2 added 24 characters in body edited Aug 27 '16 at 7:00 user36790 Let's assume it travels at a constant speed of .4c (since we don't know anything about its acceleration) and its mass is 100kg. At those speeds, the relativistic effects are minor compared to the fact that I just totally made the mass up out of thin air, so we can do a classical solution. Using $$E=1/2mv^2$$ we find that the energy is $$1/2 * 100 * 119916983^2 = 7.19004143 × 10^{17} J$$$$1/2 \times 100 \times 119916983^2 = 7.19004143 \times 10^{17} ~\mathrm J$$. Using my favorite page on the internet, Orders of Magnitude (Energy), we see that that's on par with detonating the Tsar Bomba, the largest nuclear weapon ever built. It's 6 orders of magnitude off from the energy released by the Chicxulub meteor event that is believed to have caused the extinction of the dinosaurs. Thus there's little risk of an extinction event. That and hitting a planet by accident 4 light years away without trying is pretty hard! Let's assume it travels at a constant speed of .4c (since we don't know anything about its acceleration) and its mass is 100kg. At those speeds, the relativistic effects are minor compared to the fact that I just totally made the mass up out of thin air, so we can do a classical solution. Using $$E=1/2mv^2$$ we find that the energy is $$1/2 * 100 * 119916983^2 = 7.19004143 × 10^{17} J$$. Using my favorite page on the internet, Orders of Magnitude (Energy), we see that that's on par with detonating the Tsar Bomba, the largest nuclear weapon ever built. It's 6 orders of magnitude off from the energy released by the Chicxulub meteor event that is believed to have caused the extinction of the dinosaurs. Thus there's little risk of an extinction event. That and hitting a planet by accident 4 light years away without trying is pretty hard! Let's assume it travels at a constant speed of .4c (since we don't know anything about its acceleration) and its mass is 100kg. At those speeds, the relativistic effects are minor compared to the fact that I just totally made the mass up out of thin air, so we can do a classical solution. Using $$E=1/2mv^2$$ we find that the energy is $$1/2 \times 100 \times 119916983^2 = 7.19004143 \times 10^{17} ~\mathrm J$$. Using my favorite page on the internet, Orders of Magnitude (Energy), we see that that's on par with detonating the Tsar Bomba, the largest nuclear weapon ever built. It's 6 orders of magnitude off from the energy released by the Chicxulub meteor event that is believed to have caused the extinction of the dinosaurs. Thus there's little risk of an extinction event. That and hitting a planet by accident 4 light years away without trying is pretty hard! 1 answered Aug 26 '16 at 6:14 Cort Ammon 26.2k45488 Let's assume it travels at a constant speed of .4c (since we don't know anything about its acceleration) and its mass is 100kg. At those speeds, the relativistic effects are minor compared to the fact that I just totally made the mass up out of thin air, so we can do a classical solution. Using $$E=1/2mv^2$$ we find that the energy is $$1/2 * 100 * 119916983^2 = 7.19004143 × 10^{17} J$$. Using my favorite page on the internet, Orders of Magnitude (Energy), we see that that's on par with detonating the Tsar Bomba, the largest nuclear weapon ever built. It's 6 orders of magnitude off from the energy released by the Chicxulub meteor event that is believed to have caused the extinction of the dinosaurs. Thus there's little risk of an extinction event. That and hitting a planet by accident 4 light years away without trying is pretty hard!