Detecting Asteroid Collisions: 'Oumuamua & Radio Telescopes

[|Glitch|]

Reducing the speed by a factor 2 (assuming you used 40 km/s) shouldn’t reduce the energy by a factor of more than 10 unless you are not considering the full energy.

I apologize, I should have been more clear. The 6.96 x 10¹⁵ Joules of energy (1.66 MegaTons TNT) released by the 30 meter meteorite traveling at 20 km/s was the energy released upon impact. Before entering the atmosphere that meteorite would have had 2.37 x 10¹⁶ Joules (5.67 MegaTons TNT) of energy. I have fixed my prior post.

 

[jim wolfe]

Radar simple answer -- disregarding the power issue which is huge --
if I send a RF pulse out to an object months away ( not light months, just 2 months away at say 30Km/s or 60Km/s ) the travel to and return path is some number of
seconds. in those seconds the Earth has rotated on its axis and the Earth has moved in it orbit, the antenna is no longer in a position to see the return, and no separate antennas is not going to solve the problem. Off the top of my head, let's says two months is 150 million KM - the distance to the sun - that is about 8 light minutes each way, consider how much the Earth rotated and what our orbital movement is in 15 minutes (hint the antenna is moving up to 450 m/sec and the Earth moved in its orbit 30 Km/sec

 

[mfb]

Radar is scattered back in all directions. The motion of the antenna on Earth is completely irrelevant. You don't even need the same antenna as receiver, you can use one thousands of kilometers away if you want. It is just more convenient to do both at the same place as the antenna points in that direction anyway.

Absolute motion doesn't exist anyway. You can use the antenna as reference frame (an inertial frame to a very good approximation) and then it doesn't move.

 

[jim wolfe]

Bistatic radar does work, but trying to do any real computation with side scatter radar reflections is challenging at best. Trying to do doppler on the return to see if the reflector was heading our way or even Earth orbit crossing i think is beyond the limits of current technology. Something light minutes away outside the main bean would have a terrible S/N ratio.

 

[roineust]

Would it be better in anyway to have a radar in outer space at a convenient location, be it some kind of Earth stationary orbit, Lagrange point or other?

As well, there is something i don't understand in the context of radars and detecting asteroids: In the Wikipedia entry for Albedo, under the title Astronomical albedo in the first paragraph, there is a reference to detecting asteroids Albedo by radar ('Radar albedo'). Why is a radar mentioned there and can such existing systems, contribute to asteroid collision early detection? Or perhaps radar albedo is a term byitslef, that actually has nothing to do with a radar machine? Or maybe it is only with asteroids of high metal composition, that radar can and are used for detection?

 

[mfb]

Bistatic radar does work, but trying to do any real computation with side scatter radar reflections is challenging at best. Trying to do doppler on the return to see if the reflector was heading our way or even Earth orbit crossing i think is beyond the limits of current technology. Something light minutes away outside the main bean would have a terrible S/N ratio.

We are talking about an angle of typically 0.1 millirad with respect to a direct reflection - it is not "side-scattering" in the conventional sense.

Would it be better in anyway to have a radar in outer space at a convenient location, be it some kind of Earth stationary orbit, Lagrange point or other?

No. For the same price you get a much smaller antenna and orders of magnitude lower power, which means the range gets much shorter. The probability that something would fly through the volume where the space-based radar happens to be is tiny.

In the Wikipedia entry for Albedo, under the title Astronomical albedo in the first paragraph, there is a reference to detecting asteroids Albedo by radar ('Radar albedo').

Don't misrepresent the article please. It is not talking about asteroid detection there, it is talking about measuring known asteroids. The amount of radar power they reflect tells us something about their composition. This requires them to be in range and at a known place, however.

 

[jim wolfe]

i was responding to:
i guess people are not relating to my 'why not radar?' question, because somehow by just bringing this up, is demonstrated a lack of even basic understanding of how astronomical equipment is used for that goal. But yet can anyone please try and explain in simple words, how far away is current technology from being able to detect by radar at least 2 months ahead,

I am used to radar antenna with very small effective aperture - fighter aircraft -- so if i moved my receive antenna 350 Km east after i transmit a pulse straight up i would not expect much return signal to be received even though it is only 0.07 radians of Earth rotation

either way, you are a mentor, i am just a radar engineer - so I concede

We are talking about an angle of typically 0.1 millirad with respect to a direct reflection - it is not "side-scattering" in the conventional sense.No. For the same price you get a much smaller antenna and orders of magnitude lower power, which means the range gets much shorter. The probability that something would fly through the volume where the space-based radar happens to be is tiny.Don't misrepresent the article please. It is not talking about asteroid detection there, it is talking about measuring known asteroids. The amount of radar power they reflect tells us something about their composition. This requires them to be in range and at a known place, however.

 

[websterling]

so if i moved my receive antenna 350 Km east after i transmit a pulse straight up i would not expect much return signal to be received even though it is only 0.07 radians of Earth rotation

You need to consider the distance to the target. If you're looking at say 4 light minutes to the target, that's about 72 million km. Even if the antenna moves 350 km east, that's still a very small angle relative to the target distance, ~0.005 milliradian.

 

[mfb]

I am used to radar antenna with very small effective aperture - fighter aircraft -- so if i moved my receive antenna 350 Km east after i transmit a pulse straight up i would not expect much return signal to be received even though it is only 0.07 radians of Earth rotation

If you produce a pulse straight up you are looking for things at a distance of 10 km, maybe 30 km - if you move your antenna by 350 km that is a huge difference, of course. The equivalent in the solar system would be to move the antenna to Saturn.

The systems scale quite nicely. An interstellar asteroid in the inner solar system will have a typical distance of maybe 150 million km. Your radar pointing up looks for things with a typical distance of maybe 15 km, a factor 10 million closer. To keep the angular velocity the same, we have to scale down the typical velocity of 50 km/s by the same factor, and we get 5 mm/s. Surely a radar system can track an object moving at 5 mm/s. Or, exactly equivalently, a radar system moving at 5 mm/s can track a stationary object. Ships move more than a factor 1000 faster, and fighter jets move a factor 100,000 faster. The non-inertial reference frame of the antenna, scaled down as well, adds an acceleration of 3*10^-9 m/s², completely negligible as well.
We didn't scale the light speed here, unfortunately scaling down everything doesn't work that nicely. To get that right, we have to keep the relative velocity. Expressed in the target frame, your antenna tracking the aircraft in 15 km distance would move by 5 meters. Expressed in the radar antenna frame, the target moves by 5 meters during the measurement. That is not an issue either.

The signal intensity scales with the diameter squared divided by the distance to the fourth power. If we move our target a factor 10 million closer, we have to make its diameter a factor 100 trillion smaller. That reduces 'Oumuamua's size to 2.5 pm, smaller than an atom. Obviously we can't scale it that way, there are no solid objects smaller than an atom and even a full atom wouldn't have radar properties similar to an asteroid. But that gives you an idea of the size of the target you would have to find with radar here on Earth to make a radar detection of 'Oumuamua at this distance possible.

 

[roineust]

"They are sensitive enough to hear a common aircraft radar transmitting to us from any of the 1000 nearest stars." Source: Breakthrough Listen.
Would such a signal be less feeble, than a radar signal bouncing off a meteorite at a distance of 5-10 light minutes away?

 

[mfb]

If we would use an aircraft radar as source in both cases, we get a factor 1/(star distance)² for the first case and (radar albedo)(asteroid radius)²/(pi (asteroid distance)⁴) in the second case, making a rough assumption about the way radar is reflected for the numerical prefactor.
There are 2000 stars within 50 light years, so let's be optimistic and use that as distance. I'll use 1 for the radar albedo, again very optimistic, https://echo.jpl.nasa.gov/~lance/asteroid_radar_properties/nea.radaralbedo.html are in the range of 0.1 to 0.5.
5 light minutes are 150 million km, and I'll use 200 m for the asteroid. In this case the first fraction is 4.5 *10^-36 m^-2, while the second one is 1.9*10^-40, or a factor 20,000 weaker even with all these optimistic assumptions.
It is unclear what exactly "aircraft radar" means. To find aircraft, or aircraft-mounted? Good aircraft-mounted radar seems to be in the range of tens of kW of power (example), ground-based radar seems to be similar (https://www.faa.gov/air_traffic/technology/asr-11/). We don't have antennas that can send with more than a few MW, or a factor 100 more. We have radio dishes that can focus a beam better than these radar installations, however.

You can probably install an even more powerful transmitter at Arecibo, or use multiple antennas to listen to the signal, or repeat the measurements over and over again, and it might become possible to detect a faint echo of the asteroid - if you aim the beam precisely at its position. It is still not useful to find it! You can only measure it once you know where to send the signal to.

 

[Vanadium 50]

They are sensitive enough to hear a common aircraft radar transmitting to us from any of the 1000 nearest stars.

I don't believe this. TCAS is 70 W at 1.03 GHz. At 35 ly (roughly where there are 1000 stars) this works out to 10^-10 photons per second per square meter at the earth. You won't see a signal at all, much less pull it out of the noise.

 

[mfb]

Did you take the directionality of the antenna into account?
70 W is significantly lower than what I found.

The number is still quite low.

 

[Vanadium 50]

Did you take the directionality of the antenna into account?

Yes, but I took their claim very literally. Aircraft radar means "one aircraft", it means "civilian aircraft" (e.g. not an AWACS), it means "mounted on the aircraft" and it means "pointing where the aircraft wants", which in turn means integrated over time it's covering 4π.

 

[websterling]

TCAS is 70 W at 1.03 GHz

TCAS isn't radar. It's a transponder based interrogtate-and-respond system, basically just a radio system. I think some do aircraft have weather radars that operate at 10k+ Watts. Maybe this is what they're talking about? Their website really doesn't explain much.

Traffic collision avoidance system

 

[Vanadium 50]

TCAS isn't radar. It's a transponder based interrogtate-and-respond system, basically just a radio system.

I would argue it is radar, although via transponder and not reflection. But I agree that the website is sufficiently vague to keep us guessing. I would say that to take a military radar out of the plane, and hooking it up to a polar mounted radio telescope and to transmit towards Earth is either a) not what they mean, or b) what they mean, but they are being misleading.

 

[websterling]

This was posted on the arXiv last evening.
Breakthrough Listen Observations of 1I/'Oumuamua with the GBT

We have conducted a search for radio emission consistent with an artificial source targeting 1I/'Oumuamua with the Robert C. Byrd Green Bank Telescope (GBT) between 1.1 and 11.6 GHz. We searched the data for narrowband signals and found none. Given the close proximity to this interstellar object, we can place limits to putative transmitters with extremely low power (0.08 W).

Eight hours of observations didn't find any artificial transmissions. While it's good to know that it probably wasn't aliens, I can think of better uses for the GBT.