Asteroids Missing Earth: What Means “Close Call”?

Every now and then you read about asteroids passing Earth in a certain distance but nobody gives you a feeling of how close such an encounter is. Allow me to fill in this gap.

Say an asteroid passes in a distance x from Earth, the radius of which we will call r. We now can ask how likely it is that an object that hits a disk of radius r+x also hits a disk of radius r provided that it any point on the larger disk is hit with equal likelihood.

The likelihood then is p = A(r)/A(r+x) where A is the area of a disk of the given radius, In other words p = πr²/π(r+x)² = 1/(1+x/r)². If we now define ξ=x/r (which is the distance in units of Earth's radius we get a quite simple formula: p = 1/(1+ξ)².

Using ξ is advantageous as it is a value you actually find in tables. Let’s try a couple of values; LD means Lunar distance and is the distance in terms of the average distance between Earth and Moon:

Distance in		ξ	p in %
LD	km
1.65614	636619.77	100	0.098
0.49684	190985.93	30	0.104
0.16561	63661.98	10	0.826
0.04968	19098.59	3	6.250
0.01656	6366.20	1	25.000
0.00497	1909.86	0.3	59.172
0.00166	636.62	0.1	82.645
0.00050	190.99	0.03	94.260
0.00017	63.66	0.01	98.030

Please note that the closer an encounter is the less meaningless this rough estimate becomes as the asteroid by no means randomly hits the disk of radius r+x but follows a clearly determined path.

Crowded Skies

I knew that there is quite a number of artificial satellites in space but I did not know how annoying they can be for people photographing stars. Take a look at this Sixty Symbols video introducing the Deep Sky Videos youtube channel.

Spy Satellites (from Deep Sky Videos)

The History of Our Universe Made Easy

Two videos from Potholer54 on the history of our universe and how scientists little by little learned about it.

The History of Our Universe Made Easy, Part 1

The History of Our Universe Made Easy, Part 2

Simply Explained: 2011 Nobel Price in Physics

Physics Nobel Prize 2011 - Sixty Symbols: “Studying supernovae and the accelerating universe - and a bet over a bottle of whisky. It's our take on the 2011 Nobel Prize in Physics, won by Saul Perlmutter, Brian P Schmidt and Adam G Riess.”

Physics Nobel Prize 2011 - Sixty Symbols

Searching Eos

Scientists have come up with the theory that the current Earth-Moon system is not a direct result of the collision of a Mars-sized object with the young Earth but that for some time there have been two moons that eventually and considerably after the collision themselves collided to form our current Moon. Chances are that NASA’s GRAIL mission will be able to answer the question whether Eos actually existed.

“Eos?” you ask? I must admit that this is just my idea for naming this former celestial body; however, in Greek mythology, Eos is the sister of Luna and the goddess of dawn. The name seems to be perfect as Luna is associated with the Moon and the lost celestial body dates back to the dawn of Earth’s history. Moreover, in antiquity Eos was not associated with an actual celestial body.

ScienceCast: Did Earth Have Two Moons?