The observation of water, H₂O, by radioastronomy is often hampered because of the presence of water in Earth's amosphere. Only comparatively weak transitions can be detected as there will be non-zero transmittance at these frequencies through the atmosphere. Nevertheless, water was one of the first molecules to be detected in the interstellar medium. The J_{K_aK_c} = 6₁₆ – 5₂₃ ortho rotational transition of water, H₂O, was observed in Orion, SgrB2, and W49 displaying maser activity:
A. C. Cheung, D. M. Rank, C. H. Townes, D. D. Thornton, and W. J. Welch,
Detection of Water in Interstellar Regions by its Microwave Radiation
Nature 221, 626–628 (1969).
At the observed intensity levels, the detection could be viewed as secure. In the meantime, many additional transitions have been reported.

Singly deterated water, HDO, was reported first by
B. E. Turner, N. Fourikis, M. Morris, P. Palmer, and B. Zuckerman,
Microwave Detection of Interstellar HDO
Astrophys. J. 198, L125–L128 (1975).
The 1₁₁ – 1₁₀ transition was observed in emission toward Orion KL. Several other transitions have been reported in the meantime.

H₂¹⁸O was detected in emission through its 3₁₃ – 2₂₀ transition toward G34.3+0.2:
T. Jacq, C. Henkel, C. M. Walmsley, P. R. Jewell, and A. Baudry,
H₂¹⁸O in Hot Dense Molecular Cloud Cores
Astron. Astrophys. 199, L5–L8 (1988).
Emission features were also detected toward Orion KL, Sgr B2(N), and W51 e1/e2, but not unambiguous because possible overlap with lines of other molecules. In the meantime, other transitions have been detected, too.

H₂¹⁷O was detected in emission through its 1₁₀ – 1₁₀ transition toward Orion KL:
Å. Hjalmarson, P. Bergman, N. Biver, H.-G. Flor&eacite;n, U. Frisk, T. Hasegawa, K. Justtanont, B. Larsson, S. Lundin, M. Olberg, H. Olofsson, G. Persson, G. Rydbeck, A. Sandqvist,
Recent Astronomy Highlights from the Odin Satellite
Adv. Space Res. 36, 1031–1047 (2005).

The 1₁₀ – 1₁₀ transition of doubly deuterated water was detected in emission and absorption toward IRAS 16293-2422, a low-mass protostellar binary system:
H. M. Butner, S. B. Charnley, C. Ceccarelli, S. D. Rodgers, J. R. Pardo, B. Parise, J. Cernicharo, and G. R. Davis,
Discovery of Interstellar Heavy Water
Astrophys. J. 659, L137–L140 (2007).

Water in its v₂ = 1 excited vibrational state were detected in emission first towards VY CMa and W Hya:
K. M. Menten and G. J. Melnick,
Hot Water around Late-Type Stars – Detection of Two Millimeter-Wave Emission Lines from the //v//₂ Vibrationally Excited State
Astrophys. J. 341, L91–L94 (1989).

Contributor(s): H. S. P. Müller; 04, 2007