# Could Earth Capture a SECOND Satellite the size of the Moon?

1. Dec 16, 2013

### Taymith

I'm doing a research paper based on mining asteroids or near earth objects.
I was wondering, could we pull/move a relatively large asteroid About half as large as the moon could we use is as an anchor to launch missions?
Thanks!

2. Dec 16, 2013

### RonanB

http://www.universetoday.com/92022/earths-other-moons/

Based on this article there have already been other natural satellites of the earth in the past. The question is whether it would be possible to keep the satellite in orbit for long enough to mine or launch missions from it.

3. Dec 16, 2013

### glappkaeft

There are no asteroids half as large as the moon so that would be a big no.

4. Dec 16, 2013

### RonanB

True all the asteroids in the Asteroid Belt would not equal the size of our moon. I was taking the questions as a theoretical one.

Why couldn't you have the missions launched from out moon itself instead?

That would seem more feasible.

5. Dec 16, 2013

### Bandersnatch

I think capturing something half the size of the Moon, whether by mass or by radius, is pretty close to crazy talk. Adding any sort of useful ΔV against that much inertia would be beyond daunting.

For more sensible-sized asteroids, have a look at these:
http://www.nss.org/settlement/asteroids/capture.html
http://www.kiss.caltech.edu/study/asteroid/asteroid_final_report.pdf

Also, the largest asteroid is Ceres, with less than third of the Moon's radius and 1/100th of its mass.

6. Dec 17, 2013

### snorkack

There are, since Pluto was demoted and branded with a number.

7. Dec 17, 2013

### Bandersnatch

It was demoted to dwarf planet, not to asteroid, though.

8. Dec 20, 2013

### mrspeedybob

There aren't any such objects in the solar system but there may be such objects in interstellar space with trajectories that will bring them through our solar system at some time.

9. Dec 20, 2013

### TumblingDice

@Glappkaeft: Good point.

@Mrspeedybob: Nearly 70% of the mass of the interstellar medium is made up of hydrogen atoms. The rest is mostly helium and heavier atoms, molecular clouds, and dust. Asteroids orbit the Sun, and most of them are inside the orbit of Jupiter.

10. Dec 20, 2013

### SteamKing

Staff Emeritus
11. Jan 8, 2014

### nikkkom

Correction. Most *known* asteriods are inside the orbit of Jupiter.

Kuiper belt is quite farther out than Jupiter.

While we detected many sub-kilometer object inside the orbit of Jupiter, in KB we only detected some of the largest stuff - and already we know about six objects larger than Ceres (>1000 km in diameter). It looks like in reality, "most asteriods are outside of orbit of Neptune".

12. Jan 8, 2014

### syedbau

Thank you for posting important links! These are really helpful to know.

Best Regards!

13. Jan 8, 2014

### syedbau

You are right. :)

14. Jan 8, 2014

### TumblingDice

That doesn't make sense. If there were more *unknown* asteroids, how could you know?

That's quite the opposite of what NASA/JPL indicates and Wiki clarifies
Whatever the bodies outside of Neptune are called, most Kuiper belt bodies are made up of ices. Those wouldn't remain solid anywhere near Earth.

15. Jan 9, 2014

### snorkack

Most of them are not massive enough to melt near Earth.

16. Jan 10, 2014

### bahamagreen

When you ask, "Could Earth capture..." you have to add more about whether it is being guided with direct manipulations (meaning space tugs or whatever), or whether (at least in the final approach) it is allowed to try to insert into Earth orbit "hands off".

With enough technology, I think it could be done with direct manipulation.

But, another way of interpreting the question would be, could we manipulate the motion of a distant large object so that it naturally approaches the Earth and inserts into a stable orbit? This is similar to asking if a passing large body could enter the solar system, approach the Earth, and adopt a stable orbit.

For these later cases of the object "passively" (at least for the final approach ) inserting into a stable orbit without direct additional manipulation, I'm not so sure... especially if the body is coming from higher than the Earth's orbit around the Sun.

Imagine an object that is already in stable Earth orbit... now imagine how you would move it out of that orbit - you would have to apply an acceleration to do so. To me I think this means that the body had to decelerate in order to insert into the orbit...

What would cause a body approaching the Earth and increasing attraction with approach to decelerate other than direct manipulation?

17. Jan 10, 2014

### Staff: Mentor

Large satellites orbiting planets have some interesting properties when it comes to the dynamics of the system. I believe I have read somewhere (possibly even on PF) that lack of other satellites of the Earth is due to the fact Moon interferes with their orbits, effectively ejecting them into space.

IOW, it is not just a matter of putting something on the orbit, it is also a matter of keeping it in that orbit and not destroying the already existing system.

18. Jan 10, 2014

### Keln

This is what I understand to be a major "problem" in having a second large satellite in the Earth system. As moons go, our Moon is quite dense and massive, and simply inserting a large satellite into the system isn't going to fly. Small objects for mining and the like seem feasible, since they could possibly be constantly manipulated to keep them in orbit, but putting, say, Ceres into orbit, not even considering the energy required to do that, seems destined for failure over the long term. The interaction between the Earth, the moon, and the smaller, yet large satellite would, I think, end up with the smaller object getting booted out.

19. Jan 21, 2014

### tony873004

I think we have discussed this here before. I like simulating this stuff. There are no prograde orbits around Earth exterior to the Moon that are stable. Any object put in any prograde orbit exterior to the Moon gets ejected from the system in just a few orbits. Retrograde orbits exterior to the Moon are fine. But they're not too common in the solar system. Prograde orbits interior to the Moons are stable. But the Moon was once much closer to Earth. It would have swept that area (volume?) clear as it migrated to where it is now.

20. Jan 26, 2014

### |Glitch|

I agree. Just out of curiosity, I created a simulation using the Sun, Earth, Moon, and Ceres. The only way I could keep Ceres in a stable orbit was by placing it in Earth-Moon Lagrange Points 3, 4, and 5. Everywhere else (including Earth-Moon Lagrange Points 1 and 2), Ceres either crashes into the Moon, or gets flung out of the Earth-Moon orbit.

In absolutely every case (even with Ceres in the Earth-Moon Lagrange Points 3, 4, and 5), the extra mass was sufficient to slightly slow down the Earth's velocity around the Sun (from 29.8 km/s to 29.3 km/s), causing Earth's orbit to increase slightly (1.02 AU SMA) and increasing the length of a year (adding about 8.4 hours per year).

The amount of energy required to move an object the size of Ceres would be huge. Well beyond all the energy of our combined nuclear weapons. However, once in either the Earth-Moon Lagrange Points 3, 4, or 5, no additional energy would be required to keep it in a stable orbit.

21. Jan 26, 2014

### D H

Staff Emeritus
Of course an object at the first and second Lagrange points is going to be booted out of the system. Those points aren't stable. So is the third. Only the two triangular Lagrange points are stable.

That you couldn't find any stable orbits suggests that you might just be seeing an artifact of your numerical integrator rather than something real. If you are using the classical fourth order Runge-Kutta integrator, you can almost certainly blame the booting on the integrator rather than reality. The same applies for low order symplectic integrators. The large truncation errors associated with these techniques means the results are pure fiction after a few dozen orbits. A good integrator for the four body problem (the Sun is going to be a big perturber) is hard to come by.

There has been a lot of interest of late in selenocentric distant retrograde orbits (google that term). These orbits appear to be stable for hundreds of revolutions given the right evection angle (between -90 and +90 degrees, more or less). Some are very distant indeed. At a distance of 70,000 km from the Moon, these orbits are anything but elliptical when viewed from the perspective of a Moon-centered frame, and they are rather exotic when viewed from the perspective of an Earth-centered frame.

22. Jan 26, 2014

### Staff: Mentor

Most known planets are in our galaxy, but we know our galaxy is not unusual in that respect - other galaxies have stars with planets as well, we just don't have the technology to detect them (with a few candidates as exception).
In the same way, you can know that the Kuiper belt has many objects we did not discover so far, as our telescopes are not good enough and/or did not scan the whole sky with the same precision.

23. Jan 26, 2014

### tinypositrons

It would screw up our lives. That's all I can say.

24. Jan 26, 2014

### |Glitch|

I did not consider putting Ceres in a lunar orbit. When I place Ceres 70,000 km in a lunar orbit (30.1 km/s with an eccentricity of 0.014, or 19.1 days per orbit around the Moon), using RK4, in about 4.5 (~90 days) orbits the eccentricity of Ceres' orbit increases to 0.56 and is flung out of the Earth-Moon orbit into an orbit around the Sun. Furthermore, the eccentricity of the Moon increased, bringing the Moon almost 100,000 km closer to Earth at its semi-minor axis.

25. Jan 26, 2014

### D H

Staff Emeritus
You can't use RK4 and expect anything remotely resembling reality after even one orbit. It is not a stable integrator.