It would just be too much money and too challenging

When NASA’s next big rocket launches for the first time, chances are good it won’t have people on board.

For the last two months, the space agency has been studying what it would take to fly a crewof two on the maiden flight of the Space Launch System, or SLS — the monster rocket that NASA has been developing to take people into deep space and on to Mars. Specifically, NASA wanted to know if such a crewed flight could be done safely by 2019. But after figuring out the costs and challenges associated with putting astronauts on that inaugural mission, called EM-1, NASA says doing a crewed flight first wouldn’t be the best way to go.

“IT IS DIFFICULT TO ACCOMMODATE CHANGES NEEDED FOR A CREWED EM-1 MISSION AT THIS TIME.”

“After evaluating cost, risk, and technical factors in a project of this magnitude, it is difficult to accommodate changes needed for a crewed EM-1 mission at this time,” NASA’s acting administrator, Robert Lightfoot, sent in an email to agency employees that was obtained by The Verge.

So for now, it seems that NASA will stick with its original strategy for debuting the SLS: doing an uncrewed flight first, followed by a crewed mission. However, the vehicle’s targeted launch dates are probably going to slip a bit. Originally, NASA had planned to launch the first SLS mission in November 2018, sending an uncrewed spacecraft called Orion on a three-week trip around the Moon. That mission plan still stands, but technical challenges and limited budgets are forcing NASA to push back the launch date to 2019. An exact date will be decided upon in the coming months. A follow-up flight with crew inside Orion is tentatively scheduled for no earlier than 2021, but will probably be pushed back as well.

NASA only started thinking about changing the mission plan earlier this year, after Lightfoot sent out a memo to employees saying NASA would study the feasibility of putting people on the first fight of the SLS. NASA later admitted that appointees for the Trump administration had directed the agency to look into that possibility, according to Space News.

Lightfoot said in press conference today that putting a crew on the first flight of the SLS was technically feasible. However, a lot of technologies would have to be accelerated, such as a life support system to keep the crew alive and abort system that could save the crew in case of an emergency during flight. Plus it would have cost a lot more money, approximately between $600 million and $900 million extra, and NASA would have needed additional time to pull it off. Lightfoot said that a 2020 flight was more likely if people needed to be on board. So NASA conferred with the White House and agreed that the best route would be stick with its original plan all along.

PUTTING CREW ON THE FIRST FLIGHT OF A NEW VEHICLE ISN’T USUALLY HOW NASA DOES THINGS

Putting crew on the first flight of a new vehicle isn’t usually how NASA does things, though. Typically, the agency does an uncrewed test flight first to determine if a vehicle is safe. The Saturn V rocket, for instance, did a mission without passengers before crew rode on board, and the future space taxis that SpaceX and Boeing are developing to take NASA astronauts to and from the International Space Station will fly empty first to make sure they are safe for crew. However, one notable exception to this rule was the first flight of the Space Shuttle, which carried a crew of two into orbit around Earth. It was a bold move at the time, but NASA needed a pilot in the Shuttle to control the vehicle’s landing.

It’s been bad news for the SLS program in recent weeks. NASA admitted that part of a propellant tank needed for testing out the rocket was damaged at the agency’s Michoud Assembly Facility in New Orleans. And one of NASA’s associate administrators, Bill Gerstenmaier, acknowledged to the Government Accountability Office two weeks ago that the first flight of SLS would likely slip to 2019.

Astronomers studying pulsars may have finally unravelled why these mysterious pulsating objects appear to send different kinds of signals into space – by tracking the immense cloud trails of their nebulae.

Thanks to images taken by NASA's Chandra X-ray Observatory, a new analysis of two very contrasting pulsars shows that pulsars' energy emissions may only be visible to Earth from certain angles – which could explain a mystery that's puzzled scientists up until now.

Pulsars are a kind of neutron star – an extremely dense core of matter left over once a massive star has exploded in a supernova.

What sets pulsars apart from other neutron stars are their pulsating radio waveemissions – a tightly focused beam of electromagnetic radiation that rapidly rotates around the pulsar much like the light shone from a lighthouse.

While pulsars were long categorised by the emission of these radio waves, more recently researchers have found that some pulsars also generate a different kind of energy signal – gamma rays – and the distinction has puzzled astronomers for almost a decade.

Geminga, artist's impression. Credit: Nahks Tr'Ehnl

"It's not fully understood why there are variations between different pulsars," says researcher Bettina Posselt from Pennsylvania State University.

"One of the main ideas here is that pulse differences have a lot to do with geometry – and it also depends on how the pulsar's spin and magnetic axes are oriented with respect to line of sight whether you see certain pulsars or not, as well as how you see them."

One of the ways scientists can study pulsars is by tracking what's called pulsar wind nebulae – clouds of energetic particles that form doughnut-shaped rings called tori, which stretch out into tail-like structures as they spin off from the rapidly rotating pulsar.

Thanks to the Chandra X-ray Observatory, scientists can now study these nebulae, which helps us understand the different kinds of magnetic activity that pulsars exhibit – and also why we only pick up certain emissions from them.

"This is one of the nicest results of our larger study of pulsar wind nebulae," says one of the team, physicist Roger W. Romani from Stanford University.

"By making the 3D structure of these winds visible, we have shown how one can trace back to the plasma injected by the pulsar at the centre."

Two very divergent kinds of pulsar wind nebulae can be seen in the pulsars Geminga – which is approximately 800 light-years from Earth – and BO355+54, which is about 3,300 light years away.

In the case of Geminga (shown above), you can see the pulsar has three distinct tails making up its nebulae. Of these, the two long tails are called lateral tails, and are thought to spin off from Geminga's magnetic poles.

These lateral tails stretch out for more than half a light-year from Geminga, while a third, shorter tail also emanates from the pulsar.

The researchers think the positioning of Geminga's poles is why we can only pick up gamma rays from the pulsar on Earth – but not radio emissions.

"The tails seem to tell us why that is," says Posselt. "For Geminga, we view the bright gamma ray pulses and the edge of the pulsar wind nebula torus, but the radio beams near the jets point off to the sides and remain unseen."

Top Left X-ray: NASA/CXC/PSU/B.Posselt et al; Infrared: NASA/JPL-Caltech. Top Right X-ray: NASA/CXC/GWU/N.Klinger et al; Infrared: NASA/JPL-Caltech. Illustrations by Nahks Tr'Ehnl.

By contrast, B0355+54 (pictured above alongside Geminga) has its radio waves angled at Earth, which is how we're able to detect it – even though we can't pick up its gamma rays.

"For B0355+54, a jet points nearly at us so we detect the bright radio pulses while most of the gamma-ray emission is directed in the plane of the sky and misses the Earth," says one of the team, Oleg Kargaltsev from George Washington University.

"This implies that the pulsar's spin axis direction is close to our line-of-sight direction and that the pulsar is moving nearly perpendicularly to its spin axis."

While there's still a lot to learn about pulsars, the new discoveries help explain why we've only picked up distinct emissions from these kinds of cosmic objects before – and lets researchers study particle physics in ways that would be impossible to replicate in a lab on Earth.