It’s a bit of a mixed bag for planetary science fans today. On the one hand, Cassini has just returned the closest ever images of Saturn. On the other, the Dawn spacecraft is in a spot of bother.
One of the two remaining working reaction wheels used to orient the spacecraft currently in orbit around Ceres has malfunctioned, according to NASA. This essentially means the spacecraft is not able to turn itself properly anymore.
The wheel (one of four) stopped working on April 23, with engineers discovering the issue a day later. However, they’ve been able to use thrusters on the spacecraft to replace the job of this wheel, so apparently the rest of the mission will not be affected.
"We already have a great deal of experience flying Dawn without reaction wheels," Marc Rayman, chief engineer for Dawn, told IFLScience. "We are well prepared to continue the mission without the use of wheels."
Reaction wheels are used by spacecraft to change their orientation in a particular axis. They are essentially gyroscopes, spinning in a particular direction to change which way a spacecraft is looking.
They are incredibly useful, as they need little or no fuel, but they are not without problems. Perhaps the most infamous reaction wheel issue occurred on NASA’s planet-hunting Kepler telescope. When it lost three of its four wheels, scientists instead had to use the force of the Sun’s photons hitting the spacecraft as a makeshift wheel.
On Dawn, it has hydrazine thrusters that can replace the job of this wheel, and the team had apparently been prepared for this event. Its other two failed wheels stopped working in 2010 and 2012, so Rayman said they were ready for this third wheel failing, and the mission can continue as planned.
Dawn is still in orbit around Ceres, where it has been for more than two years, and it is about to perform a new study on the mysterious Occator Crater, which has a lot of bright material in it.
By positioning itself between the crater and the Sun, it’s hoped we’ll get new insight into what this material is exactly. The spacecraft will be brought down into an orbit about 25,400 kilometers (15,800 miles) above Ceres.
Dawn is expected to continue operations around Ceres until later this year, when it will be turned off and left in orbit around the dwarf planet. In July 2016, it was denied an exciting new mission to go and explore an asteroid called 145 Adeona in May 2019. Instead, NASA decided to keep the spacecraft in orbit around Ceres.