UPDATE: In a new release put out today, NASA announced that it had ultimately decided against plans to send the Dawn spacecraft to visit an additional asteroid. Apparently whoever posted yesterday's Dawn journal was not informed of this decision. "The long-term monitoring of Ceres, particularly as it gets closer to perihelion—the part of its orbit with the shortest distance to the Sun—has the potential to provide more significant science discoveries than a flyby of Adeona,” said NASA Director of Planetary Science Jim Green.

The announcement does come with good news for fans of the unknown: New Horizons has been approved for a visit to a Kuiper Belt object known as 2014 MU69. That rendezvous will take place in 2019. Given the unexpected nature of Pluto, visiting a second body in the Kuiper Belt will provide some much-needed perspective on these worlds.

Original story:

Thursday marked the official end of the primary mission of NASA's Dawn spacecraft. Dawn demonstrated the potential of ion engines in the exploration of the Solar System, as it was able to rendezvous with and enter orbit around two different asteroids, Vesta and Ceres. Scientifically, its findings have changed what we thought we knew about some of the bodies of the asteroid belt.

Dawn isn't going to shut down now that its mission is over, as it's in reasonable working order and still observing Ceres. But there's a hint that NASA has bigger plans for the spacecraft.

Yesterday, the Jet Propulsion Lab (which operates Dawn) put up and then removed what it calls a "Dawn Journal" entry. The entry described the future plans for Dawn, and they don't involve staying in orbit around Ceres. Instead, the craft's ion engines will be used to gradually nudge it away from Ceres. The low-power, high-efficiency engines will take until the end of the year to get the spacecraft free of the dwarf planet's gravitational pull.

Back in orbit around the Sun, the craft will head toward a 2019 rendezvous with yet another asteroid, this one called 145 Adeona. This asteroid, discovered in 1875, is rather large at 150km in diameter, but it's much more typical of the bodies in the asteroid belt than either Ceres or Vesta. Plans for the approach would allow for a relatively slow pass at 4,700 km/hour—one-tenth the speed of New Horizons' Pluto flyby.

Dawn has had two reaction wheels fail, which means that it is burning some of its maneuvering fuel (hydrazine) in order to remain oriented to communicate with Earth. Heading back to deep space should allow the craft to minimize fuel use, extending its lifespan well beyond what would be possible if it were to remain parked near Ceres.

Presumably, NASA will re-announce this plan sometime in the next couple of days once formal approval has been given.