WHICH IS THE MOST VULNERABLE BODY IN THE SOLAR SYSTEM?
In the beginning our Solar System resembled a cosmic pool table, in which planet-sized objects were flying about in all directions, sometimes resulting in the most enormous of collisions. It was chaotic. Thankfully things have since calmed down as the planets and moons settled into (more or less) uniform orbits.
Stability of this system ensures that although minor objects such as small asteroids and comets may sometimes come to grief on a planet's surface, most of the larger heavenly bodies have a reasonably secure future for aeons to come, until indeed the Sun itself exhausts it's supply of nuclear fuel.
However, though planets seemingly endlessly orbit the sun, and moons endlessly orbit their respective planet, some moons orbit rather too close. These include one of the most impressive, which I have already described. Triton has a decaying revolution and so, in about 100 million years, it will stray too close to mighty Neptune and it will break up.
Mars has two strange moons which are irregular in shape, and ultra small just a few kilometres in length. They may in fact be captured asteroids. Orbiting at only 6000km from the Martian surface, one of these objects Phobos is the closest moon to its parent planet in the Solar System (at least that we know of), and its fate is sealed. The orbit is reducing by nearly 2cm per year, and in 10 to 50 million years it is destined to break up or crash into Mars (with a thud).
Vulnerability of these moons is measured in terms of self destruction. But there is another way in which vulnerability can be gauged - the value of loss. If Phobos crashes into Mars in 50 million years, a lump of rock will be lost. we're talking one lump of rock in 50 million years time.
But on one planet a huge amount more than rock could be lost within mere decades. Thousands of living species. The most vulnerable planet in terms of the scale of what could be lost within the lifetime of people alive today is EARTH.