An upper limit on gas production from 3200 Phaethon

Asteroid 3200 Phaethon resembles a comet in some ways, including a highly-eccentric orbit (e∼0.89e∼0.89) and a strong associated meteor shower (the Geminids). Yet this object has never been observed to exhibit any cometary activity, i.e., gas production. We observed 3200 Phaethon with the Caltech Submillimeter Observatory on two occasions, once while it was near its closest approach to Earth as it neared perihelion, and another while it was further from Earth post-perihelion. Observations of the J=2→1J=2→1 and J=3→2J=3→2 rotational transitions of 12CO, typically strong lines in comets and indicative of gas production, yielded no detection. Upper limits on the 12CO production of 1.8×10281.8×1028 and 7.6×1028 moleculess−1 for Phaethon were determined on these two occasions.