Tracking Luna 24 from Florida and Stockholm

Luna 24 turned out to be the last of the Soviet lunar probe series started in 1969. It was a very ambitious flight but provided some baffling experiences for space trackers. The spacecraft carried a drill that took samples in a long core sample to a depth of 2 meters. The core sample was in a long flexible tube that was rolled up inside the return capsule.  (See a picture below of the sample return capsules from Luna 16, 20, and 24 on display at the NPO Lavochkin Museum. Courtesy of Alexander Chernov and the Virtual Space Museum).

The flight profile of the flight is given in the table on the left. The major difference between Luna 24 and previous two lunar sample-return flights was the duration of the return flight from the moon. Also, this longer flight time led to a landing at a much northern latitude after an approach to earth over the north pole.

When this flight was launched the Kettering Group swung into action to try to find the signals from Luna 24. Since Sven Grahn tracked the Luna 20 return flight four years earlier, Dick Flagg had put a 9 meter dish into operation at the University of Florida and this antenna could now be used to monitor the flight on both 922 MHz and 183 MHz. Sven Grahn still had the same equipment that was used to receive Luna 20 on 183 MHz.

According to a telephone conversation that Dick had with "a well known space tracking facility" during the flight of Luna 24 it  transmitted on 922.764 MHz with "6 kHz FM-telemetry [6 kHz = subcarrier frequency?] and a 3 kHz ranging tone". It transmitted on this frequency for about 20 minutes on each orbit around the Moon. The 183 MHz signal would "come up after lift-off" from the Moon according to Dick's source. The return flight signals on 183.6 MHz was our main tracking target. We did receive signals, but they were somewhat baffling.

Here is an extract from a letter from Dick Flagg to Sven Grahn dated 26 September 1976:

"...Luna 24 was a very strange flight in several ways. It was gratifying to see that the launch time plotted very near to Luna 18 on my AGE-Declination Phase plane. It is an interesting observation that all known sample-return flights have been launched when the declination phase was between 312 and 322 degrees. What about Luna 23 then - was it a different type of mision?

At any rate - the Luna 24 mission profile seemed normal until liftoff from the moon - which was a few hours early. Stay time on the surface was 0.95 days for Luna 24 compared with 1.15 days for Luna 20 and 1.1 days for Luna 16. This few hour shorter stay meant that the liftoff was viewable from Yevpatoria. And now the most interesting thing that the return flight was 3.52 days rather than 2.84 days for Luna 20 or the 2.9 days for Luna 16.

For Luna 20 and Luna 16 the re-entry to earth was along a direct path from the moon to the earth. When the probe was launched from the moon, the moon was over the Atlantic. During the 2.8 day flight the moon moved eastward about 35 degrees and so when the probe landed on earth the moon was above the longitude of the landing site.

But Luna 24 was different - the extra return flight time coupled with the early launch put the landing site on the back-side of the earth as viewed from the moon. This is the same geometry that existed for Zonds 6 and 7. I don't necessaerily mean that it was a skip type reentry, I am only saying the geometry was the same as when skip reentries were used. It was a definite departure from the direct ballistic return of Luna 16 and Luna 20......

........Of course one thing directly affected by the longer return flight was our tracking. Here is what I got.

Aug 19 [Launch from the moon took place at 0525 UT]
0724.00-0725.50 UT very weak CW carrier on 922.76 MHz using 30 ft dish, helical feed with 2.3 dB NF preamp right at feed. At 0822 UT I installed 183 feed (2 el yagi and 3.1 dB NF preamp) on 30 ft dish. 0826.40 UT very strong signals on 183.512 MHz +/- 5 or 6 kHz - exactly what Sven heard from Luna 20 -no question about it. Signals very strong S/N>15 dB, dish gain about 22 B. 0833 UT command-off. The lift-off took place Aug 19 at 0525 UT. My signals from the returner heard about 3 hours after liftoff and I assume the 922 was from the descent stage still operating after the returner blasted off.

Aug 22 [Earth landing took place at 1755 UT]
0033 UT tracking at home and there is areally band sounding "warbling FSK" on 183.512 MHz - not sweeping like signals on Aug 19. To use a local expression - sound like a "raw mule fart". However I did strip out a bit after frequency demodulation with HP500BR and there is a periodic structure. Signals fade out at 0100.40 UT - command off, not fade out. Once again signals were very sgrong about 15 dB S/N.

During the next few hours I listen every 20-30 minutes and at 0421 UT note the same signal as before except now its freq is 183.35 MHz - a change of some 160 kHz. Strip out shows same structure. 0452.10 UT command-off. Did you hear this same thing Sven? What the hell was it?...."

In a letter to Richard Flagg dated 29 August 1976 Sven Grahn wrote:

Luna 24
".....I carefully did all the calculations and found that the landing should take place Sunday [22 August] at about 0700 UT, and I listened on 183.54 MHz and other frequencies for 5 hours before the time and 2 hours after. Nothing - absolutely nothing similar to what I got from Luna 20. The only thing I had was an unsteady carrier which faded out at 0751 UT. Frequency 183.50 MHz. ....I don't understand a thing! What the hell happened?......"
 

Analysis

Why did the carrier of the radio transmitter on 183.6 MHz "warble"? I think that the periods when this happened represent the flight phases when there was no ranging uplink transmitted and the onboard oscillator was free-running (and obviously unstable). The periodic pattern that Dick Flagg detected may have been the result of the slow roll of the return vehicle. Of course it is gratifying that the frequency sweeps for ranging purposes were again observed.