# Jupiter as a Gravitational lens?

1. Nov 27, 2005

### Azael

Jupiter as a Gravitational lens??

I remember reading in a swedish science mag about a planed probe to the solar foci 550 AU from the sun. When there it would use the gravity lensing of the sun as a telescop.

But getting a probe out there and the cost of the project seems like to much of a obstacle to be overcom in the close future.

So my question is. Why not use jupiter in the same way? Is Jupiters mass just to small to give a lensing effect worth the money to put a probe there??

2. Nov 27, 2005

### Staff: Mentor

If the gravitational pull of the sun is so weak that it requires a 550 AU focal length for the scope, a scope using Jupiter as the lens would need to be thousands - maybe millions - of AU in length.

3. Nov 27, 2005

### Azael

doh!!
For some reason I though a scope could be closer with smaller masses. But obviously its like you say when I think about it :)

4. Nov 27, 2005

### SpaceTiger

Staff Emeritus
If I'm interpreting this correctly, then russ is right. Gravitational lenses are not, unfortunately, set up such that they can focus all parallel light beams to a point. However, you can focus all parallel beams within an annulus around the source. This is how we get the Einstein Ring effect (see here, for example). The distance of the focal point from the lens depends upon radius of the annulus (or, equivalently, the impact parameter of the rays) and the smaller this radius, the closer the focal point. However, if the annulus of light has a radius less than the physical radius of the sun, then it will simply be absorbed. Thus, we must move to a distance from the sun such that we reach the focal point of light rays passing within a solar radius from the center of the sun. Light is deflected from a point source by an angle:

$$\alpha=\frac{4GM}{rc^2}$$

where r is the impact parameter of the light and M is the mass of the lens. Doing a little trigonometry, we find that all parallel light beams coming in at r will be focused to a point at a distance:

$$d=\frac{r^2c^2}{4GM}$$

Plugging in the radius and mass of the sun, you can see for yourself that this gives about 550 AU.

Now the question is easy to answer. To find the distance you'd need to be from Jupiter to see this effect, you just plug in its mass and radius. Since the mass is about 1000 times smaller thans the sun's and the radius about 10 times smaller, you'd only have to go about 10 times further (~5,000 AU) to see the effect. On the other hand, the advantage of this would be that Jupiter is much dimmer than the sun and would be less of a contaminant in the observations.

5. Nov 29, 2005

### Chronos

Jupiter makes a better lense than the sun because the sun is blindingly brighter than jupiter across virtually all of the the EM spectrum. In other words, what ST said.

6. Nov 29, 2005

### Azael

Yes ST you are interpreting it correctly :) I was thinking backwards. Thanks for the explanation.

If anyone knows anything about that scope project that the mag I read writes about(I seem to remember it beeing called FOCAL). How are they going to get around the suns brightness?
Is this just wild speculations at this point or do we have the technology to use the suns lensing as a scope if we put it out there? In that case why are we waiting??

7. Nov 29, 2005

### Vast

Because it’s not the best space based telescope idea put forward. We don’t have the technology to pull this one off, or the money. I’d say it would take decades of R&D, plus you’re limited by the number of places you can actually observe. (Always being pointed towards to sun)

This might be of interest nonetheless.

8. Nov 30, 2005

### japam

supertelescope

wow ,so when this project be completed is gonna be THE supertelescope, i read it could detect continents on extrasolar planets of the size of the earth, and be billions time more precise in the cosmic background irregularities, i hope to live till that time