How are radars used in the study of NEOs?

 

Radar is a powerful source of information about asteroids' orbits and physical properties.
The use of Radars in the study of asteroids began with the detection of 1566 Icarus in 1968. But of course, since radars beams are necessarily tiny, the best applications of these instruments is not the discovery of new asteroids. Instead, with the discovery of delay-Doppler imaging, radars are today used to build geologically detailed 3 dimensional models and to define the rotation state with precision, as can be seen from this image. Radars are also used to understand the object's internal density distribution.


Radar images of asteroid Toutatis (courtesy of Nasa )

But how does a radar work? Radar stands for Radio Detection And Ranging. It works transmitting a radio wave toward some object (this is the transmission). This wave is reflected by the object and some of this reflected wave is received back by the antenna. Receiving this wave means making a detection, or in other words, it means detecting that some object stands in the direction of propagation of the wave. The beam is somewhat like the beam of a laser light: it is emitted at a single frequency and is coherent, meaning that the phase of the beam is the same across the entire wavefront. One of the greatest advantages is that the beam's characteristics (such as its frequency, its duration and its strength) are perfectly known and can be varied.

The delay-Doppler imaging
But radars give much more scientific information than mere detection of objects, and this first of all thanks to Delay-Doppler imaging. Let's start by giving a definition of the time delay, which is the time it takes for the wave to reach the object and come back to the antenna. Measuring this time, it is possible to have a measure of the distance of the object. Furthermore, thanks to time delay, it is possible to detect the position of the object (making an orbital measurement) and to make a 2 dimensional image of the object (see the animation below).

If the observed object is moving, the frequency of the reflected wave is different from the frequency of the initial wave (this is called Doppler effect). Measuring this new frequency it is possible to know the speed of the observed object. Combining this doppler shift with the time delay information, it is possible to obtain two dimensional delay-Doppler images. These 2d images obtained are one of the most powerful sources of information. Furthermore, they can be combined together obtaining a 3 dimensional model of the object.


a radar image of asteroid Toutatis (NASA)


In this animation waves emitted by the radar are represented as balls, with wavelength corresponding to their color. The balls, all emitted at the same time, will hit the moving asteroid in different places, and therefore will come back to the radar with different delay times. Using this information it is possible to make a 2 dimensional picture of the object (as can be seen in the image of Toutatis above).
Since the asteroid is spinning, some parts of it are moving toward the radar, and therefore change wavelength (becoming blue in the animation) and some others are moving away from it (becoming red). This is a representation of the Doppler effect.

 

Characteristics obtained form radar measures

Since the characteristics of the emitted beam are perfectly known, comparing them with the characteristics of the reflected beam, it is also possible to derive a considerable amount of information about the surface's composition and texture. In fact the roughness of the asteroid's surface affects the way in which the radar beam is scattered (reflected): a smooth surface tends to conserve the coherency of the beam while a rough surface tends to make the echo beam become incoherent (where coherence means that the beams has a constant phase). The material composition is also very important: bare metallic objects are strong radar reflector while dusty insulating surfaces reflect poorly.

 

The history of radars in asteroid science

Delay-doppler imaging with enough resolution to perform physically detailed modeling of asteroids first occurred in 1989, at the Jet Propulsion Laboratory, on 4769 Castalia. Today, there are only two radars in the world that can make a delay-Doppler image of an asteroid.

The NASA's Goldstone Solar System Radar, situated in California, is part of the Deep Space Network. The radar is made of two dishes : DSS-14, the largest dish of 70 meters and the second DSS-13. The Radar woks at 8510 MHz, transmitting 500 kW. Reception can be performed by both of the two dishes, making it possible to use this radar in bistatic mode.

photo courtesy NASA

The Arecibo Radar Telescope is operated by NAIC. The 30 m fixed antenna represented is near Arecibo (Puerto Rico). It transmits 1MW at 2380 MHz. It mainly operates in monostatic mode but it can also work in bistatic mode, with the NRAO's Greenbank antenna.

photo courtesy NAIC

 

Lidar: a light-radar

A Lidar is a particular, light radar. This instrument sends a pulse of light to the target body and measures the round trip time required for the light signal to travel to and from the bounce point on the target body. By measuring this so-called "light travel time," the distance between the spacecraft and target body can be accurately determined. This instrument is often used to define the shape of the target body and to help navigate the spacecraft when it is in close proximity to the target body.