Meitner continues to be one of the most notable scientists not to have won a Nobel Prize. The omission stands out especially because Hahn won it in 1945. By all accounts Meitner and Hahn enjoyed a very productive working relationship for almost thirty years before the tide of fascism tore their lives apart. Their relationship was warm and friendly, but still formal; both were after all products of the rigid and hierarchical German society of their time.
Meitner's lack of a Nobel Prize is stark not only because of the seminal importance of nuclear fission, but because a search of the Nobel Prize nomination database reveals a striking fact: Meitner was nominated for the Nobel Prize in physics or chemistry no less than 48 times between 1937 and 1948 alone. That's almost five nominations a year. Other great scientists have also received dozens of nominations - for instance the chemist R. B. Woodward had 92 before he finally won - but Meitner's is certainly on the higher side. The database runs to 1965, and it's rather curious to see no nominations after 1948.
The list of scientists nominating her is a roster of the who's who of twentieth century physics: Max Planck, Niels Bohr, Werner Heisenberg, Arthur Compton and Max Born all nominated her multiple times. Hahn nominated her once. Curiously, Albert Einstein who thought highly of Meitner does not seem to have nominated her even once; given Einstein's freethinking views and liberal persona this is rather strange.
Given the number of prominent personalities advocating her work, Meitner's omission from a Nobel Prize will continue to be a blot on the history of the prizes. This hole stands out even more because Meitner was otherwise highly decorated and publicly recognized, receiving for instance the prestigious Enrico Fermi Award from the United States Atomic Energy Commission. A combination of factors likely contributed to the failure of the prize committee. Sexism, certainly, but I don't think sexism played as prominent a role as it did in the careers of some other deserving female scientists. Part of the reason why I don't think it played an overriding role is that nuclear physics was the only field until then in which two women - Marie Curie (who shares a birthday today with Meitner) and Irene Joliot-Curie - had received Nobel Prizes. The physicist Maria Goeppert Mayer would very soon become only the second woman to win a physics Nobel Prize, again for nuclear physics. Also, Otto Frisch and Fritz Strassmann who were instrumental in both discoveries also did not receive the prize, and their omission of course cannot be ascribed to gender discrimination.
The real reason remains muddy and is likely a collage of fuzzy factors. Some have speculated it was anti-semitism, although given the number of Jewish prize recipients until then it would appear to be a minor determinant. Others think that the relatively low opinion of her work held by some prominent members of the Nobel committee might have contributed. Personally I also think it might have been an honest albeit misguided view of the contributions of the four scientists involved in discovering fission: Hahn's work might have been regarded as a proper "discovery" while Meitner and Frisch's might have been regarded as a mere "explanation". Strassmann could have been omitted because he was presumably Hahn's "assistant" (which he wasn't).
All these factors likely played a role, and no single factor might have been dominant. At the very least, Meitner's lack of a prize is as disappointing as Frisch and Strassmann's. In fact I always think that if there is an underappreciated hero of the nuclear fission story, it's not Meitner but Fritz Strassmann. This quiet and industrious man did much of the tedious grunt work that was key to solving the puzzle of the breakup of uranium. The kind of craftsmanship that he exhibited is too often overlooked, by the public as well as by prize committees. Morally too he was a hero; during those perilous years he hid a Jewish friend in his apartment for many years at considerable risk to his life.
Meanwhile, Frisch was instrumental in helping his aunt work out the exact mathematics of the fission process and then performing the first fission experiment outside Berlin. Working with Rudolf Peierls in Birmingham, he later established the first value for the critical mass of a bomb and helped convince key members of then slow-moving Manhattan project that nuclear weapons were possible. He also fine tuned this critical mass value at Los Alamos by performing dangerous experiments which were christened 'tickling the dragon's tail'.
Niels Bohr rightly suggested that the physics Nobel Prize should have been split between Meitner and Frisch, the chemistry prize between Hahn and Strassmann. But the prize is a human institution after all, and a fallible human committee created by fallible human beings does not always obey the logical dictates of history. Hahn and Meitner are now largely remembered, Strassmann and Frisch are now largely forgotten. But Meitner stands out, for her brilliance and experimental acumen, for her perseverance and doggedness, for her stoic will during tumultuous and painful times, for great personal fortitude. She may not have won a Nobel Prize, but her 48 nominations provide a tribute to her remarkable personality as a scientist and human being. She deserves to be constantly remembered and celebrated.