# (How) would we detect a passing-by relativistic object?

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1. Nov 27, 2015

### xpell

Hi, I'm really curious about this one. :) Let's imagine a massive, large object with a very low albedo traveling at a slightly sub-c speed enters the solar system and passes through it before leaving on the opposite side. Please, would us earthlings detect it as it approaches, passes by and leaves, and if so, how and when? Would we optically see it and what aspect would it have? (I'm thinking in a passing-by object, not one hurling towards the Earth, which I assume we wouldn't detect prior to impact)

Thank you all in advance! :)

Note: Since I think this question can be kind of "interdisciplinary", I've posted it here in the general forum. If you mods think it's more appropriate in some other sub-forum, please feel free to move it, no qualms. ;)

2. Nov 27, 2015

### Buzz Bloom

Hi xpell:

From the way you have posed the question, I assume you want to allow no awareness of the object by means of photons. (If I am wrong about this, I would suggest that radar might detect the object.) With this assumption, together with the assumption that the object doesn't hit anything the only way the object might be noticed is if it had sufficient mass so it would gravitationally alter the motions of known objects unexpectedly to a noticeable extent.

Regards,
Buzz

3. Nov 27, 2015

### xpell

Hi, Buzz, thank you! Yes, I am mostly interested in the "intrinsic" emissions or reflections of an object at such speed (if there are any) that would allow us to detect (by wathever means) or even see it optically (with the naked eye or telescopes) as it passes by. But I'm also open to other realistic possibilities. ;) (Like emissions from the continuous impact with the interplanetary medium?)

Radar radioastronomy sounds quite fine to me, but I understand it has a limited range, and further we would only detect it until it's very close to the nearest "passing-by point", wouldn't we?

(And not, the object I was thinking it is not large enough to cause obvious gravitational anomalies... but I'm also open to read about what would happen if it was)

Last edited: Nov 27, 2015
4. Nov 27, 2015

### Buzz Bloom

Hi xpell:

I hope you won't mind my asking. Are you writing a fictional story in which such an object passes through the solar system?

Regarding "radar", I don't know the limits of the technology, so I have to pass on commenting about
Re
I assume that the relativistic speed is expected to play a role in making detection of the object difficult. That is, at a non-relativistic speed, you might expect the object to be easily detected. Is that correct?

Regards,
Buzz

5. Nov 27, 2015

### Staff: Mentor

The solar system is only about ten light hours across, so we wouldn't have very long to detect such an object.... but I'd expect that such an object would be very hot from interaction with blue-shifted radiation and the odd dust particle and we'd be easily able to see it by its own light. Remember, "low albedo" is just another way of saying "absorbs and is heated by the light that reaches it".

Round numbers: an object with a cross-sectional area of 100 square meters (ten meters on a side, more like a spaceship than a wandering planetoid) passing through the solar system would encounter something on the order of 1022 stray gas particles (mostly hydrogen and helium). At 99.99% of light speed collisions with these would release energy roughly equal to the explosion of a one-megaton nuclear weapon. Even though it will be spread out over a ten-hour transit time, we'd probably notice that.... and that's ignoring the possibility that the object encounters a few grains of sand or dust that will produce some serious fireworks.

(It would be wise to check my calculations above... I've been known to misplace a few factors of ten in doing this sort of back of the envelope calculation.)

6. Nov 27, 2015

### xpell

I don't mind at all and you nailed it... ;-) but I'm not writing it, right now I'm just considering the possibility. If I write it, I'd like it to be as real as possible, and a fair part of the plot would revolve around the "human interaction" (via detection) of such an object. So before starting to even devising the plot, I'd need to know if such detection would be possible and probable, how could it happen and if such "pass-bys" could have happened many times in the past without us having detected them before or there would be a "clear, obvious signal" giving away such passages to our detectors (any of them), or if it would be totally serendipitous. :-)

Well... yes, more or less like any other low-albedo object (let's say of about 1 to 5 miles in size) traveling through the Solar System.

Best regards!

7. Nov 27, 2015

### xpell

Hi and thank you, Nugatory. :-) Yes, I thought about this (even if maybe it would start interacting with the interplanetary medium after crossing the heliopause...?), but I assume that given its almost-c speed we wouldn't be able to detect until it's really close to the maximum approach point or even after it is already leaving... for a few hours. Please correct me if I'm wrong!

Will do!!! ;-) BTW, what kind of emissions would be caused by this interaction? Infrarred? X or gamma rays?

8. Nov 27, 2015

### Staff: Mentor

We'd easily see it coming, although it would arrive not long after we first saw it - when the light reaches us the approaching object will not be far behind.
At relativistic velocities, even the very weak cosmic microwave background of interstellar space is blue-shifted into very energetic gamma radiation - this is just the doppler effect acting at very high relative speeds. And inside the solar system there are many more sources of even more energetic electromagnetic radiation, and all of this will also blue-shift enough to transfer substantial amounts of energy to an object moving at relativistic speeds.

9. Nov 27, 2015

### Staff: Mentor

Using the speed of 99.99% from Nugatory's post, I get a relativistic gamma-factor of 70. To reproduce the number of particles, I have to assume a density of the interplanetary medium that is similar to the density close to Earth, but over 10 light hours. Something smaller is probably more realistic. Either way, I cannot reproduce the megaton estimate, I get ~30 ktons with Nugatory's number and ~1 kton using my own estimate.

Let's scale the object up to 300m x 300m front surface, or 1000 times the surface area of the spacecraft-sized asteroid (the object has been described as "large"), so we recover the megaton nuclear weapon. That is 4*1015 J, with a peak power of something like 1 TW (strongly depending on the minimal distance of the object to sun, the number is for roughly an Earth-like perihelion), about 1/1000 of the power that Mars receives from the sun (order of magnitude). It won't radiate everything as visible light, but Mars doesn't re-emit everything as light either. At a similar distance as Mars (note that Mars' distance changes significantly), its apparent magnitude would be 7.5 lower. There would be a reasonable chance to see the object with the naked eye (although as very faint object) if it crosses the inner solar system close to Earth. I don't know how frequent whole-sky surveys are done, but if any telescope would point in the right direction, it is hard to miss such an object.

If the object hits a grain of sand, I guess could be as bright as the moon for a brief moment.

10. Nov 27, 2015

### Staff: Mentor

Hmmm.... I took the diameter of the solar system to be $10^{10}$ kilometers which is $10^{15}$ cm. With 100 square meters or $10^6$ square centimeters of surface area in the spaceship that's a total swept volume of $10^{21}$ cubic centimeters. Assume ten particles per cubic centimeter and we're around .1 mole, which is $10^{-4}$ kilograms if it's all atomic hydrogen. Multiply by the gamma factor and I'm at $10^{-2}$, multiply by $c^2=10^{17}$ and I'm at $10^{15}$ Joules.

You're right, that's well under a megaton ($5\times{10}^{15}$ joules), and that's before allowing for the factor of 6 I lost going to moles and the additional loss from rounding gamme up from 70 to 100.

11. Nov 27, 2015

### xpell

Thank you all very much. :-) If anyone can think of any other way to detect such an object, I'd love to read about it too.

12. Nov 30, 2015

### newjerseyrunner

I imagine something traveling at such fantastic speeds would have to push through the solar wind and interstellar debris, collisions at that speed should produce very high intensity light, we might be able to detect a streak of gamma rays against the cold background of space.

13. Nov 30, 2015

### Staff: Mentor

That's what we calculated in the previous posts.