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"Maybe the big take-home point on this one is it's a pretty big object and it was only found 10 or 12 days before closest approach," I've said this before. The

"Maybe the big take-home point on this one is it's a pretty big object and it was only found 10 or 12 days before closest approach," I've said this before. The way they find these things is looking for movement against the distant background / starfield. That works pretty well for finding things, but there is one problem. Any object on a collision course with earth will NOT be moving across the background in the days leading up to the collision. It will appear stationary against the stars, so will not be detected by these systems. To some extent it also doesn't matter that we are orbiting the sun because while near, earth and asteroid will be in the same accelerating reference frame. Another way to model it is to think of firing a projectile from earth into deep space and see how that tracks against the background. That'd be the reverse scenario and there will be no relative motion until it's far enough to not get the same influence from the sun.

rbanffy 2y ago
One more reason for deep space telescopes. A PHA about to impact will not have much movement in relation to the background stars when seen from Earth, but will be moving when seen from a different vantage point.
kemiller 2y ago
Sounds like we need a monitoring station on Mars.
s1artibartfast 2y ago
>It will appear stationary against the stars, so will not be detected by these systems. To some extent it also doesn't matter that we are orbiting the sun because while near, earth and asteroid will be in the same accelerating reference frame. I dont think that is true aside from the case where the asteroid vector is the exact opposite of the earth. Even if the earth and the asteroid are in the same accelerating reference plane, the background is not. If you have two race cars going in around a track and at one from the other, it still appears to be moving relative to the stands.
- mike_d 2y ago
  > If you have two race cars going in around a track and at one from the other, it still appears to be moving relative to the stands. Right. Now put the stands on another planet and you start to understand the problem. We need to be able to see an asteroid at a minimum of of 3-4x the distance between the earth and the sun to have enough time to do anything about it based on the average speed of 40k mph. Now keep in mind that at that distance you are looking for something as "tiny" as 60 miles across, the accepted size that would unquestionably end life on earth. Something ~1km across would cause mass famine. In 1908 an asteroid less than 200 feet across flattened 80 million trees over an 830 square mile area.
  - s1artibartfast 2y ago
    Even if the stands are far away, even light years away, you pan through 360° every year. The fact that the background is far away means that it moves even quicker relative to a nearby object in a rotational frame. If you spin around in a circle, hold your hand out and look at it, it will be moving very quickly against a distant background, like the horizon. My point is that the distance to the background helps detection, not hurts it. If the background was nearby and co-rotating around the Sun, detection would be a near impossible.
  - delta_p_delta_x 2y ago
    Firstly, the several different unit systems in this comment give me a headache. Next, > We need to be able to see an asteroid at a minimum of of 3-4x the distance between the earth and the sun to have enough time to do anything about it based on the average speed of 40k mph. Not really. The important variable is not distance, but rather the mass and velocity vector. There could exist an asteroid that is currently barely 1 moon orbit away (~400 000 km), but has a velocity vector almost equal to that of Earth's. This asteroid could then be predicted to impact upon the next close encounter with Earth. In general, the more distant the predicted impact date, the less the delta-v required to push the asteroid off its current path and avoid an impact in future. > Now keep in mind that at that distance you are looking for something as "tiny" as 60 miles across NASA and ESA have databases of some 35000 near-Earth asteroids, and many of them are orders of magnitude smaller than 100 km[1]. Our detection capability has significantly improved since efforts first started in the 1980s. Asteroid impact avoidance is amongst the few natural disasters humanity has the technological, financial, and engineering capability to completely avoid. We have both the firepower and the lift capability (if not now, then in the near future, dictated by the urgency of the asteroid impact) to deflect even a 12 km, Chicxulub-size impactor if detected sufficiently early. About 5-10 years' warning would be enough time for a large nuclear device to deflect such an asteroid, which would be a major achievement for humanity. [1]: https://cneos.jpl.nasa.gov/sentry/ https://cneos.jpl.nasa.gov/sentry/
EA-3167 2y ago
What you're describing is known as Constant Bearing, Decreasing Range (CBDR), https://en.wikipedia.org/wiki/Constant_bearing,_decreasing_range https://en.wikipedia.org/wiki/Constant_bearing,_decreasing_r... and generally doesn't apply to to the case of a nearby asteroid on a collision course with Earth. For the parallax created by our orbit to be negligible, the object would have to be very distant.
- mjcohen 2y ago
  The way I heard this is, at a military site, "No change in azimuth. No change in elevation. Let's get the hell out of here!"
- phkahler 2y ago
  CBDR only requires constant velocity. Similar triangles are all you need. On short time scales even orbits are approximately straight. It still applies and may well be the reason this one was missed until so close.
- Modified3019 2y ago
  Sounds related to motion camouflage, which dragonflies use to hunt. Allows them to appear stationary, with the only sign they are on an intercept course is looming as their image slowly grows larger. https://en.m.wikipedia.org/wiki/Motion_camouflage https://en.m.wikipedia.org/wiki/Motion_camouflage
pocketstar 2y ago
This is an incredibly naive approach. All asteroids emit light in the infrared, to detect and track asteroids just takes an infrared telescope making apparent motion against any background completely irrelevant. NASA is working on this: https://en.m.wikipedia.org/wiki/NEO_Surveyor https://en.m.wikipedia.org/wiki/NEO_Surveyor
- dard 2y ago
  I work on NEO Surveyor, I replied to the parent comment largely agreeing with you, however I wanted to mention that we do rely on apparent motion for determining if a source is moving (asteroid/comet). However IR is ideal for spotting asteroids for a few different reasons, a big one is that asteroids have a wide range of surface brightness. Think bright concrete to black coal, so in the visible spectrum a 100m light stony asteroid can be just as bright as a 500m coal colored one. It is very difficult to determine size from the brightness alone. However in IR everything is approximately the same temperature (largely proportional to the distance from the sun), so these things all glow in the IR directly proportional to their diameter.
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squarefoot 2y ago
> Any object on a collision course with earth will NOT be moving across the background in the days leading up to the collision. Earth itself moves, so anything appearing static would be easy to spot since it's the background that also moves relative to an observer.
dard 2y ago
I actually work on NEO Surveyor telescope (surveying for dangerous asteroids), writing the simulation code use to track expected performance. So simulating +20 million asteroids and what will be visible during the mission. Motion is how we determine if there is an asteroid or just some static sky source (IE: star). However your comment is not correct, everything moves, even in the days leading up to a potential impact things are constantly in motion. The only asteroids which appear stationary actually tend to be quite far away and just happen to be moving at just the right speed. Things which are close to us, even impactors tend to have large angular velocities, very VERY few things come directly radially in. A part of this is that the Earth is rotating, if you are familiar with the parallax effect then the Earths rotation causes parallactic motion of the asteroids when close. IE, take a photo, wait 4 hours, and you, an observer on Earth, has now moved. The geometry has to be just right for close objects to be stationary, and a different observing position on Earth (or space) will see the object moving even if you see it stationary.
- phkahler 2y ago
  Earth is about 8000mi across. The moon is about 239000mi away. Looking at it from opposite sides would be about 0.016 degrees of angular displacement. And that's a couple days from impact. >> So simulating +20 million asteroids and what will be visible during the mission. How many impacts do you simulate? These are the ones we care most about, and I still think they will be the hardest to detect.
  - dard 2y ago
    Here is a simulation I did in response to your comment, this is ~6 million impactors and their on sky velocity as they approach Earth. Dotted black line is our expected detectability limit, anything to the right of that is detectable. On sky velocity is measured in degrees / day. https://www.dardahlen.com/6m_impactor_vel.png https://www.dardahlen.com/6m_impactor_vel.png
    - phkahler 2y ago
      Thanks, that's really cool. And the detectability line is under 0.01 degrees per day, so the parallax from opposite sides of the earth is well within detectable at lunar distance. You also confirm that the detectability (due to motion) is dropping as the time to impact decreases, but this is apparently not a problem. Leave it to projects run by physicists to have exquisite measurement precision ;-)

rbanffy 2y ago

One more reason for deep space telescopes. A PHA about to impact will not have much movement in relation to the background stars when seen from Earth, but will be moving when seen from a different vantage point.

kemiller 2y ago

Sounds like we need a monitoring station on Mars.

s1artibartfast 2y ago

>It will appear stationary against the stars, so will not be detected by these systems. To some extent it also doesn't matter that we are orbiting the sun because while near, earth and asteroid will be in the same accelerating reference frame. I dont think that is true aside from the case where the asteroid vector is the exact opposite of the earth. Even if the earth and the asteroid are in the same accelerating reference plane, the background is not. If you have two race cars going in around a track and at one from the other, it still appears to be moving relative to the stands.

mike_d 2y ago
> If you have two race cars going in around a track and at one from the other, it still appears to be moving relative to the stands. Right. Now put the stands on another planet and you start to understand the problem. We need to be able to see an asteroid at a minimum of of 3-4x the distance between the earth and the sun to have enough time to do anything about it based on the average speed of 40k mph. Now keep in mind that at that distance you are looking for something as "tiny" as 60 miles across, the accepted size that would unquestionably end life on earth. Something ~1km across would cause mass famine. In 1908 an asteroid less than 200 feet across flattened 80 million trees over an 830 square mile area.
- s1artibartfast 2y ago
  Even if the stands are far away, even light years away, you pan through 360° every year. The fact that the background is far away means that it moves even quicker relative to a nearby object in a rotational frame. If you spin around in a circle, hold your hand out and look at it, it will be moving very quickly against a distant background, like the horizon. My point is that the distance to the background helps detection, not hurts it. If the background was nearby and co-rotating around the Sun, detection would be a near impossible.
- delta_p_delta_x 2y ago
  Firstly, the several different unit systems in this comment give me a headache. Next, > We need to be able to see an asteroid at a minimum of of 3-4x the distance between the earth and the sun to have enough time to do anything about it based on the average speed of 40k mph. Not really. The important variable is not distance, but rather the mass and velocity vector. There could exist an asteroid that is currently barely 1 moon orbit away (~400 000 km), but has a velocity vector almost equal to that of Earth's. This asteroid could then be predicted to impact upon the next close encounter with Earth. In general, the more distant the predicted impact date, the less the delta-v required to push the asteroid off its current path and avoid an impact in future. > Now keep in mind that at that distance you are looking for something as "tiny" as 60 miles across NASA and ESA have databases of some 35000 near-Earth asteroids, and many of them are orders of magnitude smaller than 100 km[1]. Our detection capability has significantly improved since efforts first started in the 1980s. Asteroid impact avoidance is amongst the few natural disasters humanity has the technological, financial, and engineering capability to completely avoid. We have both the firepower and the lift capability (if not now, then in the near future, dictated by the urgency of the asteroid impact) to deflect even a 12 km, Chicxulub-size impactor if detected sufficiently early. About 5-10 years' warning would be enough time for a large nuclear device to deflect such an asteroid, which would be a major achievement for humanity. [1]: https://cneos.jpl.nasa.gov/sentry/ https://cneos.jpl.nasa.gov/sentry/

EA-3167 2y ago

What you're describing is known as Constant Bearing, Decreasing Range (CBDR), https://en.wikipedia.org/wiki/Constant_bearing,_decreasing_range https://en.wikipedia.org/wiki/Constant_bearing,_decreasing_r... and generally doesn't apply to to the case of a nearby asteroid on a collision course with Earth. For the parallax created by our orbit to be negligible, the object would have to be very distant.

mjcohen 2y ago
The way I heard this is, at a military site, "No change in azimuth. No change in elevation. Let's get the hell out of here!"
phkahler 2y ago
CBDR only requires constant velocity. Similar triangles are all you need. On short time scales even orbits are approximately straight. It still applies and may well be the reason this one was missed until so close.
Modified3019 2y ago
Sounds related to motion camouflage, which dragonflies use to hunt. Allows them to appear stationary, with the only sign they are on an intercept course is looming as their image slowly grows larger. https://en.m.wikipedia.org/wiki/Motion_camouflage https://en.m.wikipedia.org/wiki/Motion_camouflage

pocketstar 2y ago

This is an incredibly naive approach. All asteroids emit light in the infrared, to detect and track asteroids just takes an infrared telescope making apparent motion against any background completely irrelevant. NASA is working on this: https://en.m.wikipedia.org/wiki/NEO_Surveyor https://en.m.wikipedia.org/wiki/NEO_Surveyor

dard 2y ago
I work on NEO Surveyor, I replied to the parent comment largely agreeing with you, however I wanted to mention that we do rely on apparent motion for determining if a source is moving (asteroid/comet). However IR is ideal for spotting asteroids for a few different reasons, a big one is that asteroids have a wide range of surface brightness. Think bright concrete to black coal, so in the visible spectrum a 100m light stony asteroid can be just as bright as a 500m coal colored one. It is very difficult to determine size from the brightness alone. However in IR everything is approximately the same temperature (largely proportional to the distance from the sun), so these things all glow in the IR directly proportional to their diameter.

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squarefoot 2y ago

> Any object on a collision course with earth will NOT be moving across the background in the days leading up to the collision. Earth itself moves, so anything appearing static would be easy to spot since it's the background that also moves relative to an observer.

dard 2y ago

I actually work on NEO Surveyor telescope (surveying for dangerous asteroids), writing the simulation code use to track expected performance. So simulating +20 million asteroids and what will be visible during the mission. Motion is how we determine if there is an asteroid or just some static sky source (IE: star). However your comment is not correct, everything moves, even in the days leading up to a potential impact things are constantly in motion. The only asteroids which appear stationary actually tend to be quite far away and just happen to be moving at just the right speed. Things which are close to us, even impactors tend to have large angular velocities, very VERY few things come directly radially in. A part of this is that the Earth is rotating, if you are familiar with the parallax effect then the Earths rotation causes parallactic motion of the asteroids when close. IE, take a photo, wait 4 hours, and you, an observer on Earth, has now moved. The geometry has to be just right for close objects to be stationary, and a different observing position on Earth (or space) will see the object moving even if you see it stationary.

phkahler 2y ago
Earth is about 8000mi across. The moon is about 239000mi away. Looking at it from opposite sides would be about 0.016 degrees of angular displacement. And that's a couple days from impact. >> So simulating +20 million asteroids and what will be visible during the mission. How many impacts do you simulate? These are the ones we care most about, and I still think they will be the hardest to detect.
- dard 2y ago
  Here is a simulation I did in response to your comment, this is ~6 million impactors and their on sky velocity as they approach Earth. Dotted black line is our expected detectability limit, anything to the right of that is detectable. On sky velocity is measured in degrees / day. https://www.dardahlen.com/6m_impactor_vel.png https://www.dardahlen.com/6m_impactor_vel.png
  - phkahler 2y ago
    Thanks, that's really cool. And the detectability line is under 0.01 degrees per day, so the parallax from opposite sides of the earth is well within detectable at lunar distance. You also confirm that the detectability (due to motion) is dropping as the time to impact decreases, but this is apparently not a problem. Leave it to projects run by physicists to have exquisite measurement precision ;-)