The Parker Solar Probe just earned the title of the fastest-moving manmade object. Launched by NASA this past August, this robotic spacecraft is currently very, very near the Sun, on its way to probe the outer corona of our local star.

OK, I know you have questions. Let me just jump right into it.

How fast is it going?

According to NASA, its current speed is 153,545 mph (or 68.6 kilometers per second). But really, that just means super fast. It's nearly impossible to imagine something that fast when the fastest man-made stuff on Earth is perhaps a rail gun projectile at about 2.52 km/s. That means the Parker Solar Probe is traveling at a speed that is 27 times faster that the fastest thing we've got down here. Zoom fast.

What does this have to do with the speed of light?

Of course, light is even faster. Light has a speed of about 3 x 108 m/s (300,000 km/s). But why does that matter? You can't get an object up to (or greater than) the speed of light. Why? Let's start with an example. Suppose I have a force of 1 Newton and I push on an object at rest with a mass of 1 kg for 1 second (I'm using easy numbers). The momentum principle says that the momentum is the product of mass and velocity. Also, the force applied to an object tells us the rate of change of momentum. This means a 1 Newton force for 1 second gives a CHANGE in momentum of 1 kg*m/s (the change part is important).

This mostly works for super high speeds. The momentum principle still works as long as you use a better definition of momentum. It should look like this (in one dimension).

Rhett Allain

In this expression, the p is momentum (don't ask why) and the c represents the speed of light. Notice that as the velocity gets closer to the speed of light, you get a much smaller increase in speed for the same force. In fact, if the velocity was equal to the speed of light you would be dividing by zero—which is generally a bad thing.

Just to be clear, there aren't two models for momentum. You can always use the more complicated version of momentum. Try this: Calculate the momentum of a baseball with a mass of 0.142 kg and a speed of 35 m/s. First do this with the simple formula of mass times velocity and you get 497 kg*m/s. Now try it with the more complicated formula. Guess what? You get the same thing. I recommend using the simple formula whenever possible.