Planetary
scientists are coloring in the family portrait of our solar system as
close-up photographs and observations stream back from Pluto, a world three billion miles away with towering mountains of ice, vast smooth plains and many mysteries yet to be revealed.
The flyby of Pluto last week by NASA’s
New Horizons spacecraft is rightly celebrated as a triumph of human
ingenuity, the capstone of a mission that unfolded nearly flawlessly.
Yet it almost did not happen, which would have left Pluto as just a hazy dot of light.
New Horizons overcame skeptical NASA
officials, repeated threats to its funding, laboratory troubles that
constricted the amount of plutonium available to power the spacecraft
and an unforgiving deadline set by the clockwork of the planets. Though
none of the obstacles packed the drama of space-exploration crises like
the Apollo 13 mission, their number and magnitude seemed unbelievable.
The
story of New Horizons, the little spacecraft that could, and did, visit
a small planet that is now considered too small to be a planet, started
15 years ago when NASA called it quits on Pluto.
For
a decade, concepts for sending a mission there had been studied but
never done. In 2000, the price tag for the latest incarnation, called
Pluto-Kuiper Express, appeared to be getting out of control.
“When
it was canceled,” Dr. Stern said, “the associate administrator at the
time, Ed Weiler, held a press conference and said: ‘We’re out of the
Pluto business. It’s over. It’s dead. It’s dead. It’s dead.’ He repeated
himself three times.”
Many planetary scientists and Pluto fans reacted in dismay, especially as it seemed to be a case of then or never.
Pluto
had reached the closest point of its orbit to the sun in 1989 and was
on the outbound trek, turning colder. Scientists worried that Pluto’s
tenuous atmosphere would turn to ice and fall to the ground, making
Pluto a much less interesting place to study until it neared the sun
again — two centuries later, when they would be long gone.
There was a second orbital consideration. The quickest way to Pluto is to take a left turn at Jupiter,
using the giant planet’s gravity for acceleration, which cuts the
travel time by four years. But a launch after January 2006 would mean Jupiter would be too far out of alignment to provide a boost.
Stamatios
Krimigis, then the head of the space department at the Johns Hopkins
Applied Physics Laboratory in Maryland and a member of a committee that
advised NASA on missions to the outer planets, recalled Dr. Weiler’s
asking him in the fall of 2000 whether it would be possible to do a
low-cost Pluto mission similar to the Near Earth Asteroid Rendezvous spacecraft that the laboratory had built and operated for NASA a few years earlier.
“I said, ‘Well, we can look at that,’ ” Dr. Krimigis said in an interview. He was intrigued but uncertain.
Dr.
Krimigis pulled together a small group who worked over the Thanksgiving
holiday to come up with a cost estimate: $500 million including the
rocket. That quick study sketched out a basic design that would turn
into New Horizons.
A
few months later, NASA put out a call for proposals, a competition to
design a new Pluto mission that would arrive by 2015 and cost less than
$500 million.
The
Johns Hopkins team knew how to build spacecraft, but the science of
Pluto was not its expertise. For that, Dr. Krimigis reached out to Dr.
Stern, the head of the Southwest Research Institute’s space studies
department in Boulder, Colo.
Dr.
Stern was a member of the “Pluto Underground,” a dozen planetary
scientists who in 1989 met in a Baltimore restaurant and discussed how
to push NASA toward a Pluto mission. Over the years, he had worked on
various studies for Pluto missions, none of which had paid off.
But Dr. Stern, who rallied efforts to persuade NASA to again consider a Pluto mission, liked what he heard from Dr. Krimigis.
They discussed, compromised and then agreed.
In November 2001, NASA chose New Horizons. “We busted our butts, and we won it,” Dr. Stern said.
That
started a four-year, two-month sprint to design, build and test the
spacecraft and get it to the launching pad — but almost immediately
there was an obstacle. “Two months later, the Bush administration
canceled it,” Dr. Stern said, laughing.
The
president’s budget proposal for 2003 included no money for Pluto, the
second year in a row that the administration had tried to kill such a
mission. But Congress, persuaded by Senator Barbara A. Mikulski of
Maryland, inserted earmarks in the spending bills to keep the Pluto
mission on track.
“Every year Congress had to keep us on life support,” said Glen Fountain, New Horizons’ project manager.
In
2002, the National Academy of Sciences named Pluto a top priority for
NASA’s planetary science missions. “At that point, you could feel things
change,” Mr. Fountain said.
Managers
of spacecraft missions often talk about the trade-offs between cost,
schedule and risk. Too quick and too cheap greatly raise the chance of
failure. “We don’t believe in that,” Dr. Krimigis said.
With
just seven instruments, the craft was about the size of a grand piano.
Mr. Fountain said the philosophy at the Johns Hopkins laboratory is to
stick to proven technologies and keep the design to the essentials,
which reduce cost and avoid delays without increasing risk. The one
compromise, he said, was a digital radio receiver that would consume
less power. The Johns Hopkins laboratory had already started working on
the technology in a separate project.
“We didn’t think it was a huge risk,” Mr. Fountain said.
Development continued without any showstoppers, although the cost rose to $722 million.
Then,
in August 2004, the Department of Energy informed the New Horizons team
that it could not provide the plutonium power source. At the far
reaches of the solar system, the sun is too dim for a spacecraft to rely
on solar panels or batteries. Instead, a chunk of radioactive plutonium
generates heat that is converted to electricity.
Security
lapses and safety issues at the Los Alamos National Laboratory in New
Mexico had shut down the production of plutonium dioxide pellets for New
Horizons’ power generator. Not enough had been made to provide the 220
watts called for in the design.
“We said, ‘How much power could you deliver?’ ” Mr. Fountain said. The reply: 180 watts.
Because
of design decisions like the digital radio receiver, Mr. Fountain
thought the reduced power would be sufficient. In the end, the
Department of Energy was able to build a power generator that put out
200 watts during the flyby.
New Horizons launched on top of an Atlas 5 rocket on Jan. 19, 2006, making the fastest-ever trip out of Earth’s neighborhood.
Thirteen months later, the craft was at Jupiter
already, and the mission team put its instruments to the test. New
Horizons captured a volcanic eruption on Io, one of Jupiter’s four big
moons. That was the first ever observed from a volcano not on Earth.
Just
after the Jupiter flyby, New Horizons suffered its first computer
glitch. For spacecraft outside Earth’s protective atmosphere,
high-energy cosmic rays occasionally zip through computer memory,
causing a crash and restart. Calculations indicated that there would be
one such crash during the nine-and-a-half-year trip to Pluto.
Instead, they occurred almost once a year. But none caused lasting damage, and they proved good learning experiences.
“It was just eventful enough to keep us alert,” said Christopher Hersman, the missions systems engineer. “It actually helped.”
The
rest of the long cruise was mostly uneventful. Flinging a spacecraft to
a rendezvous at the edge of the solar system is indeed rocket science,
but not groundbreaking rocket science. The equations — the basic laws of
Isaac Newton — are the same ones that were used decades ago.
Still,
the engineers were careful with their calculations — tiny errors can
grow calamitous — and periodic checkups made sure everything was working
smoothly on the spacecraft.
Then,
on July 4, 10 days before the Pluto flyby, the spacecraft suddenly fell
silent. Alice Bowman, the mission operations manager, said years of
experience had given her a sense when a problem might be the fault of
the receiving stations and when it might be a problem with a spacecraft.
“I pretty much knew it was something on the spacecraft,” she said.
She called Mr. Hersman. “Where are you?” she said, summoning him urgently to the missions operations center.
After
an Independence Day barbecue with neighbors, Mr. Hersman was already on
the way to the office anyway. “Going in, I was thinking, ‘Remain
calm,’ ” Mr. Hersman recalled.
Ms. Bowman called Mr. Fountain. He, too, headed in.
Thoughts
of the worst popped into his mind: “Could we have been so extremely
unlucky that we hit something?” Even debris the size of a grain of rice
could destroy a delicate craft moving so fast.
But
it turned out the spacecraft’s computer had overloaded trying to do two
things at once — receive instructions for the flyby while compressing
images in its memory banks. By design, the main computer entered what
engineers call “safe” mode to avoid damage to the spacecraft, and the
backup computer kicked in.
An
hour and a half later, the ground stations detected the signal from the
backup computer. “Then I knew we could do it,” Ms. Bowman said. “The
question was, could we do it in time?”
A
nine-day sequence of commands to guide New Horizons through the flyby
was set to begin on July 7. Ms. Bowman spent two nights at the office,
taking only short naps. “You would be amazed how much that can do,” she
said. “I can’t say I slept.”
With
hours to spare, the craft was back in operation. Then the flyby
directions kicked in, and New Horizons did everything it was told to do.
Correction: July 19, 2015
Because of an editing error, an earlier version of a picture caption with this article misstated the date of a spin test of the New Horizons spacecraft. It was in 2005, not 2006.
Because of an editing error, an earlier version of a picture caption with this article misstated the date of a spin test of the New Horizons spacecraft. It was in 2005, not 2006.
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