# I 'Oumuamua detection date and ramifications for a big asteroid collision with the Earth

#### |Glitch|

Consider that finding NEOs efficiently requires robotic telescopes and software/hardware capable of processing thousands of images every day.
It also helps if you are actually looking for them, which NASA wasn't prior to being given a congressional directive in 1998.

#### DrStupid

Ivy Mike's 10Mt explosion on a small coral reef island resulted in 1.9km diameter, 50m deep crater.
Ivy Mike exploded at ground level. That's not easy with a speed of 50 km/s.

#### nikkkom

Ivy Mike exploded at ground level. That's not easy with a speed of 50 km/s.
I don't think so. Military have 100+ years of experience in developing contact fuses.

#### websterling

Military have 100+ years of experience in developing contact fuses
I don't think a contact fuse would do it. Even with a direct hit and the fuse being activated, with the speeds involved (50 km/sec for the target and another 5-10 km/sec for the warhead) the warhead would probably be destroyed before it detonated.

#### roineust

Radio telescopes configured as radar are routinely used to study near earth objects. They are useful for determining, for example, rotational rates and surface properties. They are also useful for refining the orbit of an object.

They are not useful as instruments for discovering such objects. Large radio telescopes have a very narrow beamwidth so they only see a very small area of the sky. And you have to consider the round trip time of a radar pulse. At its detection Oumuamua was about 2 light minutes from earth giving a round trip time of 4 minutes. So you would have to transmit a pulse and wait for at least 4 minutes to see if you got a return before you could move the antenna to try again in a different area of the sky.
How fundamental is that narrow beam width limitation, in terms of being a current technological barrier? Isn't it possible to develop a radar-radio-telescope, which encodes its transmitter signals just like internet packets and then not waiting with the whole apparatus for the round trip time, rather having a receiver that listens somehow to a wider range of returning signals, so any signal that returns is already encoded and thus will be known, to what region of transmission that signal belongs?

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#### websterling

How fundamental is that narrow beam limitation, in terms of being a current technological barrier? Isn't it possible to develop a radar-radio-telescope, which encodes its transmitter signals just like internet packets and then not waiting with the whole apparatus for the round trip time, rather having a receiver that listens at a much wider returning beam-width, so any signal that returns is already encoded and thus will be known, to what region of transmission that signal belongs?
It's not uncommon to have the receiver and transmitter on different antenna. And they could be separated by some distance- radars for observing meteors are typically done this way. I think any encoding would probably be scrambled by a reflection from an irregular surface. You might be able to use different frequencies rather than an encoding. Still, considering transmitted power and beamwidth constraints, I don't think you could make a very efficient system.

#### roineust

It's not uncommon to have the receiver and transmitter on different antenna. And they could be separated by some distance- radars for observing meteors are typically done this way. I think any encoding would probably be scrambled by a reflection from an irregular surface. You might be able to use different frequencies rather than an encoding. Still, considering transmitted power and beamwidth constraints, I don't think you could make a very efficient system.
If some new SETI collaboration programs claim, that they will be able to hear an encoded signal as low as 100W in power, from a distance of up to 50 light years, isn't it possible to have earth transmitters strong enough, to decode an asteroid hit round trip of up to a few hours?

#### websterling

they will be able to hear an encoded signal as low as 100W in power, from a distance of up to 50 light years
In the article they're talking about a signal from a 100W laser; entirely different physics involved.

With radar I think the limit for just detection of a 1km object is less than 1 AU, less than a 15 minute round trip time. For an object like Oumuamua it would be far less.

#### roineust

In the article they're talking about a signal from a 100W laser; entirely different physics involved.

With radar I think the limit for just detection of a 1km object is less than 1 AU, less than a 15 minute round trip time. For an object like Oumuamua it would be far less.

Yes, sorry, i read the whole article but missed the laser part when returned to quote, what about laser scanning then? (-:

It looks like LIDAR telescopes are used for atmospheric research. In what ways would it be different to scan much further away to a distance and object size of an asteroid 2 light hours from earth?

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#### mfb

Mentor
You misrepresented the project you cited, claiming it was to search for PHAs when it was actually searching for NEOs.
Uh, what?
"They looked for A and B."
"How often did they find A?"
"I said they looked for A and B, not only for A!"

Here A=PHAs, B=NEOs that are not PHAs just in case it was not clear.
NASA was established in 1958, and yet 40 years later they had only discovered ~10% of the NEOs larger than 1 km. Yet since Congress ordered NASA to locate NEOs larger than 1 km NASA has managed to find more than 98% of them in less than 20 years. Which clearly demonstrates that NASA was not specifically looking for NEOs (of any size) until ordered to track them by Congress.
No it does not demonstrate this, and even if it would, it would miss the point because I was asking about a different time frame. Telescopes and data analysis are improving rapidly, with or without congress orders.

Gaia alone is expected to roughly double the number of known asteroids in the solar system (and measure most of the discovered ones as well, of course). Without any congress order, and without NASA at all, because it is an ESA mission.

Yes, sorry, i read the whole article but missed the laser part when returned to quote, what about laser scanning then? (-:
You can't "laser scan" for asteroids. They are not nice retroreflectors that would reflect the lasers. Even with the Moon, which is nearby and where we have actual retroreflectors, we just get something like 1 photon per shot back with the best combination of ground stations and mirrors. You have to know precisely where the mirror is and you need a good estimate for the distance already, otherwise you wouldn't even find the Moon with that approach.
To get a detectable signal back from radar astronomy, the beam has to be very narrow. You cannot scan the whole sky, or even a relevant fraction of it, like that.
I expect attempting to achieve surface blast. (Subsurface would be even better, but it requires development of a penetrating warhead).

Ivy Mike's 10Mt explosion on a small coral reef island resulted in 1.9km diameter, 50m deep crater. You can fit about a hundred Oumuamua's into a crater of these dimensions.
You are still missing the atmosphere.

#### roineust

In the article they're talking about a signal from a 100W laser; entirely different physics involved.

With radar I think the limit for just detection of a 1km object is less than 1 AU, less than a 15 minute round trip time. For an object like Oumuamua it would be far less.
What are the basic technical and scientific hurdles for this 1km and less than 1 AU range/size limit?

#### websterling

What are the basic technical and scientific hurdles for this 1km and less than 1 AU range/size limit?
1) There's a limit to the transmitter power available.
2) Signal strength falls off as $1/d^2$.
3) Dust covered rocky objects are poor reflectors.
4) 1km is rather small.
5) The return also falls off as $1/d^2$.

#### roineust

In the article they're talking about a signal from a 100W laser; entirely different physics involved.

With radar I think the limit for just detection of a 1km object is less than 1 AU, less than a 15 minute round trip time. For an object like Oumuamua it would be far less.
To which specific radar type, class or model were you referring to, which have these limits (1km 1 AU) and yet are the best currently available, for such an asteroid finding task? You meant hypothetical usable radars or ones that already exist as part of an asteroid detection system? You did mention that radars are already used to search for near earth objects, you probably meant space derbies collision avoidance for the ISS or did you refer to other radars that search further away than space derbies? How come these radars are able to detect meteors, as you mentioned, while meteors are much smaller objects than asteroids? You meant only when meteors are as close to us as space derbies or less?

What i could find regarding radars and space derbies was Cobra Dane and EISCAT.

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#### russ_watters

Mentor
It also helps if you are actually looking for them, which NASA wasn't prior to being given a congressional directive in 1998.
You ignored @nikkkom 's point, so I'll expand/repeat: Robotic/automated sky surveys were at best difficult and at worst impossible until at least the 1980s. But the capabilities expanded so fast that starting in the 1990s, amateurs regularly discover comets and asteroids. Many do their own automated sky surveys, discovering dozens or even hundreds. These days, hunting for such objects is so easy it is practically a race.

Sure, there is a a bit of a chicken-or-egg issue here, but what is certainly NOT a component of it is the implication that we could have found these thousands of objects in the 60s or 70s if we had simply decided to look. We could not have. And conversely, whether Congress was the cart or horse, a significant fraction of these objects were going to be found either way.

 A quick google tells me that the first commercial CCD camera (100x100 pixels) was released in 1975 and the first telescope with a digital camera (pointed down ;) ) was launched into space in 1976. So I think it is fair to say that digital sky surveys were impossible until at least 1976.

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#### glappkaeft

You ignored @nikkkom 's point, so I'll expand/repeat: Robotic/automated sky surveys were at best difficult and at worst impossible until at least the 1980s. But the capabilities expanded so fast that starting in the 1990s, amateurs regularly discover comets and asteroids. Many do their own automated sky surveys, discovering dozens or even hundreds. These days, hunting for such objects is so easy it is practically a race.
Unfortunately (or not) those days have been over for years if you are talking about the amateurs due to the big automated surveys. It is only going to get even harder for the amateurs when LSST comes online and with GAIA's later data releases.

Most amateurs switched to followup observations or other areas like variable stars years ago. See for instance this article https://www.airspacemag.com/as-interview/aamps-interview-roy-tucker-112571/?all

#### |Glitch|

You ignored @nikkkom 's point, so I'll expand/repeat: Robotic/automated sky surveys were at best difficult and at worst impossible until at least the 1980s. But the capabilities expanded so fast that starting in the 1990s, amateurs regularly discover comets and asteroids. Many do their own automated sky surveys, discovering dozens or even hundreds. These days, hunting for such objects is so easy it is practically a race.

Sure, there is a a bit of a chicken-or-egg issue here, but what is certainly NOT a component of it is the implication that we could have found these thousands of objects in the 60s or 70s if we had simply decided to look. We could not have. And conversely, whether Congress was the cart or horse, a significant fraction of these objects were going to be found either way.

 A quick google tells me that the first commercial CCD camera (100x100 pixels) was released in 1975 and the first telescope with a digital camera (pointed down ;) ) was launched into space in 1976. So I think it is fair to say that digital sky surveys were impossible until at least 1976.
@nikkkom 's point was irrelevant. I wasn't debating the efficiency of automated searches. You pretend as if nothing could be discovered prior to CCDs, when we know that is not true. Granted, it is not as "efficient" as an automated/robotic search, but we have taken photographs of the same part of the sky at different times and then compared them to see if anything moved. How do you think Pluto was discovered?

NASA is also a government agency that does what they are directed to do by both Congress and the President. It is not like they have the initiative (or an unlimited budget) to do whatever they please. If Congress does not direct NASA to locate NEOs, and does not provide funding for that purpose, then NASA will not locate NEOs. Which is what happened between 1958 and 1998.

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#### TeethWhitener

Gold Member
Even if we assume you manage to break the asteroid apart: So what? Now you have several smaller components that still fly towards Earth with the same combined energy. But instead of one area with a massive impact crater you get many impact craters scattered over a large area. You might even increase the damage it does.
Not to mention these fragments are highly radioactive. You've traded a sniper rifle for a plutonium shotgun.

#### DrStupid

Not to mention these fragments are highly radioactive.
It would be low- or medium-reactive material, depending in the size of the warhead (the bigger the less radioactive). An explosion of the same size on Earth would produce more nuclear fallout and we had many of them in the past. The resulting increase of the background radiation would be an irrelevant side-effect of the impact. The main problem is in fact the fragmentation of the asteroid.

mfb

#### nikkkom

The main problem is in fact the fragmentation of the asteroid.
Non-fragmented asteroid is exponentially more destructive.
1-2 meter fragments will do no damage whatsoever.
20-meter fragments generally won't reach the surface too (a-la Chelyabinsk) but cause shockwave damage.
50-meter fragments can cause a city-scale destruction.
200-meter intact asteroid would leave about 5km diameter crater.

#### mfb

Mentor
64 scattered city-scale destruction areas (hard to evacuate) vs. one larger focused spot of destruction (possible to evacuate).

#### nikkkom

230x35x35m asteroid can't be separated into 64 30m fragments. Maximum eight fragments.

Fragmentation at ~500km altitude means ~25 seconds to impact. If big fragments would have ~100m/s lateral velocities, they can drift apart by ~2.5 km only. The destruction will still be localized in about the same location, not spread across half a continent.

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#### mfb

Mentor
If you break it up that late it doesn’t matter anyway.

The number of fragments scales with the cube of the length scale. It doesn’t matter which shape you assume but be consistent - don’t switch in between.

#### DrStupid

The destruction will still be localized in about the same location
And so will be the energy. Just the type of destruction will change. The larger the fragments the higher the ratio of mechanical effects (e.g. shock waves or seismic waves). The smaller the fragments the larger the ratio of radiative effects (e.g. heat radiation or emp). It depends on the circumstances which of them are more destructive. A direct hit could kill a city in any case.

#### nikkkom

And so will be the energy. Just the type of destruction will change. The larger the fragments the higher the ratio of mechanical effects (e.g. shock waves or seismic waves). The smaller the fragments the larger the ratio of radiative effects (e.g. heat radiation or emp).
Exactly.
Energy in different forms has very different destructive potential.
For example, one kilogram of burning wood releases more energy than a hand grenade.

If you turn this asteroid into 1-5 meter fragments, on entry its energy will be converted almost entirely to light and shock waves in the air.
Light flash would be spectacular but won't be intense enough to do harm.
Shock waves would cause widespread window damage and may cause moderate structural damage in some buildings.