Why does the Parker Solar Probe slow down as the distance from the Sun increases?
Image credit: Wikipedia user Phoenix777, CC BY-SA 4.0
Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It only takes a minute to sign up.
Sign up to join this communityWhy does the Parker Solar Probe slow down as the distance from the Sun increases?
Image credit: Wikipedia user Phoenix777, CC BY-SA 4.0
Why does the “parker solar probe” lose speed as the distance from the sun increases?
Because energy and angular momentum are individually conserved quantities in the two body problem. Except for where the Parker Space Probe has close fly-bys with Venus, the gravitationally interactions between the Parker Space Probe and the solar system are very closely modeled as a two body problem (the Sun and the probe), plus very small perturbations from the planets.
One way to express conservation of energy in the two body problem is the vis-viva equation, $$v^2 = \mu\left(\frac2r - \frac1a\right)$$
where
Note that the mass of the Parker Space Probe is so much less than that of the Sun that one can drop the Parker Space Probe's mass from the expression $\mu = G(M+m)$, resulting in $\mu = GM_{\text{sun}}$.
Note that the only variable on the right hand side of the vis-viva equation is the radial distance. As radial distance increases the square magnitude of the velocity vector (and thus the magnitude of the velocity vector) decreases.
Without mathematics, conservation of energy dictates that the sum of an orbiting body's kinetic energy and gravitational potential energy must remain constant. As the orbiting body moves further from the central body, the orbiting body's potential energy increases, which means its kinetic energy must correspondingly decrease. This in turn means the orbiting body's velocity must decrease.
Quite simply, the sun's gravity is pulling on the space probe at all times. As the probe moves away from the sun, the force of gravity pulls it back in, slowing it down. As the probe moves toward the sun, the force of gravity continues to pull, speeding it up. Any object orbiting the sun is always accelerating toward the sun - when that acceleration opposes motion, the object slows down, and when it's in the same direction as motion, the object speeds up.
It's no different from throwing a ball into the air and seeing it slow down as it rises, reverse direction, and then speed up again as it falls, except the primary gravitational body in that case is the earth and not the sun.
The simplest explanation is that the satellite generally obeys Kepler's second law of orbital mechanics:
A line joining a planet and the Sun sweeps out equal areas during equal intervals of time
When the satellite is further away from the sun, the area swept out remains constant only because the satellite is moving more slowly.
The physical answer, as others have pointed out, is that the satellite trades out kinetic energy for gravitational potential energy as it travels "up" the Sun's gravity well. This is no different from gradually slowing as you coast up a hill on your bike (although you are climbing up the Earth's gravity well not the Sun's).