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414 al Z A (NASA-CR-175350) RADAR INVESTIGATION OF N84-17090 Semiannual Status Report, 1 Jul. - 31 Dec. 1983 (Cornell Univ.) 14 p HC A02/MF A01 CSCL 03B Unclas G3/91 00572 CORNELL UNIVERSITY Center for Radiophysics and Space Research

ITHACA, N. Y.

SEMI-ANNUAL STATUS REPORT

to the

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

under

NASA Grant NAGW-116

Y RADAR INVESTIGATION OF ASTEROIDS D July 1, 1983 - December 31, 1983

Principal Investigator: Professor Steven J. Ostro

t

n CENTER FOR RADIOPHYSICS AND SPACE RESEARCH

CORNELL UNIVERSITY

ITHACA, NEW YORK 14853

SEMI-ANNUAL STATUS REPORT

NASA Grant NAGW-116

July 1, 1983 - December 31, 1983

RADAR INVESTIGATION OF ASTEROIDS r

Prepared January 1984

i

Professor Steven J. Ostro Principal Investigator

o^

TABLE OF CONTENTS

page

I. SUMMARY OF PROGRESS ...... 1

New Radar Observations ...... 1

Highlights or Data Analysis ...... S

I I . FUNDING STATUS ...... 8

III. PUBLICATION OF RESULTS ...... 10

A 1

I. SUMMARY OF PROGRESS

During the current report period, research supported under NASA

Grant NAGW-116 proceeded as follows:

New Radar Observations

The principal investigator conducted the initial radar

observations of asteroids 80 Sappho, 356 Liguria, 694 Ekard, and

2340 Hathor. For each target, data were taken simultaneously in the

same sense of circular polarization as transmitted (i.e., the "SC"

sense) as well as in the opposite (OC) sense. [Estimates of the radar

cross sections aoc and asc provide estimates of the circular

polarization ratio, µc = asc/ apc, and the geometric ,

_ A p = (1 + 4c)aoc/4, where the normalized OC radar cross section is

aoc - aoc/ A , with A the target's projected geometric area. Th

geometric albedo constitutes a first approximation to the target's

Fresnel, normal-incidence, power-reflection coefficient, R.

Figure 1 shows dual-polarization echo power spectra for the

main-belt objects Sappho, Liguria, and Ekard, which were easily

detected. To maximize the signal-to-noise ratio, each spectrum has

been smoothed using the indicated effective filter bandwidth (EFB).

Standard errors applied to estimates of µc correspond to the

receiver noise in these EFB's.

2

ORIGI INAL F;I.C-2 3 OF POOR QUALITY

S25 i 1200 356 1983 Oct. ti so 1983 O ct. LIGURIA EFB z 70 Hz 4 = 0 SAPPHO -E FS HZ 1000 0. 5± 0.05, j,c a 0.1310.06- —CC —OC ...... SC ..... -SC 375 800

Oo - C 600

C 225'- 400 3C a.0 i 5 - 0 200 LU W 75r 0 I V V .kvq V V A

0. j- i,. H -200

1 I I .75 1 - -400 i0 00 500 0 -500 -1000 1000 500 0 -500 -t000 Doppler Frequency ( Hz Doppler Frequency (Hz )

N.

360, 1 - 1 1 694 1983 Oct. EFB *150 Hz 300'^ EKARD FIGURE 1 µc a 0.00 t 0.1 O ^ —OC 240 SC Average OC and SC radar echo 21 power spectra, smoothed to a 180r- resolution of EFB Hz. Echo power density, in units of km 2 of radar 3: 120 cross section per frequency 60 resolution cell, is plotted Ai against Doppler frequency. The 04 central vertical bar represents

-60 plus and minus one standard deviation of the background noise.

IV -i -1202000 1000 0 -1000 Doppler Frequency ( Hz

r. 3

The values of µc obtained for Ekard and Liguria support earlier

impressions that 0 < µc < 0.1 for large C-type asteroids. On the

other hand, Sappho's polarization ratio, µc = 0.24 t 0.05, is the

largest estimated for any main belt object and is more typical of tiny

(-1 km) Earth-approaching asteroids (e.g., 1915 Quetzalcoatl, with

µc = 0.27 t 0.10). The Sappho results comprise the first

dual-polarization detection of radar echoes from an object with

diameter between 40 and 95 km, as well as the first from a mainbelt object

whose VIS/IR reflection spectrum suggests a surface mineralogy

analogous to C3 carbonaceous chondrites.

Figure 2 shows unsmoothed OC spectra for Sappho and Liguria. The

full echo bandwidths seem to be within about 10 Hz of 95 Hz and 90 Hz,

respectively. When coupled with prior es^imates of these objects'

sizes and spin periods, these bandwidths suggest pole directions

-30° and -45° from the radar line of sight. z

y

M

..-.rz

I_ lam

4

.y

OF POOR QUAIIIY

1C 300 80 1983 Oct = 10 F:z SAPPHO EFa 250 80

200

N 60 7 c 150 a^ D Dnr 40 100 a° r 50 W 20 -a Wv 0

0 -50

-20 -100 1 300 200 100 0 -100 -200 -300 300 200 1CO 0 -100 -200 -3CO Doppler Frequency (Hz ) Doppler Frequency (Hz)

FIGURE 2

Average OC radar echo power spectra for Sappho and Liguria. Echo {-

power density, in units of radar cross section per 10-1-l: frequency

resolution cell, is plotted against Doppler frequency. The central

vertical bar represents plus and minus one standard deviation of the

background noise. 5

Highlights of Data Analysis

Radar observations last June of the peculiar object 2201 Oljato

[-like orbit (a = 2.7 AU, e - 0.71, 1 - 2.5°); possible narrowband, cometary, UV emission features] reveal an unusual set of echo power spectra (Fig. 3). The albedo and polarization ratio remain fairly constant on June 13, 14, 16, and 17, but the bandwidths range from -0.8 Hz to -1.4 Hz and the spectral shapes vary dramatically. Echo characteristics within any one date's -2.5-hr observation period do not fluctuate very much. The simplest interpretation of these results is that Oljato's shape is rather grotesque at meter to hectometer scales.

1 6

ORIGINrs►. r . ;^.; OF POOR QUALi y

2201 OLJ ATO OC JUNE 1983 ...... Sc EFa = (M Nz 0.10 June 13 µt = 0.27 ±0.03 [June 14 ^^µc =0.31 t0A2 r 0.06

Z 0.02-, W

0: - 0.02 3 0.1 June 16 µc * 0.30t0.04 June 17 µc=0.3210.02 0a 0 0.0 v r. F W 0.0 iJtf r;

,+ -0.0 =2 - 1 0 1 2-2 - 1 0 1 2 DOPPLER FREQUENCY (Hz)

FIGURE 3

Average OC (solid curves) and SC (dotted curves) radar echo power spectra obtained for 2201 Oljato on four June 1983 dates.

Echo power density, in units of km 2 of radar cross section per 1-Hz resolution cell, is plotted against Doppler frequency. The standard deviation of the background noise was between 0.015 and 0.020 km2

Hz- 1 on each night. Note the relatively high amplitude of the

August 26 echo. t

7

During the first year (1980-81) of the research supported under

NASA Grant NAGW-116, it became clear that the power of radar constraints on asteroidal dimensions, spin vectors, and surface properties can be enhanced dramatically by photoelectric lightcurves acquired concurrently with the radar observations. If echo strength

is adequate, observations at various rotational phases $ can yield

coc, k, and bandwidth as functions of m. Of course, if the

P is unknown, nothing will be known about the extent

of rotational phase coverage of the radar data. Even if P had been measured at a previous apparition, epochs of extreme light are

unlikely to be predicted accurately, so the asteroid's rotational

orientation as a function of time won't be known, and inferences from

the radar data about target size and shape will suffer. The, •efore, it

is desirable that photoelectric lightcurves be obtained in tandem with

radar observations.

In this context, the principal investigator has publicized (e.g,

in the Bulletin) dates of planned asteroid radar

observations, and has encouraged optical/infrared astronomers in the

U.S. and abroad to conduct photometric and radiometric observations of

radar-targeted objects. Partially as a result of these efforts,

VIS/IR data were obtained in support of about 90% of the post-1981

radar experiments. The investigative power of combining data from

several astronomical techniques has been demonstrated most recently in

a paper, "Radar and Photoelectric Observations of Asteroid 2100

Ra-Shalom," by Ostro et al., soon to be submitted to Icarus.

0 8

During the current report period, the principal investigator finished a series of laboratory measurements at USGS and MIT of the radar-frequency electrical properties of particulate (< 40 µm) metal-plus-silicate mixtures. By combining these measurements with radar albedo estimates, once can constrain the bulk density (d) and metal weight fraction (w) in a hypothetical asteroid regolith having the same particle-size distribution as the lab samples. For example,

Fig. 4 shows contours of constant reflection coefficient (R) and contours of constant porosity in the d,w plane. The bounds (0.05 < R

< 0.3) established on the reflection coefficient for 16 Psyche (an

M-type object with the highest radar albedo of any radar-detected asteroid) require that: (i) if d < 1, then w > 0.6, so w is twice as large as the mean value for enstatite chondrites; (ii) if d > 1.5, 0 T then w can equal zero; (iii) if the surface is solid, then w must be less than 0.2 and an enstatite chondritic mineralogy is indicated for

Psyche.

II. FUNDING STATUS R'

Spending during the current report period was at the anticipated rate. A physics undergraduate, Anthony Ferro, has been hired on a :I part-time basis to assist the principal investigator with data V analysis. C+ 9

ORIGINAL PAGE: 19 Of POOR QUALITY

X13-cm REFLECTION COEFFICIENT, R

0 0.8C U C Fe «Si02 U_ 0.6C - 400 m es h ( s 38µ) 3 0.4C J Q W 0.20

0.00 U 1 2 3 4 BULK OENSITY, g cm-3

FIGURE 4

Constraints on the Fresnel, normal-incidence, power-reflect-nn

coefficient R, derived from laboratory measurements of the

radar-frequency electrical properties of metal-plus-silica particulate mixtures.

R 10

III. PUBLICATION OF RESULTS

During this period, results of the NASA-sponsored research were presented at the 15th Annual Meeting of the Division for Planetary

Sciences of the American Astronomical Society (two abstracts appended). A paper (abstract appended) on the 1981 observations of

2100 Ra-Shalom will be submitted to Icarus in February, and a paper,

"Radar Observations of Main-Belt Asteroids," is being prepared for submission to Science in May.

•