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Wikipedia's Luna 2 says:
...the spacecraft released a sodium gas cloud so the spacecraft's movement could be visually observed.
NASA's NSSDCA/COSPAR ID: 1959-014A says:
On 13 September the spacecraft released a bright orange cloud of sodium gas which aided in spacecraft tracking and acted as an experiment on the behavior of gas in space.
Question: Where was Luna 2 when the sodium was released, and where was the sodium cloud when it was observed from Earth?
There's a nice picture here but sadly unsourced and the scale is not given.
(source)
This one's a bit more detailed.
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If this page is correct, the launch was at 06:39 and the cloud was emitted at 18:39, so T+12 hours. Still not a lot of detail though. (That last page says the sodium was emitted by the upper stage of the booster, not the probe. The booster impacted on the moon 30 minutes after the probe so it must have been on a similar trajectory).
From https://www.kik-sssr.ru/Hlynin_Series_E_-_other.htm
"Натриевая комета" была создана на расстоянии трети расстояния до Луны - при этом обеспечивалась наилучшая видимость с территории СССР. Натрия заряжалось около 5 кг, хотя достаточно было и 1 кг. Общий вес системы испарения - 30-35 кг.
translates to:
"The sodium comet" was created at distance of a third of distance to the Moon - at the same time the best visibility from the territory of the USSR was provided. Sodium about 5 kg were loaded though there was also 1 kg enough. Gross weight of system of evaporation - 30-35 kg.
From https://www.kik-sssr.ru/Series_E-1.htm
Станция Е-1 №4 была запущена 2 января 1959 года. После подтверждения выхода на траекторию полёта к Луне она получила наименование «Космическая ракета», позже её стали именовать «Луна-1». (В некоторых сообщениях СМИ ее также называли «Мечта»). 3 января в 03:59 по московскому времени на расстоянии 113 тыс.км. от Земли, с третьей ступени был испарен 1 кг натрия, который образовал облако размером в сотню километров, так называемую «натриевую комету», которая должна была наблюдаться обсерваториями, но в результате неправильного расчета по времени наблюдения были сорваны.
translates to:
The E-1 No. 4 station was started on January 2, 1959. After confirmation of an exit to a flight trajectory to the Moon it received the name "Space Rocket", after its steel to call Moon-1. (In some media reports it(she) was also called "Dream"). On January 3 at 03:59 Moscow time at distance of 113 thousand km. from Earth, from the third stage(step) 1 kg of sodium which formed a cloud of one hundred kilometers in size, a so-called "sodium comet" which had to be observed by observatories was evaporated, but as a result of the wrong calculation for time of observation were broken.
