No, an objects mass only increases very near the speed of light, at much slower speeds this is not true and even close to the speed of light an object is only difficult to accelerate , it does not naturally slow down.
This is nearly, but not exactly correct. Bodies traveling even at very low speeds relative to an observer. For example, a 100 kg man walking at a typical speed of 1 m/s actually masses
That is, he gains about 625 nanograms - about the mass of a single human egg cell, or a small grain of sand.
Even an object very close to the speed of light will continue on at that speed until an outside force acts on it. In theory it could coast at 99.999999999% of the speed of light forever.
This is true only if the object is traveling in a perfect vacuum.
Interstellar space contains has a density
of about 1 hydrogen atom per 1 to 10 cubic centimeters, or about
From this, you can calculate the power per unit area of friction for a spacecraft traveling in it as
Calculating this for the lower density for craft traveling at 0.1 c (As Janus did in post #32 of “Quickest way to get to The Super Earth”
) gives a small, but troubling from a practical engineering perspective,
. Calculating it for 0.99999999999 c gives about 500 million!
To put this into some kind of perspective, if the spacecraft used a plug of water ice near absolute zero as frontal shielding, this shield would be vaporized at a rate of about 3.2 meters of thickness per second!
In terms of conservation of momentum, ignoring the practical engineering problem of shielding and cooling, the density of interstellar space would produce an acceleration of
Even for a tiny spacecraft massing only 1000 kg with an frontal area of
, traveling 0.99999999999 c, this calculates to about
. In about 1 year, it would slow by about 3 m/s, to a speed of about .99999999 c. Friction would never completely stop it relative to the interstellar medium, but over the decades and centuries, would keep knocking 9s off of its speed as a fraction of c. Using numeric approximation methods, we can calculate its speed in about 1 year as 0.99999999 c, 0.9999999 c in about 100 years, 0.999999 c in about 10,000 years, and about 0,999 c in about 10 billion years, roughly the current age of the universe.
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