Dawn Sees Dusk — NASA Asteroid Explorer Goes Dark

Dawn fires its ion engine as it approaches the dwarf planet Ceres in this artist illustration. The spacecraft recently lost its ability to point its antenna toward Earth, ending the mission. NASA/JPL-Caltech

NASA’s Dawn spacecraft missed its schedule communication session with NASA’s Deep Space Network on both Oct. 31 and Nov. 1 this past week.  After eliminating other causes, mission manages concluded that the spacecraft finally ran out of the fuel hydrazine, which it uses to point its antenna toward Earth for communications and turn its solar panels to the sun to recharge. While everything else may still function for a time, losing pointing ability and ultimately power means the end of the mission just as sure as your cellphone will die if you don’t charge the battery.

Dawn launched 11 ago to make close up studies of the asteroid belt’s two largest objects: asteroid 4 Vesta and the dwarf planet 1 Ceres. It’s still orbiting Ceres and expected to remain there for decades — 20 years for sure and probably at least 50 years. Variations in the density of material inside Ceres tugging this way and that on the spacecraft will slowly distort its orbit, which may ultimately bring it crashing down.

Vesta, a 329-mile-wide (525 km) asteroid was visited by NASA’s Dawn spacecraft from July 16, 2011 to September 4, 2012. The rocky asteroid is the brightest known and resembles the inner planets in having an iron core, mantle and crust. NASA / JPL-Caltech

Since it’s 2007 launch, Dawn’s traveled 4.3 billion miles (6.9 billion km) propelled by ion engines instead of conventional rocket fuel — the first time an ion drive has been the primarily mode of travel for a spacecraft. In an ion engine, an electric charge is applied to atoms of xenon, a gas used in electronic camera flashes. Then, an electrically-charged plate at the back of the engine accelerates the electrified atoms — called ions — out of the thruster. The ions push back against the engine as they leave, nudging the spacecraft forward.

Vesta and Ceres orbit within the asteroid belt between Mars and Jupiter at distances of 219 million miles and 257 million miles, respectively. NASA / McREL

Dawn achieved many firsts including the first probe to orbit a body in the asteroid belt and the first to go into orbit around two destinations. It arrived at Vesta on July 16, 2011 and mapped two ginormous impact basins 310 and 250 miles across, confirmed that a class of meteorites found on Earth originated from the asteroid and discovered that Vesta’s gravity field indicates it has an iron core 140 miles in diameter. That last one’s a big deal. An iron core implies that Vesta was once molten (due to heating by radioactive elements), causing it to separate into core, mantle and crust just like the Earth and other planets. Had it not stopped growing, Vesta may have become a recognized planet in its own right.

The bright spots in Occator Crater photographed by Dawn are composed of briny salts oozed up from the outer, ice-rich layer beneath Ceres’ crust. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Not quite four years later (March 6, 2015), we sat mesmerized in front of our computers as Dawn sent photo after photo of bright, white spots inside Occator Crater. After analysis by the probe’s science instruments, these turned out to be deposits of briny magnesium sulfate and ammonia-rich clays erupted from the dwarf planet’s interior, a finding that confirms Ceres’ low density of about 25% water ice by mass. It isn’t hard to imagine a scenario where an impact ruptured and temporarily heated the crust, creating passageways for mineral-rich, slushy ice to ultimately breach the surface. With no atmosphere, the water would soon vaporize, leaving the rocks coated with bright, salty minerals.

With a nod to Vesta, Dawn revealed that Ceres has a denser, rocky mantle surrounded by an ice-rich crust. Because of its ammonia-rich clay Ceres provides a link between the inner solar system, with its rocky planets, and the remote gas giants Jupiter and Saturn, which have atmospheres that contain significant amounts of ammonia.

Ahuna Mons, seen in this simulated perspective view, is Ceres’ largest mountain at about 3 miles high by 12 miles wide. Muddy water ice softened by its content of salts extruded onto the surface into recent geological time to create the feature, the elevation of which has been exaggerated by a factor of two in this image. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn also found recent and potentially current activity in several places on Ceres — an amazing discovery given the dwarf planet’s small size and its chilly location relatively far from the sun. Scientists suspect that salts, which lower the freezing point of water, may play a role in sustaining geological change over the age of the solar system.

Like the light of dawn that scrubs away the darkness to reveal the landscape around us, Dawn has illuminated our understanding of Ceres and Vesta.