Monthly Archives: September 2014


Can You Blow Up an Asteroid With a Nuclear Weapon?

Hey Dr. Zoomie – I heard that an asteroid just missed us, and then I watched a movie about an asteroid killing everyone on Earth. Then there’s that asteroid that killed the dinosaurs. Sounds like something we should be worried about – why can’t we just blow them up with nuclear weapons?

Image of Vesta Asteroid

Image of Vesta Asteroid

Good question – especially since we only live on one planet. Let’s face it – if something whacks the Earth hard enough it can destroy our civilization and push humanity back to the Stone Age. An even bigger strike might even push us into extinction along with most of the life on Earth. We know that it’s happened in the past, and there’s no doubt it’s going to happen again sometime in the future. I don’t want to face a world without Internet, tablet computers, and cable TV – not to mention high-quality medical care, air conditioning, and refrigeration – so it’s reasonable to wonder what might be done to help keep us (and our Kindles!) safe. And since nuclear weapons are the biggest things we’ve got it seems to make sense to just blast an asteroid out of the sky. Right?

Well…not always. Here’s why.

First let’s take an easy case. When a nuclear weapon goes off it creates a fireball where temperatures are thousands to millions of degrees – enough to vaporize anything inside the fireball. For a 1 megaton nuclear weapon (technically a thermonuclear weapon or hydrogen bomb) the fireball will be 1-2 miles in radius. So setting off a bomb this size could, in theory, completely vaporize an asteroid less than about a mile or two in diameter (if the bomb is sitting on the surface). But in reality, the asteroid will absorb a lot of energy from the weapon and the fireball will start to cool down – chances are that even a smaller asteroid wouldn’t be completely vaporized. Instead it might be blown apart.

OK – blowing an asteroid apart sounds good – at least we’re taking the size down a bit, right? Well…maybe. But we also run the risk that, instead of a single big asteroid, we could instead get pummeled with dozens of smaller – but still lethal – rocks. So we trade one massive hit for a dozen (or so) big hits. This could actually be worse, so this might not be the best way to go. There’s also a small legal issue – nuclear weapons are banned in space. Granted, with the fate of humanity at stake people might not be too worried about the fine points of international law, but it does prevent us from testing any systems before we’d use them – with something as complicated as trying to nudge an asteroid off of a collision course it would be nice to be able to know what we’re doing.

Another possibility might be to try to blast an asteroid off-course so that it misses us. This is maybe better than turning a single lethal rock into a dozen lethal rocks, but only if we get it right. What if, for example, the rock is weaker than we thought and instead of being pushed off-course it just breaks up? Or what if the explosion goes off at the wrong time and it doesn’t miss us at all – maybe even hits in a worse location? The bottom line is that we have to know exactly how much the asteroid weighs and how tough it is – and we have to know its orbit and how quickly it rotates – with enough precision to make sure that we are really going to steer a single asteroid in just the right direction at exactly the right time. If we get anything wrong, as I mentioned before, then we can make things much worse.

The bottom line is that under exactly the right conditions a nuclear weapon might be able to help, but under anything less than that the effects would be anything from no change at all to making things worse.

Right now the bottom line is that we just don’t have anything we can do except to keep an eye on the skies and hope we don’t see anything with our name on it. And if we do, hopefully we’ll have enough time to figure out how to convince it to miss us. Luckily we have a pretty good handle on most of the biggest rocks in our part of the Solar System and we can predict their orbits for decades or centuries in the future. One could pop up tomorrow that would give us only a few months to respond, or even smack into us out of the blue. But every year that goes by gives us a better understanding of what’s out there and a better chance to protect the Earth. But whatever it might be that we might see, I’m pretty sure that nuclear weapons won’t be part of the solution.

Are Dental and Medical X-rays for Kids Safe?

Hey Dr. Zoomie – I’ve heard that kids are much more sensitive to radiation than adults are. Should I worry about taking my kids to the dentist or the doctor?

Image of Dental X-ray

Image of Dental X-ray

When my younger son was about two years old he broke out in big red splotches and then started having some trouble breathing. My wife and I rushed him to the hospital and they decided that an x-ray was needed. As they were prepping him the technologist started explaining the risks of x-rays to me – after a few minutes I cut him off, explained that I was a radiation safety professional and fully understood the risks, and told them to go ahead and take the x-rays. Luckily there were no serious problems and he recovered over the next week. My thinking was that the radiation dose from an x-ray is low – not much more than taking a cross-country plane flight – and the risks were somewhere between very low and nothing; in exchange for gaining valuable diagnostic information. On the other hand, even if there was a long-term risk from the radiation, that would lie many years – even a few decades – in the future. Not breathing, on the other hand, can be immediately dangerous – my feeling was that it was a good idea to make sure my son didn’t have any problems that were immediately life-threatening. Any risks that the radiation might pose wouldn’t show up (if at all) for 20 or 30 years.

The bottom line is that medical x-rays (including dental x-rays) don’t give a very high radiation dose – at most, a few tens of millirem (fluoroscopy and CT scans are higher-dose procedures; let’s get back to them in a minute). And if we dig into the radiation biology we find that the health risks from a low dose of radiation is incredibly low – about as close to zero as you can get. At the same time, we also have to consider the risk from a doctor (or dentist) not having the diagnostic information they think they need – if the risk of missing this information is higher than the risk from the radiation then there’s a good case to be made for taking the x-ray.

There are some procedures that expose us to a higher radiation dose – CT scans are one and fluoroscopy is another.

A single CT scan exposes a person to about as much radiation as 100 x-rays (give or take a bit). This sounds like a lot – and it’s not a trivial radiation dose – it’s about as much as we get from nature in a few years. But even here the risk of getting cancer is really low. How low? According to one calculation it’s about one twentieth of one percent – which means that there’s a 99.95% chance that nothing will happen. And in reality the risk might be even lower – we don’t know for sure exactly how low the risk is, but we know that it’s no higher than 0.05%, which is about twenty times safer than driving. And if you look at the risks of other options – watchful waiting or exploratory surgery – the risks from a CT scan that will provide medically necessary information seem to be pretty reasonable. At least they seemed reasonable to me when I had to make this same decision for another child several years later.

The bottom line is that if we’re going to decide the right thing to do we have to keep in mind all of the risks – and we have to think about the real risks of the diagnostic radiation as opposed to what we fear might be the case. And, while we don’t want to use medical x-rays when they’re not called for (and we still want to do what we can to reduce radiation dose whenever possible), we have to remember that they provide information that often can’t be gained in any other manner – to see inside the body we only have two choices; using some form of radiation or cutting into the body. Surely we can all agree that x-rays are better than scalpels.

What is Food Irradiation? Why is it Done?

Hi, Dr. Zoomie –

I got some ground beef at the store the other day that says it was irradiated. Can you tell me what that means and why it’s done? My brother tells me irradiation makes the food dangerous – is there any truth to that?

It’s not only the ground beef that’s irradiated – a lot of spices are irradiated, along with some fruits, meats, and more. What’s happening is that the food is exposed to very high levels of radiation; that radiation is enough to kill off any bacteria that might be inside (or on the outside)of the food. We all know that radiation can harm people – what many don’t realize that a high enough dose of radiation can kill microbes to boot. So if we slam enough radiation into food, it ought to kill all of the microbes; since those microbes are what makes the food go bad, food irradiation keeps it fresher longer.

Incidentally, it’s not only food that’s irradiated to sterilize it – there are a number of facilities in the US that use radiation to sterilize surgical supplies, some mail, and much of the blood supply.

Some types of food don’t do well under irradiation – it can cause strawberries to become mushy, for example. But in addition, some people are concerned about food irradiation because exposure to high levels of radiation can cause chemical changes in the food. And it’s true – exposing food to radiation can cause new chemicals to form, but these chemicals don’t seem to be harmful to people. And I should point out that any sort of cooking causes chemical changes to the food – that’s what cooking is. When you brown meet or caramelize onions, for example, the sugars in the food are undergoing a chemical reaction called the Maillard reaction. But what really causes chemical changes is grilling food over charcoal, wood, or even a gas flame, and some of these changes (especially the ones that include the soot or combustion products from the flame) can create chemicals that are potentially much more dangerous than anything caused by radiation. This isn’t to say that grilling out is dangerous – more to put the changes caused by irradiation in perspective. The bottom line is that cooking, grilling, and food irradiation can all cause chemical changes in your food – and irradiation causes fewer changes than the other techniques.

Something else that people worry about is that irradiation might cause food to become radioactive. Luckily there’s nothing to this at all – the gamma radiation from cobalt-60 or cesium-137 is simply unable to cause food to become radioactive (neither can the x-rays or electron beams that are sometimes used). And the food doesn’t somehow store up the radiation – no more than, say, your furniture stores up light and glows when you turn the lights off. For that matter, the only type of radiation that is effective at causing things to become radioactive is neutron radiation, and neutron radiation isn’t used for irradiating food.

Something else to keep in mind is that the whole purpose of food irradiation is to kill microbes. Think of all the food recalls and food poisoning outbreaks that have been in the news – millions of pounds of ground beef have been recalled, contaminated spinach caused illness and death, and much more. The bottom line is that food irradiation makes food safer – another of the GAO’s findings.

When you put it all together, you can reassure your brother that we get a lot more benefit than risk from food irradiation. In fact, the greatest risk is to the people working at the food irradiation facility – there have been injuries and fatalities from people who just didn’t work safely with the high levels of radioactivity that’s present. But as long as the workers are careful to follow the proper safety precautions they’ll stay safe also.