# Homework Help: Solar probe

1. Oct 28, 2014

### Batracho

Got this problem on a standardized test
Suppose scientists wanted to land a probe ON the Sun.
The probe is being launched from the Earth.

What direction should they aim launch the probe if they are launching it from the Earth?
1) Directly towards the sun
2) Directly away from the Sun — obv wrong
3) In the direction of Earth's velocity
4) In the opposite direction to Earth's velocity

My thoughts.
While Sun is certainly attracting the probe gravitationally, probe should still keep following the Earth's velocity, so it shouldn't land directly on the Sun, but rather sway off it, following direction of Earth's rotation. So, I chose 4), to, presumably, counteract earths rotation, letting solar gravity pull the probe directly towards it.

Thanks.

2. Oct 28, 2014

### collinsmark

Hello Batracho,

Welcome to Physics Forums! :)

You mentioned the Earth's "rotation." But maybe you chose the wrong wording in your explanation, and you meant revolution. I don't think the Earth's rotation has anything significant to do with this. [Edit: Well, maybe a little bit. But nothing of great significance is my point.]

On the other hand, the Earth's revolution around the Sun is of particular importance.

I interpret choice 4) as being, "In the opposite direction of Earth's orbital velocity. (As in the opposite direction of Earth's velocity relative to the Sun.)"

Is that what you meant?

Last edited: Oct 28, 2014
3. Oct 28, 2014

### Batracho

Yes, I think your correction is valid.
I meant Earth's revolution around the sun, not its rotation around its own axis.
And yes, I fully agree with your interpretation of 4)

4. Oct 28, 2014

### Staff: Mentor

(4) is right.

Well, right in the sense of the problem statement. We do not have the rocket technology for such a course. What you would really do is a much more complicated path, using the planets to change the course multiple times (probably including a fly-by at Jupiter).
Hitting the sun is harder than leaving the solar system.

5. Oct 28, 2014

### collinsmark

Then yes, your choice is correct. :)

The reason Earth orbits the Sun in a roughly circular orbit is because of its orbital velocity is such that the Earth's "centrifugal" force (around the Sun) balances that of the Sun's gravity (or if you don't like the concept of "centrifugal" force, you can look at it as the Sun's gravity provides the balancing centripetal force for a roughly circular orbit, given the Earth's velocity and distance; either way). If you were to launch a probe in the opposite direction of the Earth's orbital velocity it would obtain a more eccentric (i.e., more "elliptical") orbit around the Sun than the Earth, with a lower periapsis (closer to the Sun). In the special case where the probe's velocity with respect to Earth was equal and opposite that of the Earth's orbital velocity, then the probe's velocity would be 0 with respect to the Sun; and in that case the probe would fall directly toward the Sun.

So yes, the correct answer is 4).

[Edit: And what mfb said. :)]

Last edited: Oct 28, 2014
6. Oct 28, 2014

### Batracho

Yes!
I had a hunch — I googled this problem and got a feeling that the rocket tech needed to fully counteract earths revolution doesn't exist, and they were mentioning Jupiter fly bys for a gravitational tug.