Keilim 17:14. Among the creations of the first day there is tumah, of the second day there is no tumah, of the third day there is tumah, and of the fourth and fifth days there is no tumah, except for the wing of the osprey and the coated shell of the ostrich egg… And everything created on the sixth day is tamei.
כלים פרק יז, משנה יד. ויש במה שנברא ביום הראשון טומאה. בשני אין בו טומאה. בשלישי יש בו טומאה. ברביעי ובחמישי אין בהם טומאה. חוץ מכנף העוז וביצת נעמית המצופה.
ופירש הרע”ב: הכי קאמר, יש דברים שנבראו ביום ראשון שהעושה כלי מהם יש בהם טומאה, כגון הארץ שנבראת ביום ראשון, וכלי חרס הנעשים ממנה טמאים. בשני אין בו טומאה שבו נברא רקיע ואין בו טומאה. בשלישי נבראו אילנות, וכלי עץ הנעשים מהם מקבלים טומאה. ברביעי נתלו המאורות ואין בהם טומאה. בחמישי עופות ודגים ואם עשה מהן כלים אין מקבלים טומאה.
The Bartenura explains that if one makes a container out of the dust of the earth, which was created on the first day, it is tamei; but the rakia, which was created on the second day, has no tumah. If one makes a container out of wood, which was created on the third day, it is tamei, but on the fourth day the luminaries were created and they have no tumah.
After the first men walked on the moon in 1969, Rabbi Shlomo Zalman Auerbach commented that now we understand the true meaning of this Mishnah. The Bartenura’s explanation does not fit well into the words. The Mishnah is apparently contrasting the first and third days with the second and fourth day, yet according to the Bartenura there is really no difference; it is just a change of the case. Just as the rakia and the luminaries are not tamei, so too the earth and the trees in their natural state are not tamei; it is only the containers made from them that are tamei.
But now that men have gone to the moon and brought back moon dust, we can understand the Mishnah quite simply. This moon dust, if made into a container similar to earthenware, would not be tamei. This is the difference between earth dust, created on the first day, and moon dust, created on the fourth day. This is what the Mishnah originally meant when it was given at Sinai. It was only in the course of the generations that the meaning was forgotten, and scholars learning the Mishnah could not imagine bringing dust down from the moon, hence the Bartenura’s explanation.
[It would seem that the same applies to a container made of dust from a meteorite that landed on earth, a case that was relevant before man reached the moon.]
Rabbi Menachem Kasher quotes the above explanation in his sefer about the moon, and disagrees. After all, the Mishnah also says that what was created on the second day is not tamei. The rakia, which he understands to mean the atmosphere, cannot be made into a container.
[However, we do not really know what the rakia is, since the Torah describes it as a barrier between the waters above and the waters below.]
Also, granted the Bartenura’s explanation seems forced, but the Mishnah flows well as the Rambam explains it in his Commentary on the Mishnayos: the entire Mishnah is speaking about the creations in their natural state, not man-made containers fashioned from them. Day 1: Water can become tamei. Day 2: The rakia cannot become tamei. Day 3: Fruits and vegetables can become tamei. Day 4: The luminaries cannot become tamei.
Furthermore, there is a three-way dispute in the Midrash (Bereishis Rabbah 12:11): one opinion holds that the moon (as well as the sun) was actually fashioned from the earth. At least according to this opinion, the same laws of tumah and taharah should apply to vessels made from its dust. Another opinion holds that the heavenly bodies were created separately from the heavens, and yet a third holds that even the earth was created from the heavens.
Source: Ha’adam Al Hayareiach, pp. 65-72. (Rabbi Kasher quotes the other writer anonymously, but Rabbi Moshe Zoberman, in a shiur on Taanis 9b, said it was R’ Shlomo Zalman who held this position.)