This week on Yuri’s Night (April 12th), Stephen Hawking (who needs no introduction) and Yuri Milner (Russian entrepreneur and physicist) announced a truly remarkable and inspiring project. They announced a bold plan to send a swarm of small iPhone sized spacecraft to the nearest star system, Alpha Centauri, at one-fifth of the speed of light. This would mark humanity’s first ever spacecraft sent to another star beyond our own solar system, and could lay the ground work for other robotic missions to nearby stars, and perhaps even one day sending the first human settlers out into Cosmos. I wholeheartedly support this project, and I hope they can pull it off. However, that said, there is one thing that they did not address that I think should be taken into account for a mission such as this, namely, Planetary Protection.
Planetary Protection is a policy that NASA has established to prevent us from contaminating another planetary body, such as a planet or moon, which may harbor life. For example next year, in 2017, the Cassini spacecraft will be directed to burn up in Saturn’s atmosphere so that it can never accidentally crash on either Titan or Enceladus (moons of Saturn) because of the possibility that life may be present in their subsurface oceans. If Alpha Centauri has planets, as most star systems do, then we must take precautions to ensure that we do not contaminate or damage them.
The first such danger comes from the (albeit remote) possibility that microbes from Earth may hitch a ride on the spacecraft. If the spacecraft encounters a planetary body (i.e. crashes into it), we may be contaminating that body with our microbes, thus disrupting the natural processes there. We now know, from our own solar system, that liquid water seems to be quite prevalent. And anywhere on Earth where there is water, we find life. This doesn’t mean it’s a sure thing that life would be present there. But if it is, our microbes (bacteria, viruses, etc.) that hitched along for the ride, could be harmful or even catastrophic to that alien ecosystem.
The second danger is the impact itself, should one of our probes accidentally collide with a planet or moon in the Alpha Centauri system. Which brings me to the fun part of the article. Physics! Below we’ll calculate exactly how much energy an impact from one of these spacecraft will produce. Don’t worry, I’ll explain the ideas presented, so don’t be intimated by the math. There won’t be a test or anything at the end. 🙂
For this exercise, let’s make a comparison to meteorite strikes on the earth. For example, the famous meteor explosion over Chelyabinsk Russia in 2013. You might recall seeing on the news that this meteor exploded over Chelyabinsk and damaged thousands of buildings and blew out countless windows. According to NASA, the asteroid was about 11,000 metric-tons, and was moving about 18 kilo-meters per second (or about 2236 miles per hour), and released about 440 kilo-tons of energy as it impacted the Earth’s atmosphere (or about 20 times the amount of energy released in the atomic bomb exploded over Hiroshima in 1945). The first question in your mind is likely, “But Josh, why are we comparing this massive asteroid strike to these small iPhone sized spacecraft? Surely this tiny spacecraft wouldn’t pose any real danger to any planet that it could hit”. But the thing to remember here, is that it’s not about the size of the object, it’s about the kinetic energy of the object. This “starshot” spacecraft will be moving at one-fifth the speed of light! That’s well over 3,000 times faster. And kinetic energy scales by the square of the velocity. All that means is, that if you double the velocity, you quadruple your energy. So 3,000 times faster, means your energy is 9,000,000 times greater! Now, lets see the math.
Here I’ll use what’s known as scientific-notation. Meaning that we will express large numbers by powers of 10. For example:
The equation for kinetic energy is given by:
Notice that it depends greatly on the velocity. However, at higher velocities, namely close to the speed of light, you need to take into account relativistic effects. This means applying Einstein’s theory of special-relativity. The kinetic energy then becomes:
If we assume that the spacecraft has the mass of an iPhone 6s Plus, then we can calculate the energy of the spacecraft moving at one-fifth the speed of light.
A Joule is a unit of energy. From here we can compare with the energy of the Chelyabinsk meteor explosion:
This means that the spacecraft would have roughly 20%, or one-fifth, the energy of a Chelyabinsk sized impact! That’s pretty huge really when you think about it, but again, it’s the total kinetic-energy that matters. The velocity is one-fifth the speed of light after all. If this hit a planet like Earth, it would most likely burn up in the atmosphere without too much damage to any potential lifeforms below. But imagine if it hit a moon like Europa which has a subsurface ocean, with no atmosphere. Then it could cause a lot of damage to the surface of the moon, and perhaps wind up infecting the subsurface ocean with our nasty Earth microbes. This is something to consider when sending probes to other destinations in the galaxy, or even in our own solar system. Especially in the off-chance that an intelligent civilization inhabits that star system. If our probe hit something else, perhaps an orbiting space station of some kind, it would completely destroy said station. Again, this would be an extremely unlikely event.
All that said, I’m not overly worried about hitting a planet in the Alpha Centauri system. The probability of hitting a planet or moon, is vanishingly small. However, care should be taken to ensure that the spacecraft have some ability to maneuver so that they can avoid any planetary objects. It’s the right thing to do. After all, we wouldn’t want to get a bad reputation around the Cosmos would we? 🙂