Voyage to the stars
Lance
By Leonard David
posted: 06 July 2005
06:41 am ET
To send a spacecraft where none has gone before is a dream assignment
for any space scientist and engineer.
The idea of dispatching a dedicated robotic probe on an interstellar
trek has been seriously advocated for nearly 30 years. A recently
finished appraisal of how to build such a craft shows that it is within
reach -- but nonetheless remains a challenging task.
A NASA-sponsored study team has blueprinted an Innovative Interstellar
Explorer (IIE).
Goal of the IIE would be to plunge outward some 200 Astronomical Units.
An astronomical unit (AU) is the mean distance between the center of the
Earth and the center of the Sun - approximately 93 million miles (150
million kilometers).
Put that in your cosmic calculator and you'll get: 18,591,186,595 miles
(29,919,614,600 kilometers).
No matter how you figure it, that's a lot of infrequent flyer miles to
travel.
Pull out all the stops
"I think we're converging on something that's doable," said Ralph
McNutt, the principal investigator for the Innovative Interstellar
Explorer study. He is a space scientist at The Johns Hopkins University
Applied Physics Laboratory (APL) in Laurel, Maryland, and also the
project scientist for APL's Messenger mission to Mercury.
If launched in 2014, the probe would arrive at the 200 AU distance in
about 2044.
Clearly, along with doses of technology and a requisite high velocity,
IIE demands a good deal of patience from awaiting scientists here on Earth.
McNutt said the IIE concept has moved forward over the last several
years as a NASA Vision Mission project, through a Team X study group at
Jet Propulsion Laboratory (JPL), and by way of NASA Institute for
Advanced Concepts study grants.
"There is always interest in keeping an interstellar probe going through
various studies. But the problem that we've always run up against is
propulsion. That's because you're trying to pull out all the stops and
get out of the Solar System as fast as you can," McNutt told SPACE.com.
Continuous acceleration
In sketching out IIE, McNutt said the relatively small spacecraft would
use radioisotope power sources that energize small electric thrusters.
"You need to make a spacecraft that is Ômassless' as possible," he said,
something akin to "unobtanium".
Still, in taking a hard look at conjuring up an interstellar probe --
and the kind of science that could be done by the craft -- things do
look promising.
"We can start building them tomorrow, if we had the money," McNutt said.
Efficient, lightweight electric propulsion and power systems are part of
the key.
The Innovative Interstellar Explorer would carry a compact science
payload weighing all of 66 pounds (30 kilograms). It would be boosted
from Earth atop a Delta 4 heavy launcher. A custom upper stage is also
required.
That initial launch energy in 2014 would push IIE to an arrival at
Jupiter two years later where the probe would acquire a gravity assist
from the massive planet. Long-term, low-thrust, continuous acceleration
of the probe would be provided by a kilowatt-class ion thruster running
off electricity provided by advanced Stirling radioisotope generators.
Burnout speed
The interstellar probe would cruise out of the solar system at about 7.8
AU per year. It would accelerate to a "burnout" speed of 9.5 AU per year
at 103 AU in November 2029.
In contrast, Voyager 1 is speeding about 3.6 AU per year. Its twin,
Voyager 2, is a slow poke at about 3.3 AU per year.
"The IIE would end up being at 200 AU in about 2044," McNutt noted.
"Yes, I know that sounds like a long time from now. But you have to keep
this into perspective. When we started out with the Voyagers, none would
have believed they'd still be going," he said.
In fact, the dual Voyager spacecraft launched in 1977 are now dubbed the
Voyager Interstellar Mission (VIM), investigating the outermost edge of
the Sun's domain ... and beyond. The Voyager probes are expected to go
dead around 2025 as their plutonium-fueled generators are fully discharged.
Decades of lifetime
The 30-year mission by IIE to address fundamental scientific questions
about the interaction of the Sun with the interstellar medium, McNutt
said, can only be answered via on-the-spot measurements that such a
mission would provide.
"The idea here is to try to get as close to true interstellar space as
possible ... past the heliopause and past what we think is an external
bow shock," McNutt pointed out.
But traveling for three decades to do so is tough going.
"Designing something that's going to be good for 30 years ... that
scares the daylights out of people," McNutt admitted. "How to qualify
all the parts for decades of lifetime is a hard problem. But it's
something that we're going to eventually come to terms with. We'll
figure out how to deal with it."
Receiver farms
Pumping back science data from afar is also a challenge. And Earth will
also have to keep up its end of the bargain.
Already in the works is a hoped for upgrade of NASA's Deep Space
Network, turning huge antenna dish complexes into "receiver farms" --
that is, multiple numbers of smaller antennas. That could enable
reception of downlink signal from IIE of about five kilobytes per second
out at 200 AU, McNutt said.
At some point, as IIE keeps moving onward and outward, it will
increasingly become more difficult for ground controllers on Earth to
reprogram the spacecraft's onboard data processing unit. "The probe will
then have to decide for itself when to phone home. It's not going to be
you asking for the call," McNutt said.
Pulling the plug on the Voyagers
The recent chorus of disapproval about turning off the long-distance
Voyager craft may be a good sign of support for interstellar probe
development. The very idea of shutting down the venerable Voyager probes
to save a few million dollars per year sparked public and Congressional
comment.
"I can't answer why there was an outcry," McNutt said, "but I'm
certainly glad there was one."
Indeed, Voyager 1 relayed in May evidence that it reached the
heliosheath - an area just where the solar wind crashes into the thin
interstellar gas of the galaxy.
In a House of Representatives hearing last month, Congressman Vernon
Ehlers of Michigan implored NASA chief, Michael Griffin, to keep a
scientific ear attuned to Voyager. "It seems foolish to wait all these
years to reach the heliosheath and then suddenly pull the plug," Ehlers
told Griffin.
"I assure you that I also think it's rather dumb to be turning off
Voyagers 1 and 2," Griffin responded. Nonetheless, a NASA assessment on
what spacecraft will be kept in operation and which ones will not is to
be wrapped up later this year, he added.
Step to the stars
While the Innovative Interstellar Explorer is a real study, not real
hardware, McNutt said the reviews are favorable for such a probe to be
lofted within a reasonable period of time.
"This would be a stepping stone. A lot of people resonate with the
science fiction side about taking the first step to the stars," McNutt said.
Given a go-ahead, McNutt senses that the Innovative Interstellar
Explorer could be done for less than $2 billion. While some might
consider that inexpensive, there would also likely be a debate about
whether the first dedicated step to the stars is worth it, he said.
"Basically, it's kind of like throwing a note out in a bottle ...
throwing it out to sea and seeing what comes back," McNutt said. "If we
really want to understand the world around us, we ought to be out there
making those measurements."