# “Stationary” Space Probe

Could we place a satellite in the wake of the Earth's orbit so that when the Earth completes its orbit of the sun it would effectively meet back up with said satellite?

• Can you elaborate what you mean by "place"? Do you mean stationary or could it be moving? Is the difficulty of placing it there part of the question, or only keeping it there? – uhoh Aug 13 '16 at 14:06

No. At least not the way you're thinking of.

The reason that the Earth orbits around the Sun is that the Sun's gravity is always pulling it inwards. This is not unlike an object being swung around in a circle on a rope: if you do this, you always have to pull the object inwards to maintain the circular path.

A satellite that was somehow made "stationary" in the solar system would still feel the Sun's gravity, but wouldn't be moving around in a circle. So it would just fall straight into the Sun. You could, if you wanted to, put a satellite on an elliptical orbit that had a period of one year. This satellite would leave Earth, travel on its own orbit through the solar system, and then would happen to meet up with Earth a year later. But this wouldn't be a "stationary" satellite.

• Or unfurl a very (very) large reflective solar sail while simultaneously accelerating (braking) with a total delta-v of 30,000 m/s so that the force of the Suns photons and gravity cancel. – uhoh Aug 12 '16 at 20:01
• That would take one hell of a solar sale... – TaylorAllred Aug 12 '16 at 20:43
• @TaylorAllred Solar radiation pressure on a reflector at Earth's orbit is 9E-06 N/m^2. Gravitational force is GMsun/$\text{a}^2$ where a = 1AU = 1.5E+11m is 6E-03 N/kg. To set the two forces equal you need 670 m^2 for every 1kg of satellite+sail. If most of the mass were the sail, that would be about 1.5 microns thick, about 1/8 as thick as plastic wrap. Yep it's a big sail, but not impossibly big. – uhoh Aug 13 '16 at 3:38
• @TaylorAllred don't know if you are interested, but this answer and associated comments may be of interest. – uhoh Aug 13 '16 at 12:49

No.

If you place a space probe in the wake of earth, outside earth sphere of influence, it will just follow the earth, orbiting at a similar speed, never reaching the sun.

You cannot orbit the earth and remain in it's wake (you wouldn't be orbiting).

To reach the sun from earth, you would need to cancel the speed of your orbit around the sun, not "orbiting in it's wake"

• Earth's L5 point would be a trailing orbit, though I believe L4 and L5 are not particularly stable. – BillDOe Aug 12 '16 at 20:20
• @BillOertell: Yes, but a satellite placed at L5 (or L4) wouldn't return to Earth after a year in the way that the OP is envisioning. – Michael Seifert Aug 15 '16 at 13:18

Yes, but it wouldn't be a traditional satellite.

The Earth is constantly being pulled towards the Sun by the Sun's gravity. It orbits because it's also moving sideways fast enough that gravity only acts to bend its motion into a circle. If you launched a satellite in a way to cancel the speed it had from being on Earth, it would fall into the Sun in short order.

Since your probe isn't able to stay out of the Sun by moving sideways, you need some other way to deal with the Sun's gravity. Fortunately, there is such a way: the solar wind is a constant flow of high-energy particles moving away from the Sun, while light itself can exert a force on objects. You can harvest these forces with a magnetic sail or solar sail to let your probe stay in place while waiting for the Earth to meet up with it again.

• The "Could we place a satellite..." part is also a tough one - how to dump 30,000 m/s of delta-v from the earth's orbit first. – uhoh Aug 13 '16 at 14:04