Radar_astronomy - Wikiparency

# Radar astronomy **Radar astronomy** is the technique of studying nearby astronomical bodies, chiefly the Moon, the planets and small bodies passing close to Earth, by transmitting [[Radio_wave|radio waves]] or microwaves toward a target and analysing the echo that comes back, rather than by only collecting whatever radiation the object happens to emit or reflect on its own. Because it transmits as well as receives, radar astronomy can fix a target's distance and speed directly from the timing and frequency of its own signal, a precision passive observation cannot match, at the cost of needing enormous transmitter power to reach anything beyond the nearer parts of the solar system. A three.js companion, a variant of the [[Radar]] sketch, renders the same pulse-timing idea aimed at a planetary target rather than a nearby aircraft or ship. The technique has run for roughly six decades, applied to targets from the Moon and the nearest planets to comets and asteroids passing within a few lunar distances of Earth, and it supplies several kinds of information no other ground-based method can: a body's exact distance and radial velocity, a three-dimensional model of its shape and spin built from its own reflected echo, and, in a few celebrated cases, tests of fundamental physics that depend on timing a radio signal's travel through curved spacetime with extreme precision. ## Advantages Because the observer chooses the transmitted signal, its frequency, timing and polarisation are all known exactly in advance, so whatever the echo has done to that signal, delayed it, shifted its frequency, changed its polarisation, can be read as a direct measurement of the target rather than inferred indirectly from a source whose own emission was never under the observer's control. That control yields extremely precise range and radial-velocity measurements: bouncing a signal off a planet and timing the round trip refines that planet's orbit and, through it, the scale of the solar system itself far more tightly than positional, angle-only measurements from optical telescopes ever could. The same timing precision let radar astronomy test [[General_relativity|general relativity]] directly, by measuring the extra delay a radio signal grazing the Sun's gravity picks up on its way to and from Mercury or Venus, a delay general relativity predicts and Newtonian gravity does not.[^shapiro] Radar also builds up an image of a target's shape and rotation from the delay and Doppler spread of its echo alone, a technique that has produced detailed models of asteroids too small and too far away for any optical telescope to resolve as anything but a point of light, and because radio waves penetrate cloud and are unaffected by daylight, radar observation does not depend on a clear, dark sky the way optical astronomy does. ## Disadvantages The same activeness that gives radar astronomy its precision also sets its sharpest limit: echo strength falls with the inverse fourth power of distance, since the signal weakens once travelling out and again travelling back, where a passively observed source's brightness falls only with the inverse square, one weakening rather than two. A target twice as far away therefore returns an echo sixteen times fainter rather than four times fainter, which confines practical radar astronomy to the solar system's nearer bodies, the Moon, the inner planets and passing asteroids and comets, rather than anything at stellar or galactic distances, where the far gentler inverse-square law still lets passive telescopes see. Reaching even those distances demands transmitters of hundreds of kilowatts to a megawatt or more feeding some of the largest steerable or fixed dishes ever built, concentrated in only a handful of facilities worldwide, so the technique is expensive, comparatively rare, and vulnerable to the loss of any single instrument. Because the receiver must still pull an already weak echo out of a noise floor set partly by the sky's own brightness temperature as seen through the antenna's beam, the same noise-figure and antenna-temperature bookkeeping that limits any sensitive radio receiver's reach applies here in an unusually unforgiving form.[^ell] Radar astronomy is also inherently limited to bodies solid or dense enough to reflect a usable echo in the first place, and cannot say anything about an object, a star for instance, that radar signals simply pass through or that lies far too distant for any conceivable transmitter to reach and return from. ## History The first confirmed radar contact with another astronomical body came in 1946, when a United States Army Signal Corps project bounced a radio pulse off the Moon and detected its echo roughly two and a half seconds later, matching the light-travel time to and from the lunar surface; the Hungarian physicist Zoltán Bay ran an independent Moon-radar experiment at almost the same time.[^diana] Radar astronomy then moved outward to the planets over the following two decades: ranging to Venus in the early 1960s refined the value of the astronomical unit far more precisely than optical methods had managed and, by timing the planet's radar echo over many rotations, gave the first direct measurement of Venus's own slow, retrograde spin.[^venus] Irwin Shapiro's proposal in the mid-1960s to test general relativity by timing radar echoes from Mercury and Venus as they passed nearly behind the Sun, and the measurements that followed, gave the technique one of its most consequential results, a direct confirmation of a relativistic effect that no optical observation could have produced.[^shapiro2] Radar study of asteroids and comets developed steadily over the following decades as transmitter power and receiver sensitivity both improved, turning what had begun as an experiment in reaching the Moon into a working method for characterising bodies that pass close to Earth. ## Asteroids and comets A near-Earth asteroid is usually no more than a point of light in even a large optical telescope, too small and too far away for its shape to be resolved directly, but a radar echo from the same object carries far more information than its brightness alone: delaying the return by range and spreading it in frequency by the target's own rotation builds up a delay-Doppler image that plays much the same role for a spinning body that [[Synthetic-aperture_radar|synthetic-aperture radar]] plays for a moving one, and the closely related [[Inverse_synthetic-aperture_radar|inverse synthetic-aperture]] technique, which forms an image from a target's own rotation rather than the radar platform's motion, is essentially the same idea turned around. Radar imaging of this kind has resolved the shapes, spin states and, for some objects, the presence of an orbiting companion, of numerous near-Earth asteroids, including well-studied cases such as 4179 Toutatis, among the objects most extensively imaged by ground-based planetary radar.[^toutatis] Because radar gives both a precise range and a precise range-rate in a single measurement, rather than only a position on the sky, it sharpens an asteroid's known orbit far more effectively than additional optical observations of the same object usually can, which is why a newly discovered asteroid on a close approach is a priority radar target wherever the geometry allows it, directly informing how confidently its future path, and any impact risk, can be predicted. ## Telescopes and facilities For most of radar astronomy's history the dominant instrument was the Arecibo Observatory in Puerto Rico, whose enormous fixed dish and powerful transmitter made it the most sensitive planetary radar in the world until structural failures destroyed its instrument platform in December 2020, ending its observing career.[^arecibo] The Goldstone Solar System Radar in California, part of [[NASA|NASA]]'s Deep Space Network, has operated for decades alongside Arecibo and continues to transmit planetary and asteroid radar signals on its own. Because a target's echo is always far weaker than the outgoing pulse, modern planetary radar increasingly works bistatically, transmitting from one large dish and receiving the faint returning signal at a separate, often even larger, radio telescope pointed at the same target, which spreads the work across more of the world's largest instruments than any single facility could otherwise support alone. ## Microsims This article carries no p5.js sketch of its own. A three.js companion, a variant of the Radar sketch, instead renders the same pulse-timing idea described above aimed at a planetary target, standing in for the vastly longer ranges and delays involved without changing the underlying arithmetic. The neighbouring Radar and Doppler effect articles carry sketches that model the same timing and frequency-shift measurements this article's history and asteroid sections depend on, at ordinary, terrestrial scale. *Try:* in the [[Radar]] sketch, read the header formula `R = c·τ/2`; radar astronomy solves exactly the same equation for a delay of seconds, for the Moon, rather than the sketch's microsecond-scale ranges. *Try:* in the [[Doppler_effect]] sketch, raise the source speed and read the two observer markers' frequency shifts; delay-Doppler asteroid imaging spreads a single echo across a whole range of such shifts at once, one for every part of the target's surface moving at a slightly different speed as the body rotates. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Radar_astronomy) : [Wikitube](https://en.wikitube.io/wiki/Radar_astronomy) Skeleton mirrored at revision 1369799561. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Radar]] - [[Synthetic-aperture_radar]] - [[General_relativity]] - [[Doppler_effect]] - [[Moon]] - [[Venus]] - [[NASA]] - [[Radar_altimeter]] ## References Standard radar-range-equation reasoning, that echo power falls with the inverse fourth power of distance while passively observed brightness falls only with the inverse square, is textbook material and is not separately footnoted, per the Wikitube style guide's §6.1. Page numbers below are PDF pages of the open edition linked in the Ellingson citation. [^shapiro]: Citation needed: Irwin Shapiro's original theoretical prediction of the relativistic time-delay effect has not been pinned to its exact publication details in this pass; see also the Shapiro (1964) measurement paper cited below. [^diana]: Citation needed: the United States Army Signal Corps's Project Diana report and Zoltán Bay's independent 1946 account would confirm the exact dates and measured delay claimed here. [^venus]: Citation needed: the specific early-1960s Venus radar campaigns and their published astronomical-unit and rotation-period results have not been pinned to a primary source in this pass. [^shapiro2]: Shapiro, I. I. "Fourth Test of General Relativity." *Physical Review Letters*, vol. 13, no. 26, 1964, pp. 789-791. [^toutatis]: Citation needed: the specific radar campaigns and facilities that imaged 4179 Toutatis, and their published shape and spin results, would confirm the details asserted here. [^arecibo]: Citation needed: the National Science Foundation's own incident report on the December 2020 Arecibo platform collapse would confirm the exact date and technical cause. [^ell]: Ellingson, S. *Radio Systems Engineering, Revised First Edition*. 2023, pp. 96-107 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. ## External links - This article carries no p5.js or three.js sketch of its own yet; the three.js companion described above will be linked here once it is published.