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Planned Ulysses Radio Observations



The Ulysses spacecraft is in a high inclination orbit relative to the ecliptic plane, which will carry it under the south pole of the Sun in September 1994. Its payload includes sensitive radio receivers that may be able to observe both the immediate consequences of the collisions of Comet Shoemaker-Levy 9 fragments with Jupiter and the long-term effects on the Jovian magnetosphere.

VIEWING GEOMETRY: The impacts of the principal components of Comet Shoemaker-Levy 9 will occur from July 16-22, 1994. Ulysses will be in a relatively favorable position for observing consequences of the impacts because it will be about 2.5 AU below the ecliptic plane. It will be 5.33 A.U. from Jupiter, 8 degrees duskward of Jovian local noon, and (most importantly) at 31 degrees South Jovian latitude. This means that the most recent determination of the impact locations yields Ulysses-Jupiter-fragment (U-J-F) angles of 86-90 deg. Radio-emitting ejecta would only need to achieve a height of about 2000 km (<0.03 Jovian radii) to be detectible above the intervening ionosphere (for emissions at 1 MHz). Plumes reaching this height have been predicted.

The viewing geometry for Ulysses is based on the predictions of Yeomans and Chodas, posted 93/12/17. The fragment numbers are as designated by Jewitt.

                    Impact      Impact    Ulysses    Ulysses    
  #  Impact Date  Jovicentric  Meridian Jovicentric  Meridian    U-J-F
       (July)      Latitude     Angle     Latitude    Angle      Angle
                     (deg)      (deg)      (deg)      (deg)      (deg)
 17        17.6      -43.8       62.3      -31.0      188.4       90.4
 15        18.3      -43.7       65.9      -31.0      188.3       88.6
 14        18.8      -43.5       67.2      -30.9      188.2       88.1
 12        19.4      -43.6       69.1      -30.9      188.1       87.0
 11        19.9      -43.9       68.6      -30.9      188.1       87.0
  7        20.8      -44.0       68.5      -30.8      188.0       87.0
  6        21.3      -42.7       75.1      -30.8      187.9       84.1
  5        21.6      -44.8       66.5      -30.8      187.9       87.5
  1        22.3      -44.4       69.4      -30.8      187.8       86.2

URAP OBSERVATIONS: The Ulysses Unified Radio and Plasma Wave experiment (URAP) provides the most sensitive radio observations at frequencies less than 1 MHz of any active spacecraft. At the current distance of about 4.7 AU from Jupiter, URAP detects Jovian radio emissions on a near-daily basis. These radio emissions include broad-band kilometric emission, quasi-periodic bursts, and nonthermal continuum, each of which are emitted from different regions in the Jovian magnetosphere.

In addition to providing observations of the effects of the comet debris on the magnetospheric radio emissions, it is possible that URAP will be able to observe radio emission related to the impacts and subsequent "fireballs". If so, this will provide important constraints on the timings of the events, the heights to which the materials are ejected, and some indication of the amounts of material ejected for each observed impact. The observing sensitivity of URAP is such that only 1.E-12 (one-trillionth (U.S.)) of an impact's kinetic energy needs to be converted to radio emission (above the ionosphere) in the URAP frequency band for Ulysses to detect the impact. (This assumes that the kinetic energy released in an impact will be 1.E+29 ergs.) For comparison, the efficiency of converting total lightning energy into radio "sferics" is typically 1.E-5 to 1.E-6.

This detection limit is calculated using the following assumptions. The smallest event easily detected by URAP would be of the order of the background level of 1.E-21 W/m^2/Hz, although it may be possible to detect emission 10 to 100 times weaker. For radio waves uniformly emitted into 4 Pi steradians, the surface area of a sphere at the distance of Ulysses from Jupiter (5.33 AU) would be 8.0E+24 m^2. The radio bandwidth of Ulysses is approximately 1.E6 Hz. Multiplying these three numbers we obtain 8.0E+9 W for the minimum emitted power, which could be detected by URAP. Assuming that the emission lasts 1 sec, which would permit it to be detected in several data samples, yields 8.0E+9 Joules or 8.0E+16 ergs.

The 2 URAP radio receivers cover the following frequency bands:

 "LOW"   1.25-48.5 kHz   64 frequencies swept in 128 sec
 "HIGH"     52-940 kHz   12 frequencies swept in 144 sec

The modulation of the received signal by spacecraft spin permits determining the arrival direction of the emission, which will serve to confirm that any emission detected did indeed come from Jupiter. A combination of real-time DSN coverage and tape recorded data routinely provides nearly continuous data coverage for Ulysses.

R. MacDowall, M. Kaiser, and R.G. Stone
NASA/Goddard Space Flight Center
Code 695
Greenbelt, MD 20771

e-mail: or 15466::macd
phone: 301-286-2608
revised 4 January 1994