[meteorite-list] Dawn Mission Expected To Go Into Overtime at Ceres
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 12 Apr 2016 12:30:14 -0700 (PDT)
Dawn mission expected to go into overtime at Ceres
April 6, 2016
NASA's robotic Dawn spacecraft is now getting its closest look at the
dwarf planet Ceres, the largest resident of the asteroid belt, and mission
managers say the probe has enough leftover propellant to keep flying into
early 2017, several months beyond the prescribed end of its survey.
A year into its global study of Ceres, the Dawn spacecraft is now flying
in its closest orbit around the airless world at an average altitude of
240 miles (385 kilometers) and getting its most detailed snapshots of
the dwarf planet's alien surface.
Ceres is the Dawn mission's second destination after a visit to the
giant asteroid Vesta. Powered by an efficient ion propulsion system, Dawn
is the first spacecraft explore either of the diminutive worlds.
Scientists have identified many craters, fractures, mountains and ridges
across Ceres' landscape, which appears gray to the naked eye but is
rich in subtle color variations revealed by Dawn's instrument suite.
"Our first surprise was the topography," said Chris Russell, the Dawn
mission's principal investigator from the University of California,
Researchers expected Ceres - the largest object in the asteroid belt
between the orbits of Mars and Jupiter - to be relatively smooth except
for craters. Instead, Dawn found a rugged world with a rumpled, wavy-looking
horizon as viewed from Dawn's camera in orbit.
Many planetary scientists thought Ceres could have an icy outer shell,
a characteristic that would tend to favor a flatter surface.
"We found more topographic relief than we had expected," Russell said.
"It's about half that of Vesta, so it's more subdued than Vesta
but it's still more than we would have expected if that crust had been
an icy crust."
Scientists studying data from a sensor measuring the energy emitted as
cosmic radiation bombards the surface of Ceres have discovered the dwarf
planet is "loaded" with hydrogen in the high latitudes of its northern
hemisphere, said Tom Prettyman, principal investigator of Dawn's gamma
ray and neutron detector instrument from the Planetary Science Institute.
"If that's the case, what form does it take? We know that the main
reservoirs of hydrogen should be hydrogen-bound minerals and water ice,"
he said in a presentation last month at the Lunar and Planetary Science
Conference near Houston.
Prettyman said the extrapolation of preliminary data from the gamma ray
and neutron detector, which has only been operating a few months, indicates
water ice is at or near the surface near Ceres' north pole.
"Boy, is it a wonderful dataset," he said.
Textured imagery of Occator crater, a 57-mile (92-kilometer) basin first
glimpsed as Dawn approached Ceres in early 2015, has revealed that mysterious
bright spots there are irregular and likely made of salts left behind
as water evaporated from the crater.
"When we approached Ceres, it got ever-larger and we saw that ever-intriguing
bright spot on the surface, and the bright spot has continued to beguile
us," Russell told an audience at the Lunar and Planetary Science Conference.
A close-up view shows a dome sitting inside a smooth-walled pit in the
bright center of the crater, scientists said.
Scientists are still trying to determine the exact composition of the
material, which Russell describes as a white powder made of chlorides,
sulfides, carbonates, or a mixture of minerals.
"It is not a dropped flour bag," Russell joked, referring to one popular
description of the bright region's appearance.
Carol Raymond, Dawn's deputy principal investigator from NASA's Jet
Propulsion Laboratory, said Occator crater formed 70 to 80 million years
ago when an asteroid or comet slammed into Ceres.
The leading theoretical explanation for the formation of the bright spots
in Occator crater, and other impact sites on Ceres, is that the heat generated
by the collision melted a mixture of rock, ice and salts lurking inside
the frozen world's crust.
"That's a theory or a hypothesis that we're investigating closely
to try to explain whether we could get enough heat for a long enough period
of time, what type of material would react to that amount of heat and
become a cryo-magma - basically an ice- or salt-based magma that could
come to the surface and produce the kinds of strange features that we're
seeing," Raymond said.
But what caused the salts to persist at the surface when the water and
magma dried up in the aftermath of the impact?
"One of the issues that is a bit of a paradox is that the crater that
you saw is dated to be about 80 million years old," Raymond said. "The
bright material is extremely bright, and it's hard to keep things bright
on a planetary surface over time. Suffice it to say that we have a lot
of work to do to work out the detailed composition of the different areas
even within that crater, tie it back to the physics of the impact process
mobilizing subsurface material, and then trying to explain the persistence
of that bright material over geologic time."
One impact cavity, called Oxo crater, contains water ice on the surface.
So far, observations of the more famous Occator site have turned up no
signature of exposed ice.
Some craters show dried flow patterns outside their rims, evidence that
the impacts stirred up water and rock that streamed on Ceres' surface
for some period of time, according to Ralf Jaumann, the lead scientist
for Dawn's framing camera from DLR, the German Aerospace Center.
Raymond said Ceres appears to be a former ocean world and could have once
been similar to Europa or Enceladus, the icy moons of Jupiter and Saturn.
"One of the things that we anticipated about Ceres before getting there
is that it's a former ocean world," Raymond said. "We're so interested
in going to Europa and Enceladus, and these other interesting objects
in the outer solar system because we think they harbor subsurface oceans
at present, and possible habitable environments, and possibly even locations
where there's extant life.
"Ceres appears to have been one of those objects in the past, when it
was younger and hotter," Raymond said. "What we're looking at now
is, we believe, the remnant of a frozen ocean. The salt is left over from
the brines that were concentrated as the ocean froze out, so it's all
a fairly consistent story that Ceres is a former ocean (world) where the
ocean froze, and now we're interrogating the chemistry, essentially,
of that ocean-rock interface through the subsurface layers that we're
detecting on Ceres."
As Dawn's data continues to roll in to research labs and science teams,
engineers are focused on preserving the spacecraft's finite supply of
hydrazine rocket fuel in a bid to keep the mission running through early
The probe's primary mission ends June 30, but mission managers expect
Dawn to keep going for several months longer.
Engineers have battled issues with Dawn's reaction wheels, rapidly-spinning
devices which use momentum to keep the spacecraft pointed in the right
direction. Two of the four wheels have failed, causing the craft to rely
on its small rocket jets to stay pointed in some situations.
Dawn arrived in its low-altitude mapping orbit in December, and the mission's
ground team scripted the flight plan to complete all of the probe's
critical science observations in three months. Ceres' uneven gravitational
field disrupts the spacecraft's orbit, requiring occasional course corrections
that consume fuel, so engineers did not count on Dawn continuing beyond
Raymond said Dawn is now in position to keep going through at least June
"We are operating incredibly elegantly with two reaction wheels and
hydrazine jets," she told reporters last month. "We can finish our
primary mission. If we lost a wheel tomorrow, then we would continue
on using the reaction control system (jets) and be able to complete our
primary mission without any problem."
Manufactured by Orbital ATK, Dawn is currently orbiting Ceres with its
sensors pointed 5 degrees away from straight down toward the dwarf planet's
surface, an angle selected to further minimize consumption of hydrazine.
"If the wheels continue to operate, then it allows us to have more lifetime,"
Raymond said. "Right now, with the fuel on-board, we could last without
wheels through the beginning of August, and with wheels possibly about
a year from now. So there's quite a bit of uncertainty, but what we
are certain about is we can finish our primary mission."
Russell told Spaceflight Now that the Dawn science team is still discussing
plans for a potential extended mission.
If Dawn stays in its current orbit, it will likely run out of hydrazine
fuel in early 2017, Russell said.
The spacecraft could be moved back to a higher orbit, where it will burn
less hydrazine, to keep the mission going even longer in hopes of seeing
changes on Ceres' surface as it goes through a seasonal cycle, he said.
"One question we're asking is what is the time scale for changes on
the surface of Ceres? Maybe if we think we will see changes on the order
of days to years there is an interest in keeping the mission going,"
Russell said. "If the changes are only seen in millions of years, for
example, we would have to ask whether it is worth it?"
Russell said the Dawn mission could receive funding for a few extra months
without going through a peer review by a panel of independent scientists
- called a senior review - if the extension is short-term and does
not involve any major change in flight plan.
The senior review held every two years helps NASA determine which of its
operating space missions deserve extensions, and which projects have diminishing
Received on Tue 12 Apr 2016 03:30:14 PM PDT