these extremely cool types of stars that are sort of halfway between a star and a planet. We call them brown dwarfs. These are things that are almost like failed stars, if you will. They're stars that aren't quite massive enough to really shine super brightly like the Sun. Neither are they completely like Jupiter either, which is a gas giant. So, they're kind of somewhere in the middle.

So, this particular set of wavelengths was really tuned for finding these ultra-cool brown dwarfs, and we can pick them out against a huge number of confusing objects.

So the two longer wavelengths on WISE are really optimized for finding things that are around room temperature. We think of space as either being extremely cold or brutally hot, but sometimes there's stuff that's just sort of in the middle, and that's what these wavelengths of light can be good for sensing.

Things that are room temperature put out most of their energy at these longer wavelengths, between 10 and 20 microns. Now, for reference, I always think of the human eye as seeing around half a micron; that's roughly where visible light is.

So, if you take an asteroid, which is a chunk of rock, and you park it at the same distance from the Sun as the Earth, it's going to warm up to a similar temperature as the Earth. That means, it's going to radiate really brightly between 10 and 20 microns. So, even though WISE wasn't originally looking for asteroids when it was designed, it turns out it's pretty good at it.

[29:47] Narrator: Scientists looking at WISE images were able to pick out asteroids from brown dwarfs and other room-temperature objects in space because, over a series of five images, the asteroids change position compared to the more stationary background stars.

Asteroids are significantly brighter in infrared than they are in visible light, making them pop out more in images. By scanning the sky in four different infrared “channels,” WISE was like the alien in the movie, “Predator,” tracking down its prey by its heat signature.

(clip from movie trailer, “Predator.”)
“It sees the heat of our bodies, and the heat of our fear.”
“Whatever it is out there, it killed Harper, and now it wants us.”

[30:26] Narrator: In the infrared, it’s harder for asteroids to hide. And as a space-based observatory, WISE could go hunting around the clock, regardless of the time of day or cloudy weather that limits observers on Earth.

Launched in 2009, the WISE mission discovered over 34,000 new asteroids and comets in less than a year. But the mission’s lifespan was limited. It was only meant to operate for six months, plus a one-month check-out period.

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Amy Mainzer: With the two longest wavelengths in particular, we can't have the telescope and the detectors be warm, because they would sort of blind themselves with their own heat. So, you can imagine if you tried to see the stars in daytime, it's pretty hard. You really can't see very much.

So, in our case, the telescope and the instrument had to be cooled down to about eight degrees above absolute zero to get those two longest wavelength channels to work, and that required this frozen solid hydrogen. So it was basically a couple of tanks full of this hydrogen ice, and it didn't last forever. It actually lasted longer than it was supposed to, by a few weeks, which was great. All total, I think it lasted around eight months or so.

So in that time, we were able to make this all-sky map of the sky, and then after that, when the hydrogen finally was gone, we lost the operability of the two longest wavelength channels. But it turned out that the two shorter wavelength infrared channels kept working. Because even though we were orbiting the Earth, the WISE mission is designed like a giant thermos bottle, it's really well insulated from the heat of the Earth, and even though we couldn't keep it quite cold enough to keep those longest channels operating, the two short ones were still doing just fine at about 75 Kelvin, which is 75 degrees above absolute zero. That's still really really cold by human being standards.

The two longest wavelengths were really the best, I would say, at finding the asteroids, but we still get a lot of detections in the two shorter wavelength channels too. In particular for the warmer near-Earth objects, the ones that are closer to the Sun.

When the hydrogen finally was all gone, late in the fall of 2010, we actually were able to keep it going for a few extra months, just enough time to make one complete survey of the inner part of our solar system with the two short wavelengths, and that let us fill out our map of the asteroids. After that, we thought, "Well, that's it, that's the end." We put the telescope into hibernation, so basically put it into a state where it was sleeping.

Of course, the science data analysis was just getting going, right? So in a lot of ways that's really the fun part, to go through that net and see all the wonderful things that you find. It turns out that one of the very last things we discovered was an interesting little guy called “2010 TK7,” which is the first known Earth-Trojan asteroid. And what that means is, it's an asteroid that's kind of sharing the Earth's orbit around the Sun; it's sort of gravitationally trapped by the Earth, and it gets dragged along with the Earth, effectively. So, we have a little friend, and that was a really fascinating discovery.

So, in any event, as we were analyzing all this data, NASA actually came back to us and said, "We would like you to see what it would take to turn it back on again, and this time just use it for looking for near-Earth objects."

[33:34] Narrator: At this point, the WISE telescope had been in hibernation for over a year, so turning it back on again was something of a gamble.

Amy Mainzer: It's like if you find an old laptop in a closet and you're wondering, "Okay, is this going to start?" But it did, it did, and it had warmed up because we weren't actively controlling its pointing anymore. It had gotten up to a couple hundred Kelvin.

The way we get it to keep cold right now is to continuously point it away from the Earth. We want to look at deep space, at the darkest part of space that we can. So when we put it in hibernation, we stopped doing that active control of the pointing, so basically half of every orbit, the telescope is looking right at the Earth. We're nice and toasty room temperature, so the telescope did get pretty warm, but the neat thing about this is our physics predictions had said that if you just stop looking at the Earth, it'll eventually cool back down after a few months, and that's exactly what it did. It cooled right back down to where it was before, and it's still at about 75 Kelvin.

[34:31] Narrator: The re-booted WISE mission was given a new name, NEOWISE, and it’s dedicated to searching for asteroids and comets that come close to our planet.

Amy Mainzer: It's just really neat, because when you have the heat signatures coming off of these asteroids or comets, you can use that to learn a whole bunch of different things that you can't otherwise get at very easily.

(music)

Once you know its orbit, and you know how far away it is, if you can measure the heat that comes off of its surface, now we can figure out, using math, how big that surface is, and that lets us tell some very valuable information, because for things that get close to the Earth, we really would like to know how big they are. If an object is larger, it could potentially pack more of an impact punch.

But we also want to look at the more distant objects, because, the near-Earth objects, one thing we know about them is they're a fairly temporary population. If you put an object in the space around the Earth, it doesn't stick around for that long. Well, on astronomer time scales. These near-Earth objects are only stable for a few millions to tens of millions of years. So they either collide with a planet, sometimes it's ours, they can get pulled into the Sun, or they can kind of get kicked back out into the very outer, cold parts of the solar system.

So, they don't stick around. Which means the fact that we see them today, that suggests that they are continuously getting resupplied from different source populations. Either in the main asteroid belt, between Mars and Jupiter, or potentially, also from the comet population. So coming in from way, way farther out. So, one of the things we like to do is try to compare the properties of asteroid populations to see where might they have come from in the first place.

[36:07] Narrator: An asteroid’s color provides one point of comparison.

Amy Mainzer: Asteroids are very, very gray (laughs) by most people's standards. And even the so-called red ones are really mostly grey when we look at them with our eyes. But if you really stretch the color, you can start to see very subtle differences in the colors.

[36:24] Narrator: It’s like picking out a paint color at the hardware store. If you decided to paint a wall “asteroid grey,” you could select one that is more reddish-grey, or bluish-grey.

Amy Mainzer: This one's a little warmer shade and this one's a cooler shade, exactly. So, if we can combine the infrared light with visible light, now we can tell the difference between light-colored objects and dark-colored objects.

Because the problem is, when we look at these objects with visible light alone, if we don't have any other measurements, we can get confused. Basically an object that is very light colored but small, looks the same brightness in visible light as an object that's really large but has a very dark color on its surface. One thing we've learned about asteroids from looking at WISE and other telescopes, is that there is a huge range of variation between asteroid brightnesses. Some of them are super-duper dark, they’re as dark as printer toner or charcoal, and others are really bright, like a freshly paved sidewalk almost.

So, this confusion between light, small objects and large, dark ones is a real problem if all you can do is measure them with visible light. Whereas if you have other techniques, like infrared or asteroid radar observations, that can give you a much better idea of the true size of the object. When you start to put these techniques all together, now in addition to the size you can also get the reflectivity, which tells you something about what the object is made out of.

[37:47] Narrator: Although the NEOWISE mission is currently NASA’s only space telescope dedicated to studying near-Earth asteroids and comets, its days are numbered.

Amy Mainzer: This telescope was only supposed to last for seven months, right? It's just like an old car at this point. Every day is a gift, so hopefully it'll keep working for a little longer, but eventually it will finally become inoperable.

What determines how long this particular telescope will stay in Earth orbit is drag from the atmosphere. Now, we're orbiting roughly around 500 kilometers above the surface of the Earth, but there's still a tiny little bit of atmosphere, even up that high, and that produces drag, which eventually is going to cause the telescope to burn up in the atmosphere in about 2024.

But before then, what's starting to happen is the plane of the orbit is starting to twist, and we're losing altitude. The telescope’s orbit will degrade to the point where we won't be able to keep the Sun or the Earth out of the telescope, and at that point, presumably, that'll be the end of it.

Eventually it'll become the thing it's looking for, it's going to become a shooting star someday (laughs).

[38:52] Narrator: It’ll be sad to see NEOWISE streak back down to Earth, but Amy and her colleagues already have their sights set on the next-generation asteroid hunter.

Amy Mainzer: Our key thing is to just really leverage the lessons we've learned from WISE, but make it an even wider field of view telescope so we can really cover lots of sky all at the same time.

And the other thing is, we'd like the mission to last a long time. And because of the cryogen that we had to use with WISE, those two longest wavelengths, the ones where the asteroids and comets are really really bright, it didn't last that long. And that's just because when you have to get to eight Kelvin, eight degrees above absolute zero, that's very difficult to do. It requires that you use a very elaborate type of refrigerator, or you have to use cryogen, and both of those are expensive and difficult and they don't last as long.

So, we've taken the detectors used in the two shorter wavelengths by WISE and we’ve adapted them so that they work at wavelengths that are almost as long as the longest wavelengths on WISE, but they don't need to be as cold. They can get by with being 40 Kelvin, which is still really really really cold. And so, If we can park the telescope just a little farther away, then we can hit that 40 Kelvin temperature.

So just outside the orbit of the Moon, there are these two magical orbit points, from my point of view, called the Lagrange points. These are semi-stable places where, if you put a spacecraft there, it will tend to stick there, and as the Earth goes around the Sun, the spacecraft kind of gets swept along with it. So we’re both orbiting the Sun, but the spacecraft is staying very close to the Earth.

From our perspective, we're just far enough away that the heat from the Earth doesn't have such an impact as it does on WISE, but at the same time, it's close enough that we can get all those wonderful pixels back, and we can have streaming video, effectively, from this spacecraft. If it gets too far away, what eventually starts to happen is, it gets really hard to transmit data. It's kind of no good having lots of pixels if you can't get the data back because you're so far away.

(music)

So, over the last… gosh, since around 2003, our team has been working on these detector arrays, to try to stretch the wavelengths out and make them longer and longer so that they can hit those wavelengths where the asteroids are very bright, and yet not have to be so cold.

[41:03] Narrator: The earliest possible date for this new space telescope is 2024. If it does launch, then it’ll help us push beyond our human limitations, revealing new shades of these small rocks that look, to our eyes, like so many tiny, twinkling stars.

(music)
“Twinkle, twinkle, little star
How I wonder what you are.”

[41:35] Narrator: Next week, for our final episode of season two, we go full circle back to the beginning, when asteroids and comets first became a major concern for NASA.

(Excerpt from Episode 10: Comets: A Love Story)
Paul Chodas: It really did hit. We really did get data, and it's a spot we predicted. So it looks like we're going to have 20 more of these impacts to observe. And sure enough, as bigger fragments hit, you saw the Hubble space telescope images of the big splots on Jupiter.

[42:01] Narrator: If you like this podcast, please subscribe, rate us on your favorite podcast platform, and share us on social media. We’re “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory.

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