A few quotes from various sources suggest there is quite a bit of material in the Oort Cloud, maybe enough to threaten interstellar space travel: “The Oort cloud is an immense spherical cloud surrounding the planetary system and extending approximately 3 light years, about 30 trillion kilometers from the Sun.” “The total mass of comets in the Oort cloud is estimated to be 40 times that of Earth.” “…researchers estimate, the total mass that ended up in the Oort cloud is somewhere between 0.6 and 2 Earth masses – far less than the common estimates of 10 to 40 Earth masses.” “Oort himself estimated that his cloud was composed of as many as 12 billion comets, and his estimate remains valid today.” Assuming minimums in the cloud’s extent, total mass and estimated comet size and number, what is the probability of a spacecraft transiting the Oort Cloud safely at a velocity of 0.1 C? I’m aware that enough shielding will be required to withstand particle collisions in interstellar space – space excluding the Oort Cloud. A bit ago I attempted to answer this question: Impact probability? Base Data: •One sixth of an estimated six trillion icy objects or comets are in the outer region with the remainder in the relatively dense core. The dense core that lies near the ecliptic plane. Use 1,000,000,000,000. (When I did this, I wasn’t using scientific notation. I recorded what my calculator generated.) •30,000 to much as two light-years. Use the average distance of 44,000 AU. •Within the cloud, comets are typically tens of millions of kilometers apart. Assumptions: •Place 1,000,000,000,000 comets in a spherical shell at a distance of 44,000 AU from the sun. Divide the surface area by the number of comets. This will give a density profile that can be used to estimate impact probability. •Radius in kilometers = 44,000 AU * (149,597,870.691 kilometers/AU) = 6,582,306,310,404 kilometers. •Area of a sphere = 4πR2 = 544460077987871840825190766.96785 km2. •Area of the sphere / number of comets = 1 comet per 544,460,077,987,871.84082519076696785 km2 or 544,460 trillion km2 •Taking the square root of this = 23,333,668.3 km or 14,499,541.5 miles. This represents the separation between comets in this model. Most extreme model: (2π * 6,582,306,310,404 kilometers) / 6,000,000,000,000 = 41,357,850,296,885.887234356537418759 km2 / 6,000,000,000,000 = 6.89 km separation. My calculations suggest that it is unlikely that the spacecraft will impact a “sizable” body. What does “sizable” mean? – I guess large enough to destroy the vehicle with one hit. I think, however, that this simple model misses considering the impact of smaller bodies created by collisions within the Oort Cloud. These smaller bodies and particles at 0.1 C might prove to be the greater hazard. Of course, it needs to be proven that comet collisions within the Oort Cloud have taken place. I’m also wonder if asteroids, which I assume have greater mass, exist within the Oort Cloud. I’ve seen nothing in the literature I’ve reviewed that suggests they may exist. I need better estimate of impact probability than I have calculated. If anyone has any thoughts on the composition of the Oort Cloud and it’s treat to interstellar space travel, please share them. A related question has to do with what we should expect about other stars like our sun. Will they more in like also have Oort Clouds?