Sporty sendoff planned for NASA's LADEE moon mission
BY STEPHEN CLARK
Posted: April 8, 2014
NASA's LADEE space probe is in the final weeks of its $280 million lunar research mission as engineers put the diminutive spacecraft through a series of tricky maneuvers to test its limits and maximize its scientific return.
Running low on fuel, the mission is expected to end around April 21, when LADEE crashes into the far side of the moon. The violent ending will come less than 33 weeks since the probe launched in September, but it keeps with the mission's script.
The mission was designed for a short lifetime, and its primary data-gathering phase lasted just 100 days and ended in March.
Since arriving at the moon in early October, LADEE has "overachieved" its objectives, said Joan Salute, LADEE program executive at NASA Headquarters in Washington.
LADEE's three science instruments scooped up dust particles, identified the chemical make-up of the moon's atmosphere, and looked for signs of hydrated compounds, such as water and hydroxyl (OH), migrating from the moon's middle latitudes toward polar cold traps in permanent shadow, where scientists say ice can sit undisturbed for billions of years.
The unique spacecraft also hosts a laser communications terminal, which successfully accomplished the first laser linkup between the moon and Earth, demonstrating a new optical relay paradigm that could drastically boost data return from future deep space missions.
"It's always bittersweet," said Butler Hine, LADEE's project manager at NASA's Ames Research Center in California. "The mission was such a resounding success that everybody on the team is incredibly proud of the performance of the spacecraft and the team members."
Officials will not know exactly when the end will come. The probe's trajectory is designed to impact somewhere on the far side of the moon around April 21.
"This represented six years of very hard work by a lot of people, so you're both very proud of the success and you're also a little melancholy that you won't be talking to the spacecraft again," Hine told reporters Thursday. "I think folks will actually miss talking to the spacecraft."
Controllers last week fired LADEE's main engine to lower the spacecraft's altitude, putting it on a trajectory to skim as low as 1 or 2 miles over the moon's mountaintops.
On Friday, engineers plan the mission's final engine burn to put LADEE on course to impact somewhere on the far side of the moon around April 21.
"That drops us to this relatively low spot that we can continue flying in for the rest of the mission until the planned impact," Hine said.
A major test for LADEE will come April 15, when the Earth will move between the moon and the sun. The occurrence will create a spectacular lunar eclipse visible from Earth, but it will stress LADEE near its technological limits.
LADEE was never designed to survive such an eclipse, when its solar panels will be starved of sunlight and the spacecraft must survive on stored battery power alone. LADEE will also get very cold during the four-hour eclipse, according to Hine.
"Parts of the spacecraft may get so cold that the components freeze and then don't thaw well and function afterward," Hine said. "So far, our predictions are that our power system will be fine, the on-board computer will be fine, and the sensors should be fine."
But the propulsion system is a different story.
"The plumbing and valves involved in the propulsion system may get some ice formed," Hine said, likening the risk to frozen pipes in homes on cold winter nights. "Parts of the spacecraft propulsion system may freeze and either stay frozen because we can't get heat to them afterward, or actually damage themselves by expansion of the fluid."
Hine said experts predict LADEE's rocket thrusters, tanks and plumbing will be fine, but "this is an engineering test to see what happens."
Developed and manufactured at Ames, the bullet-shaped LADEE spacecraft measures 7.7 feet long and 4.7 feet in diameter.
The exact timing of LADEE's collision with the moon is unpredictable because the lunar gravity field is lumpy, with pockets of relatively strong and weak gravity tugging on the spacecraft as it zips around the moon every two hours.
Officials do not know how the probe's orbit will change as it flies at very low altitudes, where the effect of gravity is amplified.
Hine said NASA decided to aim the spacecraft's impact on the far side of the moon to avoid the chance of falling on one of the Apollo landing sites, which are clustered on the side of the moon facing Earth.
During each orbit, LADEE is out of contact with Earth when it is on the back side of the moon. It re-establishes radio contact with ground controllers when it is again in line-of-sight communications with an antenna on Earth.
Hine said the LADEE team will learn of the mission's end when it fails to call home after a trip around the moon's far side.
LADEE will likely hit the moon at a grazing angle, either ramming into the side of a mountain or crater wall, or bouncing across a flat lunar plain like a rock skimming across a pond, according to Hine.
A high-resolution camera on NASA's Lunar Reconnaissance Orbiter may try to find LADEE's impact point.
Rick Elphic, LADEE project scientist at Ames, summed up LADEE's final moments: "If you hit anything at 1,600 meters per second [3,579 mph], that's not a landing you walk away from. It's by no means gentle. This is a very high speed impact, and even though there's the possibility of tumbling across the surface, theres nothing gentle about it. It will be destroyed."
Elphic said flying LADEE at lower altitudes in its final weeks will result in a rich scientific loot.
"We've had 100 days of nominal science plus, and now we're dropping down to an even lower altitude," Elphic said. "We're expecting to see new things in an exciting further chapter in the science of the tenuous lunar atmosphere and its dust environment -- right down to a few kilometers above the surface."
The moon's atmosphere is nothing like our own. Its atoms never collide, technically making the lunar atmosphere an exosphere.
But the moon's atmosphere is an analog to most atmospheres in the solar system, so scientists have used LADEE's observations as a proxy to understand the atmospheres of Mercury, the moons of other planets and even large asteroids.
"We didn't know much about the lunar atmosphere and dust environment before we went there," Elphic said, adding LADEE was the first mission ever to chase questions about the environment around the moon.
"By the time LADEE is over, and we have published the science results, we will have at least quadrupled the number of species known to be in the lunar atmosphere," Elphic said.
LADEE picked out "exotic" atoms in the lunar atmosphere, such as neon, magnesium, aluminum, titanium and oxygen. "That's just a short list of what we expect is there in our data," Elphic said.
The probe's dust counter charted more than 11,000 impacts from dust particles since October, according to scientists at the University of Colorado at Boulder, which leads the $6 million dust experiment.
"LADEE has discovered a dust veil that enshrouds the moon perpetually," Elphic said.
The dust is kicked up from the lunar surface by strikes of tiny micrometeoroids, the same minuscule materials that create shooting stars as seen on Earth.
"Each one of those little micrometeoroids strikes the surface and sends up a large cloud of debris, which then LADEE flies through," Elphic said.
There is more dust at lower altitudes, so Elphic expects LADEE's last act will tell scientists more about the complex interactions between meteoroid impacts and the moon's dusty halo.
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