If you don't add sideways velocity, you can't deflect an asteroid.

This is similar to the three laws of thermodynamics:-

1) If you add a tether, the critical issue is the mass and moment that you add. Generally, this is negligible.

2) It is *Just* possible to add an *extremely* long tether with a moderate mass, and allow a velocity difference on connection to cause rotation about the centre of mass, and thereby effect a deflection

3) Try doing it... #

# to arrange a "miss" instead of a "hit" you will need to deflect the path of an asteroid by at least 1,000 km. maybe 10,000 km, given gravitational focussing. For a 20-km diameter asteroid (the size we start to REALLY worry about) with a mass of 1 e16 Kg, you could acheive this with a 1-tonne mass, on the end of a tether 1 e13 * 1,000 to 10,000 km i.e 1 e 16 to 1 e 17 km long.

The planet earth orbits at a distance of 1.5 e 8 km from the sun, so you would need a tether 1 e 8 to 1 e 9 AU in length.

or you add a weight comparable to the mass of the asteroid, but if you could do that, you could deflect the asteroid anyway.

In other words, he's talking out of his hat...