CommentsGELFAND’S WORLD--We've been talking a lot about earthquake awareness of late, but there's one more topic that bears mentioning. It will seem science-fictional at first, but there's serious, real-world significance to it.
The topic is Planetary Defense and refers to the possibility of finding and then moving an asteroid (see photo above) that would otherwise collide with the earth and do momentous damage. At its worst, such a collision would be akin to the dinosaur killer of 66 million years ago, only we would be the dinosaurs. A smaller body such as the one which hit Tunguska in Siberia 111 years ago would wipe out close to a thousand square miles, including whatever and whoever happens to be living in that area.
There is something that can be done to prevent such an occurrence. Let's start with a development in space exploration that bears on this question.
A striking new step in planetary defense (and science) has been accomplished, even if most people aren't aware of the fact. A few months ago, an American spacecraft arrived at the asteroid known as Bennu. The spacecraft (with the name OSIRIS-REx) was then maneuvered so as to orbit around the asteroid. This is a great feat, particularly when you think about how far we've had to come since the original moon race, now about to celebrate its 50th anniversary on July 20. Why this latest accomplishment is so significant involves the fact that a collision between earth and one of the asteroids could be the end of much life on earth, just as it was some 66 million years ago. We're here because our remote ancestors (some of them, anyway) survived, while the dinosaurs didn't. By the way, although Bennu is not huge as asteroids go, it has a mass in excess of 150 billion pounds. If it were to strike the earth at high velocity, it would do a lot of damage.
What is becoming evident is that we could defend ourselves against an asteroid impact, even though we cannot at present imagine a way to prevent a major earthquake. The asteroid strike would be far worse than The Big One, but with foresight and a modest degree of spending on technological development, we could potentially predict asteroid impacts -- accurately enough to take action -- and then prevent them from happening.
In the smaller cases, we're talking about a collision that would be capable of destroying one of our largest cities -- Denver or Tokyo or San Francisco. In the larger case, we're imagining a collision potentially capable of destroying much of earth's surface life. We would be the dinosaurs.
The good news is that such asteroid strikes are awfully rare, as we can infer from the fact that we are still here even if the dinosaurs are not. What makes this discussion timely is that human technology has reached the part-way point in being able to nudge an asteroid away from a collision course with us. It turns out that we probably wouldn't have to go a lot further in scientific and engineering development to be able to do the actual orbital nudging.
In this, the 50th anniversary year of the moon landing, we have a remarkable story to relive. But part of this story involves continuing our progress, a task that needs a few millions of dollars to continue the development of planetary defense.
On June 29, the 2019 Planetary defense and asteroid exploration (PDAE) mini conference was held in Rancho Palos Verdes. It featured a stellar cast of speakers (OK, so shoot me for the pun) who have day jobs at Jet Propulsion Laboratory (JPL), the Aerospace Corporation, NASA, and other local centers of learning such as Caltech and UCSB. Over the course of the day, we received a comprehensive summary of how the dangers are discovered (and will be further elucidated), how we might think about reducing the dangers, and what advances in technology we would need to become fully capable planetary defenders.
First, a couple of highpoints: Dr Nahum Melamed of the Aerospace Corporation introduced the idea of planetary defense and asteroid exploration. For the moment, these terms are nearly synonymous, as we're still learning the ropes. But eventually, we will know enough and have the technical means to nudge an asteroid aside so that it won't collide with the earth. Taking up that proposal, Dr Paul Chodas from JPL spoke about a study (somewhat akin to a war game) in which space experts planned a mission to prevent an imaginary asteroid from hitting the earth 8 years from now.
It's useful to have an overall concept of what the asteroid belt is, and how it was formed. If you think of the solar system as beginning with a ball of gas and rocks and having a bit of a spin to it, you have the beginnings of understanding. As pebbles and rocks congealed under gravitational attraction to form planets, and as the sun formed out of light gases in the same way, we developed most of the solar system. But some of the rocks and debris did not quite make it to being a planet, and are still revolving around the sun, mostly out between Mars and Jupiter. Some of them are closer, and some have orbits that intersect our own orbit around the sun. These are the asteroids that are of concern and are referred to by space scientists as potential hazards.
Aside -- a single paragraph mini-summary that the math-phobic and astronomy-phobic can skip. As we all know, the earth and other planets orbit around the sun in paths that are pretty close to being circles. Technically, they are ellipses, which is a fancy word for taking a circle and stretching it along one direction. Such orbits can be described by the length of the stretched axis (it's called the major axis) and the length of the unstretched axis (called the minor axis), and finally by the directions that these axes point. Here's where it gets a little more complicated. Suppose an asteroid has an orbit that includes a long major axis (the long part of the ellipse) and that major axis overlaps our own earth's orbit at some point. That would be a serious hazard. But remember that the earth may be in a very different part of its own orbit at the moment that the asteroid intersects our orbit. Add one other fact to reach an understanding of the system. The asteroid and the earth usually take different amounts of time to traverse a single orbit. That means that the earth can catch up with the asteroid (or it can catch up with us) over a period of many orbits. Under the wrong circumstances, the earth and the dinosaur killer collide.
As an example of a smaller (non-dinosaur-killer) asteroid that gets closest to us every 6 years, there is Bennu, a top-shaped asteroid that is about as big around as the Empire State Building is tall. More about Bennu below, as it is the object of that great scientific step I mentioned above.
We know that there are lots of asteroids and that once in a great while, one of them might hit us. What's to be done?
The first thing is to remain calm, as this might not happen for hundreds, or even thousands of years. We are going to be hit with many tons of small meteorites every year, but the chances of an asteroid big enough to do significant damage is relatively slight on a year-by-year basis.
But it is a possibility, and the probability of a strike increases as the time frame gets larger.
During the day-long symposium, we saw video interviews with some asteroid researchers. The lesson they communicated was this: "Find them before they find us." This was not meant to be trite. Rather, it is part of a developing idea that goes something like this, and was the object of that planning scenario that Paul Chodas spoke about:
If we know in advance that an asteroid is likely to hit us after our next 8 earth orbits (meaning 8 of our years), then we can plan how to prevent this from happening. From our point of view, we have 8 years to do the following 3 things:
1) Get a more accurate fix on the precise orbit of the asteroid so that we can know whether or not it is going to hit us. If we find that it will miss us, we can relax. If not, we go on to step 2.
2) Send a spacecraft up to the asteroid to get a better look. This adds to the information we get in step 1, because it will give us a better understanding of exactly how big it is, something about what it is made of, and more accurate measurements of its precise orbit. That's what the spacecraft OSIRIS-REx is doing at Bennu right now.
3) At a calculated point in the asteroid's orbit, nudge it a bit by hitting it with a spacecraft or by exploding a nuclear device underneath it. It doesn't require a lot of additional velocity added to the asteroid to cause it to miss us on its next pass around the sun. In fact, the scientists presenting at the conference explained that it would only require a few centimeters per second change. In other words, changing its speed by one mile per hour would more than do the trick.
Fine tuning the plan
Interestingly, the U.S. congress long ago tasked NASA with locating at least 90 percent of the larger size asteroids. A few years after that, congress was convinced to enlarge the task so as to find 90 percent of all asteroids as large as 500 feet in diameter. It turns out that there are lots of asteroids (in the tens and hundreds of thousands) so this isn't the easiest project. But JPL and NASA and their associates are doing a pretty good job. They've got the orbits of most of the bigger asteroids mapped out already. Now they need to finish the job on the smaller ones.
How is this done? The answer is fairly simple. Telescopes on earth and in earth orbit look for the light reflected by space objects. An object that moves relative to the fixed stars is a potential asteroid. As few as 3 such sightings are enough to establish a rough estimate of the orbit. More sightings and better precision instruments allow the orbit to be determined more exactly.
The planetary defense people would like to have an orbiting space telescope that is designed to hunt asteroids. That's part of the few millions of dollars that I mentioned above.
How do you move an asteroid's orbit?
You thought I would never get to this question, didn't you? But the PDAE program included several discussions about how we might nudge that asteroid out of its path of destruction. Qicheng Zhang discussed "Directed Energy Methods for Planetary Defense." If I understand correctly, the idea is to get close enough to fire a powerful laser at the asteroid. Given enough intensity, this will heat one spot to incandescence, which will then spout out fast-moving gases which will be, in effect, like running a rocket motor strapped to the asteroid.
Two other methods were discussed. The first is simply to shoot a rocket into the asteroid. At the right velocity, the rocket will burrow into the asteroid and impart a change in momentum. Do this with enough mass and velocity (might require lots of such rockets) and you can achieve that few-centimeters-per-second change in velocity that is required to save the earth from destruction.
Building public awareness
I haven't seen a lot of blogs or newspaper articles on planetary defense. On the other hand, there have been movies and television shows about alien invasions and even a few about runaway comets or the odd asteroid. Most are unwatchable for the knowledgeable viewer.
Still, the use of visual media and the moving image (that's what the film scholars call a movie) are appropriate ways of getting the public's attention. Philip Groves is in the process of making a film on asteroid hunters that will be coming out in the near future. This column is a more humble attempt at the same thing. If you take away from this exercise the understanding that funding for the planetary defense program should continue, you've learned enough.
And for the presenters at the PDAE conference, thanks for taking the message to the public: Drs William Ailor, Davide Farnocchia, Madhu Thangavelu, Niraj Inamdar, Eric Mahr, and the above-mentioned Nahum Melamed and Paul chodas.
One more thing: Amateur volunteers in space exploration
You can apply your computer skills to the exploration of the asteroid Bennu and other celestial objects by going to CosmoQuest. Perhaps some of you may remember when volunteers were invited to provide time on their home computers to solve protein structures. This is analogous but involves using your own skills to map Bennu and other objects.
Back in the 1980s, people who were enthusiastic about the continuation of space exploration got together to talk and plan. There was a group called the L5 Society at that time, and it later merged into the group that is now known as the National Space Society. It is a nonprofit that pushes for awareness and funding for space exploration.
(Bob Gelfand writes on science, culture, and politics for CityWatch. He can be reached at [email protected])
