How astronauts attain orbital speeds for space-walks

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

08 July 2007

A question from Peter Wanjohi was left unanswered last week. He had asked; “…[if] the speed of a satellite [is] attained after the satellite is accelerated by an external agent e.g. a rocket, … who accelerates the space-walker[s] to attain the same speed necessary to enable them repair or build the stations while ‘floating’ around it?”

To get the answer, we should first ask how the astronauts get to the orbit in the first place. Of course they go there in a Space Shuttle. As they approach the orbit of the International Space Station, they accelerate the Shuttle’s speed to 28,000km/h – the same as that of the ISS.

Now, don’t forget that all this time the astronauts are inside the Shuttle. So they are also moving at its speed. (If you are in a car cruising at 100km/h, you will also be doing 100km/h!) Thus, when they catch up with the ISS, everything will be hurtling at 28,000km/h – that is, the ISS, the Space Shuttle, the astronauts inside it, its cargo etc.

Therefore, when an astronaut goes outside to do a space-walk, he will be flying at the orbital speed of 28,000km/h. Since there is nothing to slow him down (it’s a virtually a vacuum up there), he will float around weightlessly. Remember; this velocity is enough to counteract the gravitational pull.

As Peter rightly points out in his letter, this situation is similar to what we see when we throw a stone vertically upwards: It falls back where it started from. Even though the Earth is rotating at a very high speed (1,700km/h at the equator), the stone is not left behind because at the moment of throwing, it is already moving with the planet.

The reader also mentions that “Relativistic physics says gravitation is due to the warping of the space-time continuum by the presence of a mass.” He adds: “The Earth creates a ‘dip’ around itself, thus the ‘falling-down’ effect.”

Well, that is true, but the statement does not paint a clear picture of how this happens. The “space-time continuum” is phrase used in reference to the combination of length and duration. It captures the fact that when objects move in space, they also traverse through time. If you think about it, it is quite obvious!

Ordinarily, when an object is moving at constant velocity, it covers equal lengths of space in equal durations of time. If the space-time continuum is uniform, the object will travel along a straight line.

However, if there is another mass in the vicinity, the space-time continuum will no longer be uniform – it will be “warped”. Consequently, the only way the object can cover equal lengths in equal durations is if it follows a curved path.

One of the startling outcomes of this idea is that the moving “thing” does not have to be an object with mass. Even a mass-less entity like light will experience the same curving of its path. This is because it also travels “through” the warped space-time continuum… Mind-boggling, isn’t it?