[meteorite-list] NASA's Stardust: Good to the Last Drop

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 24 Mar 2011 14:55:31 -0700 (PDT)
Message-ID: <201103242155.p2OLtVaN011831_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2011-094

NASA's Stardust: Good to the Last Drop
Jet Propulsion Laboratory
March 23, 2011

On Thursday, March 24 at about 4 p.m. PDT (7 p.m. EDT), NASA's Stardust
spacecraft will perform a final burn with its main engines.

At first glance, the burn is something of an insignificant event. After
all, the venerable spacecraft has executed 40 major flight path
maneuvers since its 1999 launch, and between these main engines and the
reaction control system, its rocket motors have collectively fired more
than 2 million times. But the March 24 burn will be different from all
others. This burn will effectively end the life of NASA's most traveled
comet hunter.

"We call it a 'burn to depletion,' and that is pretty much what we're
doing - firing our rockets until there is nothing left in the tank,"
said Stardust-NExT project manager Tim Larson of NASA's Jet Propulsion
Laboratory in Pasadena, Calif. "It's a unique way for an interplanetary
spacecraft to go out. Essentially, Stardust will be providing us useful
information to the very end."

Burn to depletion will answer the question about how much fuel Stardust
had left in its tank.

"We'll take those data and compare them to what our estimates told us
was left," said Allan Cheuvront, Lockheed Martin Space Systems program
manager for Stardust-NExT. "That will give us a better idea how valid
our fuel consumption models are and make our predictions even more
accurate for future missions."

Fuel consumption models are necessary because no one has invented an
entirely reliable fuel gauge for spacecraft. Until that day arrives,
mission planners can approximate fuel usage by looking at the history of
the vehicle's flight and how many times and for how long its rocket
motors have fired.

Stardust's burn to depletion is expected to impart valuable information,
because the spacecraft has essentially been running on borrowed time --
for some time. Launched on Feb. 7, 1999, Stardust had already flown past
an asteroid (Annefrank), flown past and collected particle samples from
a comet (Wild 2), and returned those particles to Earth in a sample
return capsule in January 2006 - and in so doing racked up 4.63 billion
kilometers (2.88 billion miles) on its odometer. NASA then re-tasked the
still-healthy spacecraft to perform a flyby of comet Tempel 1, a new,
low-cost mission that required another five years and 1.04 billion
kilometers (646 million miles). After all those milestones and all that
time logged on the spacecraft, the Stardust team knew the end was near.
They just didn't know exactly how close.

Prior to this final burn, Stardust will point its medium-gain antenna at
Earth - some 312 million kilometers (194 million miles) away. As there
is no tomorrow for Stardust, the spacecraft is expected to downlink
information on the burn as it happens. The command from the spacecraft
computer ordering the rockets to fire will be sent for 45 minutes, but
the burn is expected to last only between a couple of minutes to
somewhat above 10 minutes. It is estimated the burn could accelerate the
spacecraft anywhere from 2.5 to 35.2 meters per second (6 to 79 mph).

"What we think will happen is that when the fuel reaches a critically
low level, gaseous helium will enter the thruster chambers," said
Larson. "The resulting thrust will be less than 10 percent of what was
expected. While Stardust will continue to command its rocket engines to
fire until the pre-planned firing time of 45 minutes has elapsed, the
burn is essentially over."

Twenty minutes after the engines run dry, the spacecraft's computer will
command its transmitters off. They actively shut off their radios to
preclude the remote chance that at some point down the road Stardust's
transmitter could turn on and broadcast on a frequency being used by
other operational spacecraft. Turning off the transmitter ensures that
there will be no unintended radio interference in the future.

Without fuel to power the spacecraft's attitude control system,
Stardust's solar panels will not remain pointed at the sun. When this
occurs, the spacecraft's batteries are expected to drain of power and
deplete within hours.

"When we take into account all the possibilities for how long the burn
could be and then the possible post-burn trajectories, we project that
over the next 100 years, Stardust will not get any closer than 1.7
million miles of Earth's orbit, or within 13 million miles of Mars
orbit," said Larson. "That is far enough from protected targets to meet
all of NASA's Planetary Protection directives. "

Some planetary spacecraft, like the Galileo mission to Jupiter, are
intentionally sent into the planet's atmosphere to make sure it is
destroyed in a controlled way. Others have their transmitters shut off
or just fade away, said Larson. "I think this is a fitting end for
Stardust. It's going down swinging."

Stardust-NExT is a low-cost mission to expand the investigation of comet
Tempel 1 initiated by NASA's Deep Impact spacecraft. JPL, a division of
the California Institute of Technology in Pasadena, manages the
Stardust-NExT project for the NASA Science Mission Directorate,
Washington, D.C., and is part of the Discovery Program managed by NASA's
Marshall Space Flight Center in Huntsville, Ala. Joe Veverka of Cornell
University, Ithaca, N.Y., is the mission's principal investigator.
Lockheed Martin Space Systems, Denver, built the spacecraft and manages
day-to-day mission operations.

Use this link to experience Stardust's final hour before
decommissioning, then use Eyes on the Solar System to relieve the entire
mission from 1999 to 2011: http://go.usa.gov/2ry. A free software
download is required.

For more information about Stardust-NExT, please visit:
http://stardustnext.jpl.nasa.gov.

DC Agle 818-393-9011
Jet Propulsion Laboratory, Pasadena, Calif.
agle at jpl.nasa.gov

2011-094
Received on Thu 24 Mar 2011 05:55:31 PM PDT