But don't we already have large ground arrays? Yes. Below is the Very Large Array in New Mexico. However, it is not used for asteroid imaging. It is a receive-only array.
So what's the problem? They are used to receive signals only. RADAR imaging requires both transmit and receive capabilities: putting energy on an object (transmitting) and receiving the energy reflected back from the object.
The technical expertise required to create a ground RADAR array system for asteroid imaging and characterization.
How can we improve on these systems? An array of ground antennas operating as one very large dish with 24/7 availability, maneuverability, and capability of producing quality images for near-Earth monitoring.
Image credit: NASA's Dawn spacecraft
Date: July 17, 2011
Note: Spacecraft Dawn is expected to arrive at Ceres in 2015
Imaging is important for asteroid characterization. Today the best images used in determining asteroids properties such as size, shape, surface structure, rotation, and material composition come from expensive spacecraft missions flying close to them and sending back the images. Only a handful of asteroids have been viewed up close by expensive spacecraft mission. Below is an example. Also ESA's Rosetta spacecraft imaged asteroids 2867 Steins (2008) and 21 Lutetia (2010) during a flyby on its way to land on comet 67P/Churyumov-Gerasimenko.
Asteroid 1998 KN3 zips by Orion Nebula
(Green dot in upper left)
Image credit: NASA/JPL-Caltech
Size: 1.1 km diameter
Taken by infrared telescope, NEOWISE at about 500 km above the Earth
Asteroid 2013 ET
Image Credit: Goldstone; NASA/JPL ,Caltech / GSSR
Date: March 2013
Distance away : About 1.2 million km (2.9 lunar distances)
Size: 64 x 140 m
Asteroid 2001 GQ2
Image credit: Arecibo 305 m dish
Date: April 2001 by M. Nolan
The following questions are considered at ASTEROIDS 101...
What are asteroids?
Why detect, characterize, and monitor asteroids?
Where are the asteroids?
What are Near Earth Objects (NEOs)?
What are Near Earth Asteroids (NEAs)?
What are Potentially Hazardous Asteroids (PHAs)?
How are asteroids classified?
What is albedo?
What are the main classifications of asteroids?
How big are asteroids?
How to we calculate rotation?
How to we calculate orbits?
How large an impact is needed to cause a continental fire?
What is the 1998 Congressional Directive?
What is the Asteroid Grand Challenge?
Should we lasso an asteroid and sling it into an orbit around the moon?
Mining asteroids - is it worthwhile?
Space-based optical systems: Plans are in the works for space-based infrared telescopes. Two such ideas are the Sentinel by B612 (Venus-like orbit) and NEOCam by JPL (Earth-Sun L1 Lagrange point: between the Earth and Sun - about four times the distance to the Moon). They specialize in detection and after multiple sighting can do some characterization. However, they are not capable of imaging these objects beyond a point of light (a dot).
Capabilities and limitations of the following systems are giving in table form 1) ground-based radar, (2) space-based optical, and (3) ground-based optical.
Can ground-based optical telescopes image an asteroid? Yes, but most (except for largest and nearest) still look like a dot of light moving with respect to the stars. Also, ground-based optical telescopes can only image into the night sky and it can take months before an asteroid comes into view at night due to the telescope's limited (seasonal) view of space. With multiple observations optical methods can deduce orbit, rotation rates, material composition
The Very Large Array at Socorro, New Mexico, US
Image credit: Wikipedia; Source Own work Author user: Hajor
Size: Adjustable up to 35 km diameter
This meteorite is a fragment of the large Chelyabinsk meteor which exploded over Chelyabinsk Oblast, Russia.
The 59' Russian asteroid that entered Earth's atmosphere (and became a meteor) is thought to be a piece of the 656' diameter 2011 EO40 asteroid
Image credit: EPA
Date: July 17, 2011
Asteroid 216 Kleopatra
Top row: Delay-Doppler images
Middle and bottom row: Simulated data
Dumbbell shaped; about 217 km x 94 km x 81 km
Image credit: Arecibo 305 m dish (Ostro et al 2000, Science)
Date: November 1999
Specialized Arrays Inc. 700 Wavecrest Ave Unit 103, Indialantic, FL 32903
What about the Hubble? ...still a dot of light. Availability is an issue too. Hubble's mission is to gain insight into the evolution of the universe. This space object was detected by Pan-STARRS (ground-based optical) and imaged by Hubble.
What about imaging from a ground-based RADAR antenna?
A RADAR antenna such as at Arecibo (305m) and Goldstone (70m) is capable of imaging objects after they are detected. They can image day and night, through the clouds, and 'towards' the Sun. There are some drawbacks though.
(1) They have limited availability due to other tasking.
(2) Arecibo (305m diameter) can see twice as far but has a limited view of the sky since it is not maneuverable (a hole in the ground).
(3) Goldstone is smaller (70m diamter) and less sensitive so the images are lower quality.
Four types of asteroid detection/characterization systems: (1) Space-based optical telescopes, (2) ground-based optical telescopes, (3) ground-based RADAR, and (4) space-based RADAR systems. The latter are economically infeasible and to date this type has only imaged a handful of asteroids during fly-bys (such as the DAWN spacecraft).
P/2013 P5 An active (six-tailed) asteroid in the asteroid belt imaged by Hubble. It was first spotted by Pan-STARRS.
Image credit: NASA / ESA / D. Jewitt, University of California, Los Angeles / J. Agarwal, Max Planck Institute for Solar System Research / H. Weaver, Johns Hopkins University Applied Physics Laboratory / M. Mutchler, STScI / S. Larson, University of Arizona
Date: September 10, 2013
Image credit: Pan-STARRS (PS1), Maui (PS1SC)
Date: September 16, 2010
Size: 150' diameter
Distance: 20 million miles away from Earth
Note: First potentially hazardous object discovered by Pan-STARRS