A NASA technician works on the primary mirror of the James Webb Space Telescope, scheduled for launch in October 2018. The telescope will be able to detect light from the earliest stars and galaxies and to search for evidence that distant planets have atmospheres conducive to life. (NASA)

The world’s most expensive telescope is parked for the moment in Greenbelt, Md., shrouded in a protective tent at the NASA Goddard Space Flight Center. In just two years, this long-delayed, $8 billion, cosmos-penetrating instrument is supposed to be nearly a million miles from Earth.

If it works, the James Webb Space Telescope will collect the oldest light in the universe, emitted soon after the big bang, when the first stars lit up and the first galaxies began to form. It will study black holes lurking at the center of galaxies. It will scrutinize the light from planets around distant stars and look for atmospheres you’d expect to see on worlds rioting with life.

But that’s only after an epic journey. It’s not a straight shot from the Washington suburbs to space.

The telescope first must be sealed in a climate-controlled container. Then, sometime in late March or early April, a truck will haul it very slowly and gently in the dead of night along a partially closed Capital Beltway. A lead car will watch for road obstacles and potholes.

Arriving at Joint Base Andrews, the Webb should slide, barely, into the cargo hold of a C-5C military transport plane.

An artist's illustration of the James Webb Space Telescope. The kite-shaped object beneath the telescope's gold-plated mirror is a sunshield to protect the Webb's instruments. (Northrop Grumman)

Then comes a flight to Houston, to the NASA Johnson Space Center, where it will be tested in a vacuum chamber that will simulate the environment of deep space. From Houston it will be flown to Los Angeles, to a facility run by the project’s primary contractor, Northrop Grumman, where it will be mated with its sunshield and navigational hardware.

Then comes a boat ride, one that will carry the telescope down the southwestern coast of North America and through the Panama Canal to French Guiana. That’s where, in October 2018, it will be blasted into space atop a European Ariane rocket — a quarter-century after the Webb was conceived.

What could go wrong?

“It’s wonderful and terrifying,” says astronomer Heidi Hammel of the Association of Universities for Research in Astronomy, one of six scientists guaranteed observing time with the instrument. “The terrifying part comes because we know that this is rocket science for real, we are taking our fabulous telescope — it’s a beautiful machine — and we’re going to put it on a rocket ship and light the fuse.”

Astronomers have a complicated and anguished relationship with the Webb. It’s amazing. It’s also wildly expensive. And it was supposed to be in space years ago.

John Mather, a Nobel laureate who is the senior project scientist, began working on it in 1995. He and his team had to achieve a long list of innovations to get the Webb built — things like the gold-covered mirrors, the sunshield and the means of keeping everything very cold.

“This job was just so hard, and when you’re at the beginning you don’t have the imagination to see how hard it is. No one had appreciated the difficulty of the test program,” he said this week.

(Zoeann Murphy/The Washington Post)

Cost overruns led to budget reviews, battles in Congress, near-death experiences, recrimination and, finally, to a reconfigured budget and timetable that lawmakers approved. The project devoured NASA money that might have gone to other science endeavors. It became known as the telescope that ate astronomy.

The Webb, which includes major contributions from the Canadian and European space agencies, has long been seen as the successor to the Hubble Space Telescope. The latter is still working fabulously but getting long in the tooth.

There have been rumors that the Trump administration could somehow gin up a Hubble repair mission with astronauts on a new shuttle-like spacecraft, but officials at NASA Goddard say they haven’t been asked to plan for anything like that.

The Webb is quite different from the Hubble, starting with the fact that it’s much bigger. The Hubble has a 2.4-meter mirror, but the Webb has 18 hexagonal mirrors that collectively are 6.5 meters — more than 21 feet — in diameter. It can collect seven times as much starlight.

The new telescope can observe the universe in infrared wavelengths of light that are inaccessible to the Hubble. In deep space, shaded from the sun, the Webb is designed to operate under extremely cold conditions. That’s necessary for infrared astronomy because otherwise the heat from its instruments would block out the faint light from distant objects.

As things turned out, the Webb’s problems with schedule may have been fortuitous.

The original goal was to look at very faint objects in deepest space — the very first stars and galaxies, so far away that their light, emitted about 13.7 billion years ago, is only now reaching our solar system. But the Webb may play a key role in the search for habitable worlds that are relatively nearby, orbiting stars in our own galactic neighborhood.

The very first “exoplanet” — a planet orbiting another star — was discovered 22 years ago, right about the time Mather began working on the Webb. Thousands of exoplanets have been found since.

Earlier this week, astronomers announced the remarkable discovery of a system with seven Earth-size planets around a star named Trappist 1, some 39 light-years away. Several have orbits that might allow water, if present, to be liquid at the surface.

There’s no way for the Webb or any other current telescope to see these planets directly, because they’re too close to the parent star. But as they transit the star — passing across the face of the star as seen from the telescope — the starlight will dim slightly. That’s how they were discovered.

An atmosphere around a planet will skew the wavelengths of the starlight. Astronomers can then use spectroscopy to discern which kinds of molecules make up the exoplanet atmosphere.

The Hubble has looked at the Trappist system and already has determined that the planets are probably rocky rather than gaseous like Jupiter or Saturn. But the Webb has sophistication the Hubble lacks. The Webb can detect the clear signatures of atmospheres containing water, ozone, oxygen, methane and other molecules. That could provide compelling evidence of a habitable planet.

As astrophysicist Michelle Thaller of NASA points out, while it wouldn’t be the same thing as direct detection of life, it would be a major achievement for the still-young scientific field known as astrobiology.

Closer to home, the Webb’s prime viewing targets include the Great Red Spot of Jupiter, Saturn’s intriguing moon Titan, the dwarf planet Pluto and its moon Charon, plus the gaggle of “trans-Neptunian objects” way out in the exurbs of the solar system — little-known worlds such as Sedna, Quaoar and Makemake.

But first the telescope, which is supposed to begin observing in April 2019, has to function as planned. It would be hard to repair something parked at L2, the Webb’s destination point in space, which is 930,000 miles from Earth on the opposite side of our planet from the sun.

“I can tell you that we’re doing what we need to do to make sure that it’ll work,” Mather said. “You test. And test and test.”

And look for potholes along the way — literally.