# Q: How big does an object have to be to gravitationally attract a Human or have a molten core?

The original question was:

I was reading up on Hyperion, Saturn’s moon, and one of the least dense objects in the solar system, and it hit me – what is the critical point for gravity to attract a human? In other words, if you were to make a big pile of rocks in space, at what mass would they drag a human towards them?  And if you kept adding rocks to the pile, at what mass would a heap of rocks create a hot molten core?

Physicist: The answer to the first question is a bit smart-assed.  The object needs to be bigger than you, or (most people would say) you’d be attracting it.  No matter how small an object is, if it has mass, it has gravity.  Another question might be, “How big does something have to be so that you can’t jump off of it?”  It turns out that it needs to be fairly big.  The Little Prince would have gone flying off of his planet if he so much as twitched.

"The Little Prince". Bullshit.

Even Deimos (the smaller of Mars’ two moons) has an escape velocity of only about 12.5 mph, so with a good running start you could literally jump into space.  I figure 12.5 mph is about the fastest that most people can muster in a pinch, so Deimos is about the smallest object that can hold people down, at about 8 miles across (8 miles on average, due to lumpiness).

It’s worth mentioning that “running” on something as small as Deimos is impossible.  With a gravitational pull of about 0.04% of Earth’s, the difference between Deimos’ gravity and zero gravity is academic.  You could easily pogo-stick on your pinky, and it would take so long to fall that you might lose track of which direction is down while waiting for the ground.

The main reason the size of an object is important to its core-moltenness, is because smaller objects radiate heat faster (proportionately) than larger objects, and larger objects have more nuclear fuel to work with.

As far as molten cores go, there are three main sources of heat: formation heat, tidal forces, and radioactive decay.

Tidal forces on a moon are similar to the crushing forces acting on an egg rolled on a table. In the case of Europa the end result is also similar.

Formation heat is just the left over energy you get when you let a few trillion gigatons of stuff fall together.  The formation heat of everything in the solar system was exhausted billions of years ago (except for Jupiter, which continues to slowly deflate and release heat.  Essentially it’s too fluffy).

Tidal forces only really apply to inner moons around gas giants.  The tidal forces have to be huge in order to melt the core just by “massaging” the moon in question.

The most important thing for a liquid core is a supply of radioactive material.  Given the amount of radioactive stuff left in the solar system today (it’s been draining away for the last 5 billion years) an object needs to have a mass between about 1 x 1023 kg and 3 x 1023 kg (between 0.02 and 0.05 Earths, or around 70 million “Deimoses”), give or take.

So age, size, tidal forces, and density are just some of the many variables that go into whether or not a core will be molten.  In fact, given enough time Earth’s core will eventually run out of nuclear fuel and solidify.  But don’t get too concerned, the Sun should swell up and swallow us long before then.

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### 37 Responses to Q: How big does an object have to be to gravitationally attract a Human or have a molten core?

1. Carlos Palacios says:

“given enough time Earth’s core will eventually run out of nuclear fuel and solidify”

Are you implying that there is some sort of nuclear reaction keeping the center of the earth molten? Clearly, the earth does not have enough mass to power a nuclear fusion reaction due to the force of gravity. The earth would have to be closer to the mass of the sun in order for this to happen.

2. The Physicist says:

You’re absolutely right! The nuclear reaction in the Earth’s core is a fission reaction, while the Sun is powered by a fusion reaction. The reaction in the Earth is the radioactive decay of heavier elements, primarily: Potassium 40, Uranium 238, 235, and Thorium 232.

3. ripularora says:

is formation heat, tidal forces, and radioactive decay are the reason behind gravity? and why every thing which has mass has gravity what is the reason behind this force of attraction? and how the energy for nuclear reaction can be provided by mass ?

4. ripularora says:

how formation heat is formed? is it conversion of mass into energy ?

5. The Physicist says:

The heat of an object doesn’t have anything to do with how much gravity that object has. As for why matter creates gravity at all; who knows? We know the nature of the relationship between matter and gravity in detail, but we still don’t know why that relationship exists in the first place!

6. aaron solomon says:

does an object need to be a specific shape (like a sphere) in order to create a depression in time space i.e. attract smaller stuff, and if not than why isn’t (or are there) little beads of microscopic matter sticking to my body? and what of the other object on earth that are more dense than i am? maybe the earths gravity intervening? the relationship of gravity to an object must be proportional!? but can those same conditions exist while on earth or do i have to be in zero gravity to observe the phenom?

7. The Physicist says:

Any shape works!
If you were in space, you would slowly accumulate tiny beads of ice and rock. You’d have to be careful not to move much, or you’d knock everything away (the gravity of a person’s body isn’t very strong). And you’re right: we still exert a tiny, tiny gravitational pull here on Earth, but the Earth itself exerts so much more that our paltry pull is overwhelmed.

8. aaron solomon says:

🙂 exciting!

9. aaron ssolomon says:

if i get a superconductor and an electro-magnet put them together “just so” and maybe have some gyroscopic effect, can i create a 3D rc machine. what are the physics of it? can it be airborne if i spin a coil or core within a coil in one direction for up and reverse for down?

10. The Physicist says:

Unfortunately, even with fancy physics you’re still bound by “for every action there’s an equal an opposite reaction”. Everything that flies needs to be held up by something. Either it’s lighter than air (blimp), or it pushes matter downward (helicopters, rockets).

11. aaron ssolomon says:

how does quantum trapping work? might this be useful to make something that can be engineered for airborne activity?

12. The Physicist says:

There’s a (derivable) law from electromagnetism called “Lenz’s Law” that say that any change in the magnetic field through a conductor results in a current, that creates a new magnetic field, that opposes that change. Usually the “responding” field is substantially smaller than the original field because energy is lost to resistance. However, in superconductors resistance is not an issue, so the responding field is always exactly strong enough to resist the change (the magnetic field is “trapped”).
Although, for smaller samples and fields, you can physically grab the super-conductor and move it (change the magnetic field through it).
That said, even when one of these super-conductors is hovering over a magnet, it’s not weightless. It’s pressing down on the magnet.

13. John DuBois says:

theoretically wouldn’t it be possible to create a artificial gravity field if u could spin liquid mercury in a device possible with switching the charge on electromagnets so spin in a circular fashion inside some kind of spear that contained the liquid metal and would cause the friction and movement if spun at high enough speeds to create it’s own field and then you could use the right sound waves to move the magnets or liquid witch would then move the whole field and whatever is trapped in it? so then u could fly and move through space at any speed u can move the core. have a lot of other technology ideas also and really looking for someone to talk to and work with to create and put together more ideas and figure things out. i can go deeper into this too. but need the right help to get it out there and not just stolen.

14. Gaurav Bhattacharjee says:

Ok, I’ll listen to your ideas, John DuBois. Add me to your circles in G+, and I’ll do the same.

15. Arthur says:

Hi, there!
Finally, I found the source for my childish questions.
EXAMPLE: I constructed a huge metallic ball (let’s say, 100 ft in diameter and 5 tons wieght) and sent it to space, far away from the Earth and the moon in order to minimize their gravitational effect.
QUESTION: Will smaller objects “orbit” my artificial “planet” that i sent to the space? If not, why and how huge the ball needs to be so that other objects could “orbit” it?
QUESTION2: Why don’t comets crash directly into the sun but make a narrow spin instead? Is it somehow related to the dark matter?

16. The Physicist says:

@ Arthur:
For #1, that’s a thing that happens fairly often in exactly the situation you describe: far from other, bigger stuff.
For #2, the effect of dark matter inside of star systems is completely ignorable. There are two reasons that things don’t crash into the Sun. First, because of centrifugal forces, things usually only fall into the thing they’re orbiting if they start at a near-dead-drop. Second, since orbits repeat, if a thing was going to fall into the sun it almost certainly would have already. That said, it does happen.

17. Writernaut says:

Hi, my question is how small something could be before it’s core would solidify? I’m writing a comic and the planet it’s on is a fair bit smaller then earth making travelling from one pole to the other much faster, I’m wondering how much smaller this Goldilocks planet could be before it’s core would stop being molten, and for how long would it’s core keep going if it was X times smaller then earth.

18. Dominique Potvin says:

Hello !

If we want to reactivate Mars molten core, so we can have a complete magnetic field there, could we do it by giving Mars a big moon ? How much M3 of material that would need ?

Thank you !

19. Katie says:

Hello. If any object that has mass has gravity, then does something as small as a peach, or a big ball, have it as well? Is there any household object that would exert a gravitational “pull” on any smaller household object? For example, if I have a building, would a grain of sand be pulled towards it? Why or why not? Thanks in advance.

20. The Physicist says:

@Katie
Absolutely! If you have equipment sensitive enough you can detect gravity all the way down to peach level. We’ve been doing that for a little over 200 years.

21. Jayden says:

Thank you for answering this, but I was wondering how big something would need to be to actually move a person towards it. By that, I mean a drag noticeable by the human eye. Would it be able to make an object like this out of astroids and place it on earth? If so, would this have any effect on the grand scale of our orbit? If there was a space vehicle this large (somehow this vehicle is a sphere), would it be able to pull people into space simply by coming near the earth?

I know its alot of questions, but it seems like possibilites are endless.
Not that I want to suck people into space or anything.

22. The Physicist says:

@Jayden
In order to pull things off of the Earth, you’d need something roughly Earth sized or bigger. The attempt would do a lot of damage.

23. Jonathan Pfeffer says:

My question concerns an accretion disk around a new forming star. I see how the initial clumps develop from small pieces of dust, but would you not end with lots of clumps at different positions within the orbits. What is the process that causes multiple large clumps in the same orbital lanes to coalesce into a configuartion where only one large planet occupies each large multi million km wide orbital lane.

24. Larissa says:

“We know the nature of the relationship between matter and gravity in detail, but we still don’t know why that relationship exists in the first place!”
So we don’t have an answer for the question “Why does mass attract mass” for sure? I mean, if I ask you “Why does this pencil falls?” You would say something about gravity and stuff, but we don’t have an answer for why gravity does it. Am I wrong?

PS: OMG, this is the best thing I could imagine to get rid of my tons of doubts!!!

25. James Paterson says:

My reference may make you laugh but, i saw an episode of “Miles from Tomorrow” where two planets closely orbited each other. One was dry and barren and the other an ocean planet, when the dry planet lined up in conjunction with its sun, gravity pulled the entire ocean through space onto the dry planet. Thus the effect of two planets swapping the same ocean back and forth. Brilliant piece of sci-fi for a kids show but is this theoretically possible???

26. The Physicist says:

@James Paterson
Not in a way that would be good for a story. If gravity can pull something off of the surface of a planet, then the surface is coming too. This sort of interaction would be more than a little destructive.

27. Michael Hume says:

Looking for agreement on my theory that the Earth had very much less gravity in the days of the dinosaurs. I thought this would be obvious and clearly explained how the huge beast could sustain their weight and pterodactyls could take flight. Over Billions of years the increase in gravity may have been a factor in their demise.
On todays rough calculations Earth receives about 60 tons of space dust per day; I’m no mathematician but my estimate is that this would mean the Earth weighed 5.3 trillion tons less during the early Triassic period. Of course the estimate of space dust falling to Earth may be slightly out; and furthermore may have been significantly higher in ancient times. For your consideration.

28. Bob says:

In an isolated system in zero gravity, what would be there largest thing that would successfully orbit a human body? And at what distance? Also in the same system what distance would two human bodies have to be apart to successfully orbit each other?

29. William says:

If the universe is expanding ‘like a balloon’ then don’t the stars and plants get bigger themselves as well? Would the very room were sat in be expanding to including everything in it?

30. Dahlia says:

So which would have a stronger gravitational attraction to the earth, (assuming there is an equal distance between the earth and the object), a person or an ant?

31. Emarric says:

How massive would an object have to be for a person to stand on it’s vertical surface on Earth? How dense could such an object be and not break through Earth’s crust?

32. Watch It Only says:

I think Moon is big enough and is having it own gravity but its not attracting the space like earth is doing. Why is this difference? I think its the matter properties some attracts some and not others and the same is for earth and moon. Like magnets can attracts steel.

33. Paul F Austin says:

What would be the smallest size of ‘craft’ which would be able launched from Earth to travel to and land on Mars?

34. Akash says:

the principle of relativity says that for every frame of reference speed of light remains same but if we let a man moving with a velocity ‘v’ with a ray of light than the velocity of light with respect to man is c-v but this contradicts principle of relativity? how? what’s the real matter

35. Curtis says:

@ michael hume.

The earth is currantly losing mass. It gains 40000 tons in space dust but it is off set by losing 96000 tons in gas through atmospheric escape.

36. Paul Bartlett says:

So, do we have to recalculate the speed of light acounting for the gravitational pull of giant stars and “black holes”? What happens when a particle is accelerated to the point of hitting itself after an 11 mile orbit? Is that bending time?