Pictorial representation of the ARTEMIS probes as they will orbit the moon beginning July 17. ARTEMIS P1 and P2 were the outermost two THEMIS probes before they began maneuvers on July 20, 2009, to swap an Earth orbit for a lunar orbit. Credit: UC Berkeley |
On
Sunday, July 17, 2011 the moon will acquire its second new companion in less
than a month. That’s when the second of two probes built by the
University of California, Berkeley, and part of NASA’s five-satellite
THEMIS mission will drop into a permanent lunar orbit after a
meandering, two-year journey from its original orbit around Earth.
The
first of the two probes settled into a stable orbit around the moon’s
equator on June 27. If all goes well, the second probe will assume a
similar lunar orbit, though in the opposite direction, sometime Sunday
afternoon. The two spacecraft that comprise the ARTEMIS mission will
immediately begin the first observations ever conducted by a pair of
satellites of the lunar surface, its magnetic field, and the surrounding
magnetic environment.
“With
two spacecraft orbiting in opposite directions, we can acquire a full
3D view of the structure of the magnetic fields near the moon and on
the lunar surface,” says Vassilis Angelopoulos, principal investigator
for the THEMIS and ARTEMIS missions and a professor of space physics at
UCLA. “ARTEMIS will be doing totally new science, as well as reusing
existing spacecraft to save a lot of taxpayer money.”
“These
are the most fully equipped spacecraft that have ever gone to the
moon,” adds David Sibeck, THEMIS and ARTEMIS project scientist at the
Goddard Space Flight Center (GSFC) in Maryland. “For the first time
we’re getting a unique, two-point perspective of the moon from two
spacecraft, and that will be a major component of our overall lunar
research program.”
The
transition into a lunar orbit will be handled by engineers at UC
Berkeley’s Space Sciences Laboratory (SSL), which serves as mission
control both for THEMIS (Time History of Events and Macroscale
Interactions during Substorms) and ARTEMIS (Acceleration, Reconnection,
Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun).
“We are on our way,” says Manfred Bester, SSL director of operations. “We’re committed.”
What makes the auroras dance?
The
five THEMIS satellites (or probes) were launched by NASA on Feb. 17,
2007 to explore how the sun’s magnetic field and million-mile-per-hour
solar wind interact with Earth’s magnetic field on Earth’s leeward side,
opposite the sun. Within a year and a half, they had answered the
mission’s primary question: Where and how do substorms in the Earth’s
magnetosphere—which make the auroras at the north and south poles
dance—originate?
The
answer: the storms originate deep in the planet’s shadow, about a third
of the way to the moon, where magnetic field lines snap, reconnect, and
unleash a storm of energy that funnels to the poles and makes the
atmosphere glow in reds and greens. Large storms can wreak havoc on
satellites, power grids, and communications systems.
Mission
accomplished, the THEMIS team was eager to divert two of the probes to
the moon to extend their magnetic field studies farther into space. One
key reason was that the two probes most distant from Earth would soon
die because, with too much time spent in Earth’s shadow, their
solar-powered batteries would discharge.
To
achieve this new mission, the UC Berkeley and Goddard teams, with the
assistance of experts at the Jet Propulsion Laboratory in Pasadena,
charted the 150 fuel-saving orbital maneuvers needed to boost the two
THEMIS spacecraft from Earth’s orbit into temporary orbits around the
two Earth-moon Lagrange points, which are spots in space where the
gravitational attraction from the moon and Earth are equal. That
transfer took about 18 months, after which Goddard colleagues kept the
two spacecraft in Lagrange-point orbits for several months before the
first probe (P1) was transferred into lunar orbit last month.
“That
was an engineering challenge; this is the first mission where we’ve
piloted into a lunar orbit spacecraft not designed to go there,” says
Daniel Cosgrove, the UC Berkeley engineer who controls the spacecrafts’
trajectories. The probes’ small thrusters, for example, only push down
and sideways. The probes are also spinning, which makes maneuvering even
more difficult.
Also,
last year probe P1 lost a spherical sensor from the end of one of four
long wires that protrude from the spacecraft to measure electrical
fields in space. The probable cause was a micrometeorite that cut a
10-ft section off of the 82-ft wire and caused it to retract into
its original spherical housing, sending the “little black sphere flying
through the solar system,” Bester says.
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“All
five spacecraft have been built by a very talented team with enormous
attention to detail,” he says, predicting that the ARTEMIS probes could
survive for another 10 years, longer than the three remaining THEMIS
probes, which repeatedly fly in and out of Earth’s dangerous Van Allen
radiation belt.
Lunar orbit
Once
the second probe, P2, is in orbit, the two ARTEMIS satellites will
graze the lunar surface once per orbit—approaching within a few tens
of kilometers—in a belt ranging 20 degrees above and below the equator
while recording electric and magnetic fields and ion concentrations.
“When
the moon traverses the solar wind, the magnetic field embedded in the
rocks near the surface interacts with the solar wind magnetic field,
while the surface itself absorbs the solar wind particles, creating a
cavity behind the moon,” Angelopoulos says. “We can study these complex
interactions to learn much about the moon as well as the solar wind
itself from a unique two-point vantage that reveals for the first time
3D structures and dynamics.”
Sibeck
noted that NASA’s twin STEREO spacecraft, launched in 2006, already
provide a 3D perspective on the sun’s large-scale magnetic fields. “THEMIS and ARTEMIS study the microscale processes, which we now know
run the system,” he says.
One goal of the ARTEMIS mission is to look for plasmoids, which are hot blobs of ionized gas or plasma.
“THEMIS
found evidence that magnetic reconnection propels hot blobs of plasma
both towards and away from the Earth, and we want to find out how big
they are and how much energy they carry,” Angelopoulos says. “Plasmoids
could be tens of thousands of kilometers across.”
“THEMIS found the cause and now ARTEMIS will study the consequences, which are likely massive and global,” Sibeck says.
The
spacecraft also will study the surface composition of the moon by
recording the solar wind particles reflected or scattered from the
surface and the ions sputtered out of the surface by the wind.
“These
measurements can tell us about the properties of the surface, from
which we can infer the formation and evolution of the surface over
billions of years,” Angelopoulos says.
The
two ARTEMIS probes will join NASA’s Lunar Reconnaissance Orbiter, which
has been orbiting the moon since 2009 taking high-resolution
photographs and looking for signs of water ice. In September, NASA is
scheduled to launch two GRAIL (Gravity Recovery and Interior Laboratory)
spacecraft to map the moon’s gravitational field, and in 2013, the
agency plans to launch LADEE (Lunar Atmosphere and Dust Environment
Explorer) to characterize the lunar atmosphere and dust environment.
“ARTEMIS will provide context for the LADEE mission,” Sibeck says.
Three
other non-functioning satellites remain in orbit around the moon: two
subsatellites of Japan’s lunar orbiter, Kaguya, which was guided to a
crash on the surface in 2009; and India’s Chandrayaan-1, which lost
communication with Earth that same year. China’s second lunar orbiter,
Chang’e 2, left the moon for interplanetary space on June 8.
