(Field of Streams Star Map courtesy of the Sloan Digital Sky Survey Image Gallery)
LC: So the second goal of the Large Synoptic Survey Telescope (LSST) is cataloging the solar system, and you said it’s going to detect around two orders of magnitude more objects than the Sloan Digital Sky Survey. What do you think is the importance of knowing the types and locations of these objects?
ZI: So there are two sets of issues. There is science—science comes from the fact that asteroids still retain memory of how the solar system looked in the beginning when it formed about five billion years ago. We can learn a lot about the early processes in the formation of the solar system, and by looking at comets we can learn about the outskirts of the solar system where we can never go with probes because it’s too far out. So there are lots of scientific reasons to study solar system objects, and then also some of these asteroids are potentially dangerous to all life on earth, not just civilization. If it’s a big one, it could destroy life on earth, and so you would want to know in advance if there is any object on the trajectory. At this point we know about most of those that are larger than about one kilometer, about half a mile. There are about a thousand of them and with existing telescopes it was demonstrated that none of them is on any dangerous trajectory over the next few hundred years. Those big ones, half a mile across, if they strike earth then probably the entire human race would disappear and only primitive life would remain.
If they’re half a mile large?
Dinosaurs were killed by an object that was about ten miles across, and basically it ejected so much material into the atmosphere that it became opaque and the lack of heat coming from the sun destroyed all the big vegetation. And then smaller, more primitive life forms smarted developing. That’s basically why humanoids developed, because they didn’t have competition from dinosaurs—that’s the theory at least. That’s hard to prove.
So the smallest one you worry about is about thirty meters across. That’s the smallest size you worry about because smaller guys just burn in the atmosphere. Between thirty meters and one kilometer there is huge range and there are actually about 100,000 objects in that size range that cross Earth’s orbit, and it’s not obvious whether they will strike Earth. Even if they have similar orbits, they have to be at the same place at the same time. To know if that will ever happen, you have to measure their orbits very precisely, and that’s where LSST comes. So we hope that we will catalog most of these guys that are larger than 100 meters and we will have orbits that are precise enough that we can know if they are going to strike earth within few centuries. So that’s very practical goal.
And then one other theme is studying the structure of the Milky Way. If you have twenty billion stars that you would detect with LSST in our own galaxy, then you can use their motions and their properties to uncover the history of our galaxy—how it was formed and how it evolved over the last ten or eleven billion years.
The fourth theme is the time domain, where we would look for everything that changes in the sky including asteroids, supernovae, and variable stars, which astronomers have been doing for centuries but with LSST we will be able to see much fainter objects. For example, we would discover five-hundred million variable stars, which is the total number of all the stars in SDSS. So this is a great example of big data astronomy.
What’s the difference between a variable star and a star?
Normal stars don’t vary in brightness, and variable stars can change brightness on time scales from a few minutes to a few years. Some of them are unstable so they pulsate, the whole star is oscillating. Some of them are double stars or a star and a planet and the darker thing goes in front of the bright one and then the brightness changes. Some of them explode like supernovae, and supernovae are especially useful because when they explode they are governed by the simple physics of thermonuclear explosions.
“The simple physics of thermonuclear explosions.”
Relatively simple, well there are no fudge factors, no things that we don’t understand. It is not super simple to calculate. You need to have the biggest computers on earth, but the army has them for military purposes, and the physics of supernovae is well known because it is the same physics as nuclear bombs, so there is a lot of research that was done over the last half century for weapons research.
When you observe supernovae in the sky, from their apparent brightness you can tell how far they are. So with supernovae you can measure the expansion of the universe because you know now how far it is and then you can measure the so-called redshift. When you plot these two, one versus the other, it tells you how fast the universe is accelerating and that’s one of the main cosmological themes.