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MentalAvenger
2007-Oct-08, 05:18 PM
What would we need to detect Asteroids, Comets, or Aliens?

On another thread we were discussing detecting possible Earth impacting asteroids and deflecting them. There have been many discussions about this, but usually the suggestion is “First we detect them, then………” without figuring out just how such detectors would work.

So, the questions are:
1. Where would be the idea place to put detectors?
(a) Earth Orbit
(b) On the Moon
(c) Solar Orbit between Earth and Mars
(d) On Mars
(e) Solar Orbit between Mars and Asteroid Belt

2. How would the detectors work? What kind of sensors could be used by autonomous detectors that would allow them to reliably detect and track asteroids?

Then it occurred to me that once the detectors were in place, they would be in a position to detect incoming spacecraft. If such spacecraft were entering our solar system from interstellar space, no doubt they would be decelerating heavily, having just crossed interstellar space at very high velocity. No matter how you look at it, they’d have to slow down to a veritable crawl before arriving at any of our planets. Assuming no exotic technology such as “gravity drive”, if they slowed down at the last possible moment, they’d probably light up the sky with some pretty spectacular energy signature. If they slowed down mostly in the outer reaches of our solar system (Pluto) , it would take them months or years to cross the solar system.

3. What kind of sensors could we add to our asteroid/comet detectors to give them the capability of detecting incoming spacecraft?

Larry Jacks
2007-Oct-08, 06:56 PM
For detecting NEOs, comets, etc, you need to sweep the whole sky repeatedly and do a comparison between the frames to look for changes. The typical technique is to take an image of a piece of sky and then reimage the same area some time later (an hour or so). You then do an XOR comparison on the images to detect changes. Those changes will indicate something moved between the images. Sweeping the whole sky is easiest done if the instrument has a pretty good field of view (perhaps a few degrees) instead of maximum sensitivity.

Putting the detectors in Earth orbit is relatively inexpensive and opens the opportunity to repair them, but IMO there's too much obscura (obstructions) to that location. You'd be blocked from looking towards the Earth, the sun, and the moon.

Putting the detectors on the moon (perhaps lunar farside) allows for very big instruments but that still results in a lot of the sky not being able to be scanned in a timely basis. It's also very expensive.

Personally, I'd put optical sensors in one or more LaGrange points, perhaps out where SOHO currently operates.

I think it might be possible to mount rotatable sensors on opposite sides of the spacecraft and rotate them in unison to reduce attitude control issues with the satellite. If you try to rotate the entire spacecraft (like with Hubble), then your reaction control wheels will have to do momentum dumps pretty often (at least once a day and maybe more often). Outside of Earth orbit, you can't use Hubble's trick of countertorqueing against the magnetic field to unload the momentum, so you'll have to fire thrusters. You'll have to use thrusters. It doesn't take much propellant to unload the wheels but there's the possibility of thruster spew coating the optics.

IsaacKuo
2007-Oct-08, 07:52 PM
The typical technique is to take an image of a piece of sky and then reimage the same area some time later (an hour or so). You then do an XOR comparison on the images to detect changes.[...]
Putting the detectors in Earth orbit is relatively inexpensive and opens the opportunity to repair them, but IMO there's too much obscura (obstructions) to that location. You'd be blocked from looking towards the Earth, the sun, and the moon.

For practical purposes, your view isn't significantly blocked by either the Earth or Moon. In low Earth orbit, you're spinning around the Earth. You can point your detector in any direction from half of the orbit, and an hour or so later you return to that position to take the next picture. Your motion around the Earth also gives you plenty of sideways motion to peek around the Moon. That just leaves the Sun as the unavoidable blind spot--a problem no matter where you are. (You can hide from the Sun in the shadow of the Earth, but that just replaces the Sun-sized blind spot with an even bigger one.)

With a low Earth orbit telescope, you can do a lot better than an XOR of two images from the same location (separated by an hour or so). Instead, you can take a binocular picture with pictures from opposite side of an orbit. This gives you a "left eye view" and "right eye view", where the baseline is 7000km. Any XOR differences will be an asteroid, moon, or planet. The advantage of this method is that it will detect an asteroid even if it's apparent motion is slow. It also gives you a binocular range, assuming you take at least a third image to average the parallax.

MentalAvenger
2007-Oct-08, 09:52 PM
Wouldn’t there be a big advantage in being able to detect the asteroids early? For any given asteroid, being twice as close would make it appear 4 times brighter. The corollary to that is that being twice as close would mean you could detect them 4 times sooner. If an asteroid was dislodged from its comfortable orbit in the near side of the asteroid belt, a detector in Mars orbit could detect it four times sooner than a detector in Earth orbit. In addition, as it passed Mars orbit, the transit velocity would be far greater, making the asteroid much easier to detect.

novaderrik
2007-Oct-08, 10:27 PM
but what if it comes from a location in Mars' orbit that isn't where Mars happens to be at that very moment?

MentalAvenger
2007-Oct-09, 12:32 AM
Let me clarify. In the same orbit as Mars, not in orbit around Mars. Perhaps about 10-12 detectors evenly spaced should do it. With an orbital distance of 885 million miles, 10 detectors evenly spaced would put one of them no further than about 44 million miles from any inbound asteroid in the plane of the ecliptic.

IsaacKuo
2007-Oct-09, 09:25 AM
It would make more sense to concentrate your resources on just one space telescope in low Earth orbit. A single larger telescope, with a bigger pixel array and bigger aperture can scan more volume at a time, and can do so more systematically with less overlap.

MentalAvenger
2007-Oct-09, 06:33 PM
Just one telescope in LEO would be a looooong ways from the opposite side of Earth’s orbit all the time, blocked by the Sun, leaving one section of interplanetary space not covered at all times. In addition, instead of being just 98 million miles from the near edge of the asteroid belt at that point, the telescope would be at 284 million miles away. That would put it 3 times further away, making any asteroid appear 9 times dimmer. Also, since relative transit velocity of an incoming asteroid is minimal at the two points in Earth’s orbit between its nearest approach to a given point in the asteroid belt, and the furthest point, detection ability would be reduced there also. A distribution of detectors further out, perhaps in Mars’ orbit, would be a lot better.

IsaacKuo
2007-Oct-09, 06:49 PM
Only a puny fraction of the sky is blocked by the Sun at any given time. The Sun is only 0.5 degrees across, meaning it only blocks 0.14% of the ecliptic plane (and most asteroids won't even be directly lined up to the ecliptic plane). The only asteroid which would be blocked by the Sun by more than a few days would be an asteroid in the (unstable) orbit opposite Earth at 1AU. There's no reason to expect anything there, because it's not a stable position. Even if there were something there, it's not going to hit Earth.

And being 9 times dimmer is more than compensated for by having 12 times as much collecting area (one satellite vs twelve satellites).

And as I noted, the binocular vision principle of taking pictures from opposite sides of the orbit allows seeing asteroids even if the apparent motion is very slow. The apparent motion can be zero, and the binocular vision principle will pick it up just fine. In contrast, satellites in solar orbit will just have to rely upon apparent motion for detection.

mugaliens
2007-Oct-11, 08:11 PM
For detecting NEOs, comets, etc, you need to sweep the whole sky repeatedly and do a comparison between the frames to look for changes. The typical technique is to take an image of a piece of sky and then reimage the same area some time later (an hour or so). You then do an XOR comparison on the images to detect changes. Those changes will indicate something moved between the images. Sweeping the whole sky is easiest done if the instrument has a pretty good field of view (perhaps a few degrees) instead of maximum sensitivity.

You're right. Some modern security cameras are double as motion detectors at night (museums) using precisely this method.