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Could an asteroid doom Earth?

A Spanish-language news website, OyeGente.com, conducted an exclusive interview with Jon D. Giorgini, a scientist at the Jet Propulsion Laboratory in Pasadena, Calif., who authored a study finding that an asteroid six-tenths of a mile wide could smash into Earth with the explosive force of millions of tons of TNT in 878 years. The asteroid, “1950 DA,” is the most threatening to Earth of all of the known large asteroids, though the odds are only about one in 300 that it would impact the planet. Nevertheless, the threat underscores the importance of new discoveries that could prevent a future cataclysm. Here is the exclusive English interview.

Q: What does this new study reveal about asteroids?

A: It shows that we need to know the physical properties of asteroids to predict their motion over long time spans – how they spin, their mass, their shape and how the surface reflects light and radiates heat. These things can change the position of the asteroid by millions of miles, given enough time and gravity amplification during close approaches to the planets and make the difference between a hit or a miss.



Jon D. Giorgini

For example, for 1950 DA, our radar observations could only narrow down the spin direction to two possible general zones. If one direction is true, there would be little chance of the asteroid hitting Earth. If another is true, the probability of collision would be near one in 300.

This is because light hits the asteroid and warms it. The warm side then rotates back into darkness and radiates heat. This acts like a weak rocket that pushes on the asteroid. Where this “rocket” points is determined by the direction the asteroid spins, which we don’t know and may not be able to determine for many years or decades.

Q: How long did it take to complete this study, and about how many engineers and scientists worked on it?

A: Total time involved was about six months, mostly to improve the physics and software required to do the analysis properly. Future cases could be studied more quickly as a result. This study draws on every branch of science, mathematics and computer science simultaneously, and involved trillions and trillions of sequential calculations performed using a computer. Thirteen other people are listed as co-authors for critical or significant contributions to the study. These ranged from operating the radar during the experiment to finding the original 1950 photographs of the asteroid and remeasuring its position. Everyone reviewed and commented on the manuscript to check for errors or deficiencies.

Q: How was this study conducted?

A: We used two different methods to compute the impact probability and both gave similar results. In one way, we looked at 10,000 statistically possible variations on our “highest probability” orbit solution. Each possible trajectory was extrapolated 878 years into the future using all the physics required to do that. We then counted how many of those trajectories ran into the Earth on March 16, 2880: 33 out of the 10,000, thus the 0.33 percent collision probability. For most factors, it isn’t possible to account for them exactly. For example, we don’t know precisely how much the Earth’s mass – thus gravitational pull – is. We only have a statistical confidence range there is a 99.7 percent chance it is between X and Y . Although X and Y are pretty close together! So we looked at the extremes, the maximum possible and the minimum possible. In this way, you know that the truth will be somewhere in between the bounds, even if you can’t pin it down exactly.

Q: If this asteroid, 1950 DA, were to hit planet Earth, is there an exact location where it would impact?

A: The timing is too uncertain to give an exact answer. In some ways, it doesn’t matter because an object this size would cause serious global consequences no matter where it hit. In the unlikely event it did hit, it would most likely be in the Atlantic Ocean.

Q: Is there anything that can be done to stop an asteroid from hitting Earth?

A: That’s one of the interesting results of the study. Over centuries, sunlight and heat emission are enough to move an asteroid, at least one this size. So if future generations determine the risk is increasing based on new information, they could simply coat the surface of 1950 DA with chalk or charcoal, or send a solar sail spacecraft mission that ends by running into the asteroid and collapsing around it – sort of like shrink-wrapping it. This would change the way it absorbs light and let sunlight do the work of moving it. Time is the key. If you can change the speed of an asteroid on an impact trajectory by only 1 millimeter per second 300 hundred years in advance, you can cause it to miss.

On the other hand, if like the dinosaurs, you only see the threat in the last 10 seconds as it’s burning through the atmosphere toward impact, there’s nothing you can do because it’s hundreds of millions of tons of rock moving at 14.2 kilometers per second. The dinosaurs were around for 150 million years, all big and bad, but they got burned in the worst way possible because they didn’t know their math and physics. They never recognized the threat until the final moments.

We are in a very different situation. The key is advanced warning. You want hundreds of years of warning, because 10 seconds leaves you with no options. The value of the study was in determining what we need to consider when predicting so far into the future. The goal was to find out what limits our ability to predict these events. Now we have a much better idea.

Q: What are some of the effects on planet Earth if an asteroid – the 1950 DA or a much larger one – were to hit planet Earth?

A: Something the size of 1950 DA (1.1 kilometers) moving at its encounter velocity of 14.25 kilometers per second would have 100,000 megatons of energy to dissipate upon impact. The crater would be about 10-20 kilometers wide. Molten material would be ejected, perhaps to temporarily orbit the Earth, then reenter and trigger fires around the world. If it hit in water, the tidal wave would inundate most adjoining coastal areas. Dust and steam would be injected into the atmosphere for some period of time.

Q: Have the studies regarding asteroids and other objects hitting Earth indicated that as the years go by, they are getting closer and closer to actually hitting Earth?

A: No. Each asteroid’s path through space is different, and there is no tendency for them to all get closer to the Earth. Most asteroids are in the belt between Mars and Jupiter. They will remain there for hundreds of millions of years or more and never come anywhere near the Earth.

Q: Have most scientists, including you personally, now concluded that asteroids killed off the dinosaurs? If not that, what did in your opinion?

A: Many have, because of the suddenness of their disappearance, the presence of the iridium layer in the rock laid down at the time – iridium normally being more common in asteroidal material – and the discovery of a large impact crater off the Yucatan peninsula that seems to have been created at the same time the dinosaurs disappeared and was made by an object large enough – more than 10 times larger than 1950 DA – to have caused the global extinctions that evidently occurred. That said, one shouldn’t close one’s mind to other possibilities if there is evidence to support them, or there is an explanation that better fits the evidence we have. But the conclusion regarding the dinosaurs is reasonable.

Q: Can an asteroid potentially destroy or seriously damage any of the nine planets, including Earth, in the solar system?

A: An asteroid or comet could make the surface of the Earth very difficult for life. But if you mean “shatter” a planet, then no, there are no known asteroids big enough or moving in such a way that that could happen. Events like that may have happened during the formation of the solar system and the Earth’s moon billions of years ago, but not now.

Q: What are some of the current studies, or future ones, that you plan to work on?

A: We observe three or four asteroids with radar every month. This weekend, we have an asteroid called 1999 GU3 scheduled. Earlier this week, asteroid 2002 FD6 went past and was observed using the Goldstone planetary radar.

I’m currently finishing a study to determine what asteroids the Stardust spacecraft might be able to visit as it travels through the solar system.

Jon D. Giorgini has a bachelor’s degree in aerospace engineering from Iowa State University and a master’s degree in aerospace engineering, specializing in celestial mechanics, from the University of Texas in Austin. He has worked at JPL for almost 11 years. He was navigator for the Magellan spacecraft during its mapping of Venus (1991-1993) and subsequent aerobraking, then worked for a time on asteroids. He went back to spacecraft navigation for the Mars Global Surveyor mission interplanetary phase (1995-1997), before returning to work on asteroids and NEAR navigation during its cruise and then to NEAR radio science during the mapping phase. As a member of the asteroid radar observing team, he is responsible for the orbit analysis and predictions used to operate the radar tracking and data acquisition systems. He has worked on more than 100 asteroid radar targets since 1994. He also developed JPL’s Horizons On-Line Ephemeris system that sits on the Internet and allows people to determine the position and motion of the objects in the solar system. His title is senior engineer in the Solar System Dynamics Group at the Jet Propulsion Laboratory.

Carlos Leon is editor of the Spanish-language news website OyeGente.com.