NASA's flying telescope SOFIA chases down a moon shadow

Updated

One remarkable night on board a flying telescope.

"We are level at 34,000 feet," says mission director Karina Leppik, addressing her crew on their radio headsets. "Do you guys have any issues with me opening the door?"

"No," comes the reply.

At Karina's command, a large hatch near the back of the plane slides slowly open, exposing a pristine 2.7-metre mirror to the night sky.

This is the deck of NASA's airborne observatory, a heavily modified 747 currently traversing the Tasman Sea.

It's time to put the telescope through its paces. In a few hours' time, it will fly through the shadow of a moon — and there's no second chance to get this right.

SOFIA, the Stratospheric Observatory For Infrared Astronomy, offers a unique view of the stars because it flies above most of the water vapour that impedes telescopes on the ground.

But it has one other big advantage: you can put it exactly where you want it on the planet.

"We can take it places and get to events that otherwise there's no telescope to get to," says Karina. "And that's what we're doing tonight."

The target

In a rare and fleeting coincidence, a faraway star and Saturn's huge moon Titan are about to line up perfectly in space. This means that tonight, Titan will cast a faint shadow on the surface of the Earth.

The shadow is continent-sized — several thousand kilometres wide — but it will race across the southern hemisphere at more than 20 kilometres per second.

At its very centre, there's an opportunity.

"When the star's directly behind Titan — like, right in the centre — Titan acts like a lens and focuses the starlight on us," explains Michael Person, an astronomer from MIT.

"So the starlight passes through the atmosphere, all around the limb of the planet, and all of that light reunites here at the observer."

Titan is the only other place in the solar system with seas, clouds and rain. Its rivers run with chilly methane instead of water, but the similarity to Earth's fluid cycle is important.

By collecting starlight that has passed through Titan's atmosphere, Michael and his colleagues will measure tonight's conditions on the giant moon.

"We can tell the temperature and pressure of the atmosphere from how the light is bent; we can tell how much dust is in the atmosphere from how much of the light goes away. We put all those effects together and we build a profile of the atmosphere."

If SOFIA can capture a snapshot of Titan's hazy shroud, then tonight's results can be compared to previous measurements.

"What I'm looking for in particular is long-term change in the atmosphere," says Michael.

He and his team want to understand how atmospheres evolve — and there are precious few atmospheres available for study, especially ones which bear comparison to Earth.

"Our own atmosphere has long-term changes. And how those sorts of things work is more easily understood by studying different examples, in different conditions."

Some past insights into Titan were gathered at much closer range, by the spacecraft Cassini.

But the last time Titan lined up for an observation like tonight's, known as an occultation, was back in 2003. Michael and his team have done a lot of maths to predict and confirm that another occultation will happen at 3:07am tonight, New Zealand time.

"The spin-up to an event takes years, but each event only takes three minutes," he says, with a nervous chuckle, during the tense but business-like hours before take-off from Christchurch Airport.

"It's a very focused stress."

You can understand Michael's butterflies. Tonight's mission was his idea.

After a final weather forecast and safety briefing, a crew of 20 will board SOFIA and chart a carefully plotted path to catch Titan's shadow as it flits across the South Pacific between Cairns and Vanuatu. It's a 10-hour round trip.

And a flying telescope doesn't come cheap: SOFIA's running costs, shared between NASA and the German space agency DLR, total about $US1 million per flight.

The chariot

SOFIA looms large on the tarmac in Christchurch, far from the bustle of the passenger terminal but still enveloped by the rasping din of an active airport.

Its tail, complete with enormous NASA logo, stands a metre taller than a regular 747, while the fuselage is shorter and stockier.

This is a 747SP: one of just 45 "special performance" jets made by Boeing, mostly in the late 1970s, to cover new long-haul distances.

SOFIA, number 18 off the production line, enjoyed a long first career shuttling passengers around the world from 1977 until 1994.

NASA brought the plane out of retirement in 1997 and spent the next decade adapting it to carry a large, German-built infrared telescope, exposed to the sky.

Anyone who has opened a window in a fast-moving car will understand the scale of this aerodynamic challenge.

Countless test flights, computer simulations and wind-tunnel experiments helped to refine a subtle, slightly bulbous reshaping of the rear fuselage, which sends the passing air smoothly over the top of the 4m-by-5m door.

The design is so good that co-pilot Spike Tellier has to check a light on the instrument panel to know that the door has opened.

"A lot of folks have asked me, 'Hey Spike, when they open the door, can you feel it?' And I go, 'Absolutely not.'

"We have about a 2 per cent degradation, with the door open, on fuel mileage — which is essentially nothing."

One hour after take-off, the door is now wide open and the telescope is cooling down to match the outside temperature, somewhere below -50 degrees Celsius.

The billion-dollar instrument has its own compartment, safe behind a pressure bulkhead.

Further forward in the main cabin, three telescope operators are starting to boot up and test their machine's various systems. Michael Person is watching and waiting. He's still nervous.

Upstairs, a flight engineer and a navigator are checking and double-checking the manoeuvres that will set the aircraft on its path to intercept Titan's shadow.

The chase

As mission director, Karina Leppik has to coordinate the efforts of the technicians, pilots, navigators and scientists.

"The most critical time is the lead-up to the occultation event itself, because we need to get the timing right," she says.

"Once we start the leg 40 minutes or an hour before the occultation point, the scientists need to be taking data that whole time, so we can't make course corrections."

That's because, although the telescope can swivel vertically to see from 20 to 60 degrees above the horizon, the only way to turn it horizontally is to steer the whole aeroplane. So the direction of an observing run has to be calculated — and maintained — with unerring precision.

"We're following these curved paths along the earth ... because we're tracking a star in the sky," Karina says. "If we change the timing a little bit, then the track will change."

Tonight's expedition combines that effort to stare constantly at a target star with the high-stakes pursuit of a moving shadow, to catch that star slipping behind Titan.

Even a 747 can't keep pace with a shadow that shifts 22km every second, so the flight plan intersects the predicted path of the shadow from side-on.

If all goes according to plan, the star will dim as SOFIA enters the shadow — then briefly, crucially, glimmer brighter.

"What we really want to get is at the centre line, where we have the potential of seeing a central flash, where the entire moon is equally illuminated by the star behind it," Karina says.

"For that we need to be right on the location at the right time, within a few seconds."

Jeff Wilson has the job of making that coincidence a reality. He's SOFIA's navigator but his hard-won experience comes from flying B52s in the US Air Force.

"It's the same skill set. I had to get over a target on time and drop my payload," he says.

"This is what I've done for 30 years, basically. So I've got a lot of techniques and things that I could pull out of my sleeve."

One of those well-worn tricks is called a "timing trombone". It's a flight leg that takes the aircraft away from its target, but which can be shortened or lengthened so that the final approach starts at exactly the right time.

There are two trombones in the flight plan.

"Because we're so high up, we're airspeed limited. You can't use airspeed for timing — maybe a little bit, but not much," Jeff explains.

"So you have to use geometry. It's kind of like playing pool."

About four-and-a-half hours into the flight, Jeff and the cockpit team execute their final turn. This interplanetary puzzle is approaching crunch time.

Naru Sadakuni, one of two deputy mission directors, briefs the telescope team.

"We are going to maintain 37,000 for the entire leg. And our predicted temperature is not going to change much, so you guys should be good."

On monitors right around the flight deck, the vast planet Saturn appears as a distractingly bright splash, over-exposed and bleeding light across the screen. Just to its left, the telescope operators locate Titan - and their target star, about to swing behind it.

It's time to start recording.

"OK, taking data," says instrument scientist Enrico Pfueller.

A message comes down from the cockpit that despite a troubling forecast earlier in the evening, the pilots are reporting no high clouds ahead.

"Can you see the data saving?" Michael asks.

"Yes," Enrico replies, after a pause.

It never hurts to check.

The instrument

"Fifteen minutes." Naru has started a countdown to the point where the telescope should detect a central flash, in the middle of Titan's shadow.

The whole crew starts to gather around monitors on the main deck, keen to see whether SOFIA bags the occultation.

Their mood is more inquisitive than anxious. It's the telescope, now, which has the most difficult job: maintaining a perfectly steady gaze while peering out of a hatch on a moving plane.

It is stabilised by gyroscopes and mounted on a large, spherical bearing that allows it to rotate and stay focussed, even if the plane wobbles in flight.

And although the mirror itself sits just behind the bulkhead, parts of the apparatus protrude into the main cabin, so the crew can see if the telescope shifts.

"The telescope is sitting still in the air, but we are moving around it," explains Friederike Graf, a pointing and control engineer from the Deutsches SOFIA Institut in Stuttgart.

"It's very confusing when you look at it ... but it's actually very stable."

Just in front of the bulkhead a stuffed toy, somebody's lucky mascot, sits strapped into a chair.

"Astronomers are fairly superstitious," Michael explains later.

"One of the other members of our group won't observe an occultation unless there's a chocolate pop-tart nearby."

Michael himself has donned the same pink shirt he wore on a previous, successful SOFIA mission.

These good-luck charms are a fun distraction, he says, from simply thinking about whether the mission will succeed or not.

There's a lot riding on the next few minutes.

Titan's shadow will strike the Earth shortly and race towards SOFIA. The predicted dip in the star's light is so subtle — just 1 or 2 per cent — that the team might not even be able to confirm tonight whether the telescope has picked it up.

"We may not know before we land," Karina says. "For tonight, success is: we were in the right place at the right time."

It's all about crossing the expected flash point with the telescope pointed in the right direction.

The shadow

Almost exactly five minutes out, some of the screens go dark. A ripple of tension passes through the crew.

An error in the telescope assembly has caused it to drift off target.

Mercifully, the telescope team clears the error fast and within 45 seconds the call comes: "Alright, we're back on tracking."

Michael breathes out. "That was bracing."

"Nice job, TOs," Naru tells the telescope operators.

"Four minutes."

As the critical instant approaches, everyone on board is glued to a screen, either following SOFIA's progress against the flight plan or studying one of the live read-outs from the telescope.

It's impossible to tell for sure when the plane enters the shadow.

"Thirty seconds out."

On the noisy deck of a 747 above the Coral Sea, five hours of flight time and years of preparation bear down on a single moment.

Halfway across the solar system, Titan spins indifferently in its orbit.

But a few faint rays of starlight have bounced through its atmosphere and are falling towards the open hatch of a flying telescope.

"Five seconds."

Imperceptibly, the target location slides past. It releases none of the tension.

Michael is hunched over his laptop, searching for hopeful signs among the data.

"That hump there happened at exactly the right time," he mutters.

A minute later, Karina confirms what can be established from the flight systems: the plane crossed two miles right of the predicted dead centre of the shadow, one second late. That should be more than good enough.

Even better, Michael is now almost certain he can see evidence of a central flash in the light curve.

"I think this is probably it," he says, smiling at last.

"It's distorted by the transparency changes, but we can get rid of that. Maybe this is real!"

As the last of the data trickles from the telescope onto hard drives, the scientists share relieved grins, handshakes and a packet of Tim Tams. They're already discussing the number crunching and interpretation they'll need to do in coming weeks.

"We were right there in the centre of the shadow. You can see in the preliminary analysis, there's a flash in the middle. So we're very happy," Michael says.

"It's just a wonderful feeling to see the predictions match up with reality."

For Michael and his team back at MIT, the next few months will be spent analysing, checking and re-checking data to reveal a brand new snapshot of Titan's atmosphere.

For mission director Karina, it's mission accomplished.

"This is great. We did as well as we possibly could have done," she says. "We were within one second of our timing … and preliminary results look good."

Karina is a veteran of 140 previous SOFIA flights, but only a handful of those have been occultations.

"It's a lot of fun. These are my favourite; they're operationally challenging."

The home straight

After the end of the observing run and a series of technical test measurements, all that remains is to cover the 3,000km back to Christchurch.

Almost all.

SOFIA's telescope is a delicate gadget, so getting the door closed again is critical.

"With the infrared optics on the telescope, if we put sunlight on it, we wreck it," explains co-pilot Spike Tellier. "We break a lot of things that are very expensive."

If the hatch sticks open, which has happened before, there's a contingency plan to divert to Brisbane while it's still dark. Everybody on board has packed their passport.

Thankfully, on Karina's instruction, the hatch crawls shut without incident.

"We're headed back to New Zealand," she tells the crew.

It's past 5:00am and there are three hours of flying to go. Everybody can relax — except the pilots.

For them, landing is often the trickiest part of the journey.

"When you've been up all night and you get tired, it takes a lot of effort to concentrate," Spike says.

But SOFIA is in safe hands. Like most of the contractors who take shifts in the cockpit, both pilots are ex-Air Force. Spike himself spent 11 years flying the E4 airborne command post, another old model of 747.

"I love this airplane," he says.

As the ageing jumbo inches back towards the coast, bold stripes of colour paint the windows. The sun is rising over a cloudy Tasman Sea.

The view calls to mind the object of the mission, in all its mystery. Does Titan have tufty clouds like these? Its atmosphere is thick and wet enough.

Might it harbour microbial life? And if its temperature and pressure are fluctuating, what can this teach us about our own precious atmosphere?

These are questions tonight's grand experiment will only begin to answer.

On the descent to Christchurch, pink dawn light gently clips the peaks of the Southern Alps.

It's hard to imagine a sight quite like this on any other world.

Credits:

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Topics: science-and-technology, astronomy-space, stars, telescopes

First posted