NEA Follow-up Characterization Needs
and Issues: Radar and Optical
Lance Benner
Jet Propulsion Laboratory
California Institute of Technology
What Can Radar Do?
Spatially resolve objects with up to 4-meter resolution:
Greatly exceeds any ground- or space-based optical telescope;
3-D shapes, convex hulls, sizes, surface features, spin states,
surface roughness and density, regolith distrubutions, constrain
composition; gravitational environments.
Identify binary and ternary objects: orbital parameters,
masses and bulk densities, orbital dynamics, constrain bulk
porosity.
Improve orbits: Very precise: for ARM targets, measure
distances to < 10 meters. Shrink uncertainties by many ordersof-magnitude for newly-discovered NEOs. Prevent loss of
objects. Predict motion for decades to centuries. Radar
observations of previously-known NEOs can reduce
uncertainties by several tens of percent.
Goldstone
70 m
Arecibo: 305 m
Capabilities are complementary
Arecibo Goldstone
Diameter
305 m
70 m
Transmitter power
900 kW
450 kW
Transmitter frequency
2.38 GHz 8.56 GHz
System temperature
23 K
17 K
Sky coverage
~1/3
~80%
Declination coverage
-1 to +38 > -35 deg
Finest range resolution
7.5 m
3.75 m
Arecibo is ~20x more sensitive. Goldstone sees more of the sky and can track
several times longer. Arecibo is easier to schedule on short notice. Goldstone
resolves slow rotators 3.6x more finely. Airspace coordination is necessary at
Goldstone but not at Arecibo.
Near-Earth Asteroids Observed by Radar
H > 27: N =10; 2.6% of all NEAs detected by radar
2005 YU55: 2011 Nov. 9, Goldstone
Rounded shape, evidence for boulders, an equatorial bulge, and craters
Diameter ~ 360 meters, 1.875 m x 0.005 Hz
Busch et al., in prep.
Goldstone Bistatic Radar Image of 2012 XB112
 Range
H = 29.7, D ~ 2 m, Resolution = 3.75 m x 0.25 Hz
Resolved in Doppler frequency but not in range: too small
Doppler frequency 
Target-of-opportunity piggybacked on observations of 4179 Toutatis
Radar Contributions to ARM
Radar Astrometry
Measure distances to < 10 meters; can prevent loss of objects
Facilitate detection of perturbations from solar radiation pressure:
provides area/mass ratio.
ARM targets are too small for radar images; no 3D models.
Echo Power Spectra: Resolve in Doppler Frequency
Echo bandwidths can provide rotation period, pole direction, the
convex hull, and other shape information.
If area/mass ratio is known, bandwidths constrain the mass and
bulk density.
Convex hull places upper bounds on optical and radar albedos &
constrains composition.
Radar albedo gives an estimate of surface density. Circular
polarization ratio gauges near-surface roughness and (in some
cases) composition.
Goldstone Echo from ARM
Candidate 2006 RH120
Convex Hull: Silhouette
from Echo Power Spectra
P = 2.7 minutes
E.g.: 4769 Castalia
Central 
dip

Solar radiation pressure perturbation
detected & area/mass ratio estimated.
Rough surface.
The Smallest NEAs Observed by Radar
Asteroid
2006 RH120
2007 EH
2010 AL30
2010 UM7
2011 UB
2012 BX34
2012 DH54
2012 DX
2012 XB112
2013 EC20
H
29.5
27.6
27.2
27.6
27.5
27.6
27.7
27.1
29.7
29.0
Observatory
Goldstone
Arecibo
Goldstone
Arecibo
Arecibo
Goldstone
Arecibo
Arecibo
Goldstone
Arecibo
2006 RH120 and 2012 XB112 are ~2 m in diameter
Obstacles for Scheduling Radar Observations
on Short Notice
1: Is the asteroid in the declination window?
2: Are the SNRs strong enough for a detection?
3: Are the 3s pointing uncertainties small enough (< 20 arcsec)?
If not, can optical astrometry improve them sufficiently before the
radar observations?
4: Can we get telescope time and observing personnel?
5: Goldstone: To transmit full power (450 kW), can we get
radiation clearance in time? We can’t get it on Friday, Saturday,
or Sunday and it usually takes at least two days.
6: If D < 0.008 AU, then we need to receive with a different
telescope. Can we schedule DSS-13 or Green Bank?
Other Issues: Radar
Goldstone can observe at ¼ power without radiation clearance,
enabling rapid response, but still need telescope time, observing
personnel, small pointing uncertainties, and strong SNRs.
Goldstone schedules fixed months in advance are difficult to
change on short notice. To support ARM, priority of radar
observations MUST increase.
At Goldstone, we can get radiation clearance faster if we already
have it for another NEA with overlapping view periods. (e.g.: 2012 XB112)
For ARM targets, short notice is the biggest problem for
scheduling radar observations because most targets will be close
enough to detect for only a few days at best.
Maintenance at Arecibo is suffering due to NSF budget cuts.
The VLBA (radar speckle tracking: NEA spin states) and Green
Bank (bistatic receiver for very close targets) might close due to
NSF budget cuts.
Goldstone currently has only one functional spare klystron.
Principal Techniques for Optical Characterization
Calibrated Astrometry: Improve orbits, obtain better absolute magnitudes.
H values for tiny NEAs are notoriously unreliable.
Lightcurves: Rotation periods and pole directions. NPA rotators and binaries.
Convex shape models. Best estimates of absolute magnitudes. Small
telescopes can help. Need more facilities in the southern hemisphere.
Photometric Colors: Identify optically bright and dark spectral classes.
Can obtain with smaller telescopes and at fainter magnitudes than vis-IR
spectroscopy (e.g., 0.6 m at Table Mountain). Need more help with this.
Vis-IR Spectroscopy: Spectral classes and compositions. Albedo
constraints. NASA’s IRTF has a protocol for rapid responses to TOOs. Need
access to more telescopes equipped for this, especially southern hemisphere.
IR radiometry: Albedos and diameters using thermal emission. Also:
thermal inertias. Spitzer Space Telescope, Gemini, Subaru, IRTF, et al. Not
possible with small telescopes at low elevations.
Optical Observations: Issues
International collaboration will be critical. Need to start establish
international collaborations ASAP.
Optical telescopes have much less stringent pointing requirements than radar.
Need sufficiently bright magnitudes and favorable solar elongations.
Weather is critical.
Rapid response protocols vary among observatories.
Need a central clearinghouse to track observations of each ARM candidate.
Need access to more facilities in the southern hemisphere. Las Cumbres
Observatory Global Telescope could really help with astrometry and lightcurves.
Identify mothballed or otherwise underutilized facilities to see if it would be worth
returning them to service (e.g., 100” on Mt. Wilson). Siding Spring and Kitt Peak
are threatened with closure and could thus be available to help.
Enhancing Goldstone Radar Observations of NEAs
Increase time on DSS-14
- The number of observed asteroids could be at least doubled (~50/year)
- For short-notice targets-of-opportunity radiation clearance is an obstacle
Upgrade the klystrons at DSS-14
- New X-band klystron design with 120 MHz bandwidth: 1.25 m resolution.
- Retain current transmitter power of 450 kW
- First light: 2017 (cost ~$10M)
- Increase the resolution to 1.25 m AND double the power to 900 kW
- SIGNIFICANTLY better science
- Preliminary cost estimate: $20-30M, first light ~2020
Add an 80 MHz, 80 kW transmitter to DSS-13 (34 m antenna)
- 1.875 meter resolution. 2x finer than the best currently available
- DSS-13 is largely underused; usually available.
- Receive at DSS-28; 1/16th as sensitive as DSS-14
- Niche system for really close flybys
- Receive at Arecibo. Boosts SNR substantially
1.25 m Resolution Plus Double TX Power
at DSS 14 Means:
- We are in much better position to study small (and large) asteroids
- Asteroids that used to be too weak for radar become detectable
- Asteroids that are already observable will be stronger targets and we’ll get
higher quality data
- More surface features will be visible
- Windows of opportunity to observe and improve the orbit will lengthen
- Shape models for significantly smaller objects, including some ARM targets
- Easier detection of asteroid satellites due to higher SNRs
- 1.25 m resolution facilitates detection of boulders on asteroid surfaces
- Enhaces bistatic tracks with Arecibo and Green Bank (5x and 2x increase in
sensitivity): More opportunities to utilize 1.25 m resolution
- Improves radar speckle tracking at the VLBA and VLA. More spin states.
Enhancing Arecibo Radar Observations
1. Increase telescope time with additional funding.
Easiest and fastest way to augment radar observations
significantly. Easier to accomplish than at Goldstone.
Enhanced funding will also help with overall
maintenance, which is suffering again due to budget
cuts from the NSF.
2. Switch Arecibo klystrons from S-band to 120 MHz
klystrons at X-band. 6x finer range resolution & 3.6x
finer Doppler resolution.
Major changes necessary: improve X-band receivers,
adjust shape of dish, probably others
Cost & timeline unknown
Enhancing Bistatic Radar Observations
Install new X-band datataking equipment at Arecibo and
Green Bank to receive 1.25 m transmissions from DSS-14
Install new equipment at DSS-14 to receive 7 GHz, 1.875 m
transmissions from DSS-13
Support radar speckle experiments (PI: M. W. Busch) with the
VLBA and VLA.
Looming problem: The VLBA and Green Bank are facing
budget cuts and may close. Closure of the VLBA would end most
radar speckle tracking and that technique’s potential for obtaining
significantly more NEA pole directions.