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How do radio telescopes track asteroids? I don't imagine that asteroids would be emitting radio waves, and their small size wouldn't seem to be an advantage eit

How do radio telescopes track asteroids? I don't imagine that asteroids would be emitting radio waves, and their small size wouldn't seem to be an advantage either (though, a few miles across might be plenty to interrupt/reflect radio wavelengths? I dunno)

kmm 6y ago
Just like with radar, it's an active process. The telescope emits radio waves, they are reflected by the object, and the telescope receives the reflected waves. With clever tricks they can then get a hint of the size or other parameters of the object from the shape of the reflected signal.
Medox 6y ago
> Relying upon high-powered terrestrial radars (of up to one MW[3]), radar astronomy is able to provide extremely accurate astrometric information on the structure, composition and movement of Solar System objects.[4] This aids in forming long-term predictions of asteroid-Earth impacts, as illustrated by the object 99942 Apophis. In particular, optical observations measure where an object appears in the sky, but cannot measure the distance with great accuracy (relying on parallax becomes more difficult when objects are small or poorly illuminated). Radar, on the other hand, directly measures the distance to the object (and how fast it is changing). The combination of optical and radar observations normally allows the prediction of orbits at least decades, and sometimes centuries, into the future. https://en.wikipedia.org/wiki/Radar_astronomy https://en.wikipedia.org/wiki/Radar_astronomy
- mturmon 6y ago
  Indeed, the fact that Mercury is not tidally-locked with the Sun was discovered using the Arecibo radar telescope. If I remember correctly, they used the Doppler shift imparted to the radio wave --which expresses differently across the width of the rotating planet -- to learn this. "The rotation of Mercury was not discovered until 1965. Until then the most widely accepted theory had Mercury tidal locked to the Sun. Soviet scientists bounced radar signals off the planet’s surface in 1962 verifying that the planet rotated, but it wasn’t until scientists using the Arecibo Observatory verified the planet’s sidereal rotational period of 58.647 day." (From: https://www.universetoday.com/14008/rotation-of-mercury/ https://www.universetoday.com/14008/rotation-of-mercury/)
throwaway0a5e 6y ago
The same way your eye tracks an object that is illuminated by light. They just do it with a different part of the EM spectrum.
Symmetry 6y ago
I had to look this up on Wikipedia[1], it can apparently transmit as well as listen the way that most radio telescopes do and in at up to 20 TW too. I had frankly thought the most powerful radars in the world were in the 10s of MW range and this thing has got a dish 305m in diameter compared to the 10-50m I'm used to for radars tracking satellites in orbit (I used to do software for that professionally). I wouldn't have thought doing active radar scans of an astroid out in space was possible given that radar returns fall off as the 4th power of range. But given that power and dish size, given that these are large objects being tracked, and given that they move slowly compared to the radar cone I guess it's possible after all. But then again one of the first ballistic missile defense radars accidentally mistook the rising Moon for an attack and almost caused WWIII[2] so I guess there's precedent, even though the Moon is much larger and closer. I'm seeing references to it tracking asteroids in the Apollo group, so relatively nearby[3] and plates which are much larger. I'd guess that even the biggest asteroid in the main belt, 1 Ceres, would be too dim for it to see but I'm curious. [1]https://en.wikipedia.org/wiki/Arecibo_Observatory https://en.wikipedia.org/wiki/Arecibo_Observatory [2]https://blog.ucsusa.org/david-wright/the-moon-and-nuclear-war-904 https://blog.ucsusa.org/david-wright/the-moon-and-nuclear-wa... [3]https://en.wikipedia.org/wiki/Apollo_asteroid https://en.wikipedia.org/wiki/Apollo_asteroid
- ISL 6y ago
  If anyone's transmitting at TW powers, it is in a short pulse. You might be comparing it with more-continuous systems.
- saalweachter 6y ago
  > But then again one of the first ballistic missile defense radars accidentally mistook the rising Moon for an attack... They are in good company; like 90% of UFO sightings are just people seeing the Moon and mistaking it for something else (or rather, not immediately identifying it, leaving them with impressions like "there was a bright light following me down the road" or "there was a glowing orb behind the trees").
- jaycroft 6y ago
  Regarding the 20TW from the Wikipedia page, that's referring to the EIRP - effective isotropic radiated power. This is the amount of power that you would need to radiate from a theoretical antenna with a perfectly spherically symmetric power distribution. The Arecibo antenna has a fairly narrow beam. The 20TW EIRP transmitter appears to operate at 2.38GHz. Plugging that frequency and the 270m diameter, a beamwidth calculator gives me a gain of about 75dBi. So, yes, it's like a 20TW transmitter, but only one tiny narrow little slice of it. At 75dBi gain, that's one (10^7.5)th of the total sphere, about 1 part in 30 million. Or, you could say, if the antenna were radiating the same power in every direction as it does in it's peak direction, that would be 30 million times as much power. So, dividing to find the power in that narrow beam, we get "only" about 600kW radiated in the direction it is pointing. Add in cable losses and other inefficiencies, and the power output at the amplifier is probably close to a megawatt.
kawfey 6y ago
I used to work for NRAO. Arecibo is a transmitter, which illuminates the asteroid with very high powered radio waves. The asteroid reflects some of the radio waves, which is received back on the ground at Arecibo or at other radio telescopes like Greenbank or the Very Large Array.

kmm 6y ago

Just like with radar, it's an active process. The telescope emits radio waves, they are reflected by the object, and the telescope receives the reflected waves. With clever tricks they can then get a hint of the size or other parameters of the object from the shape of the reflected signal.

Medox 6y ago

> Relying upon high-powered terrestrial radars (of up to one MW[3]), radar astronomy is able to provide extremely accurate astrometric information on the structure, composition and movement of Solar System objects.[4] This aids in forming long-term predictions of asteroid-Earth impacts, as illustrated by the object 99942 Apophis. In particular, optical observations measure where an object appears in the sky, but cannot measure the distance with great accuracy (relying on parallax becomes more difficult when objects are small or poorly illuminated). Radar, on the other hand, directly measures the distance to the object (and how fast it is changing). The combination of optical and radar observations normally allows the prediction of orbits at least decades, and sometimes centuries, into the future. https://en.wikipedia.org/wiki/Radar_astronomy https://en.wikipedia.org/wiki/Radar_astronomy

mturmon 6y ago
Indeed, the fact that Mercury is not tidally-locked with the Sun was discovered using the Arecibo radar telescope. If I remember correctly, they used the Doppler shift imparted to the radio wave --which expresses differently across the width of the rotating planet -- to learn this. "The rotation of Mercury was not discovered until 1965. Until then the most widely accepted theory had Mercury tidal locked to the Sun. Soviet scientists bounced radar signals off the planet’s surface in 1962 verifying that the planet rotated, but it wasn’t until scientists using the Arecibo Observatory verified the planet’s sidereal rotational period of 58.647 day." (From: https://www.universetoday.com/14008/rotation-of-mercury/ https://www.universetoday.com/14008/rotation-of-mercury/)

throwaway0a5e 6y ago

The same way your eye tracks an object that is illuminated by light. They just do it with a different part of the EM spectrum.

Symmetry 6y ago

I had to look this up on Wikipedia[1], it can apparently transmit as well as listen the way that most radio telescopes do and in at up to 20 TW too. I had frankly thought the most powerful radars in the world were in the 10s of MW range and this thing has got a dish 305m in diameter compared to the 10-50m I'm used to for radars tracking satellites in orbit (I used to do software for that professionally). I wouldn't have thought doing active radar scans of an astroid out in space was possible given that radar returns fall off as the 4th power of range. But given that power and dish size, given that these are large objects being tracked, and given that they move slowly compared to the radar cone I guess it's possible after all. But then again one of the first ballistic missile defense radars accidentally mistook the rising Moon for an attack and almost caused WWIII[2] so I guess there's precedent, even though the Moon is much larger and closer. I'm seeing references to it tracking asteroids in the Apollo group, so relatively nearby[3] and plates which are much larger. I'd guess that even the biggest asteroid in the main belt, 1 Ceres, would be too dim for it to see but I'm curious. [1]https://en.wikipedia.org/wiki/Arecibo_Observatory https://en.wikipedia.org/wiki/Arecibo_Observatory [2]https://blog.ucsusa.org/david-wright/the-moon-and-nuclear-war-904 https://blog.ucsusa.org/david-wright/the-moon-and-nuclear-wa... [3]https://en.wikipedia.org/wiki/Apollo_asteroid https://en.wikipedia.org/wiki/Apollo_asteroid

ISL 6y ago
If anyone's transmitting at TW powers, it is in a short pulse. You might be comparing it with more-continuous systems.
saalweachter 6y ago
> But then again one of the first ballistic missile defense radars accidentally mistook the rising Moon for an attack... They are in good company; like 90% of UFO sightings are just people seeing the Moon and mistaking it for something else (or rather, not immediately identifying it, leaving them with impressions like "there was a bright light following me down the road" or "there was a glowing orb behind the trees").
jaycroft 6y ago
Regarding the 20TW from the Wikipedia page, that's referring to the EIRP - effective isotropic radiated power. This is the amount of power that you would need to radiate from a theoretical antenna with a perfectly spherically symmetric power distribution. The Arecibo antenna has a fairly narrow beam. The 20TW EIRP transmitter appears to operate at 2.38GHz. Plugging that frequency and the 270m diameter, a beamwidth calculator gives me a gain of about 75dBi. So, yes, it's like a 20TW transmitter, but only one tiny narrow little slice of it. At 75dBi gain, that's one (10^7.5)th of the total sphere, about 1 part in 30 million. Or, you could say, if the antenna were radiating the same power in every direction as it does in it's peak direction, that would be 30 million times as much power. So, dividing to find the power in that narrow beam, we get "only" about 600kW radiated in the direction it is pointing. Add in cable losses and other inefficiencies, and the power output at the amplifier is probably close to a megawatt.

kawfey 6y ago

I used to work for NRAO. Arecibo is a transmitter, which illuminates the asteroid with very high powered radio waves. The asteroid reflects some of the radio waves, which is received back on the ground at Arecibo or at other radio telescopes like Greenbank or the Very Large Array.