NASA has invented a new type
of autonomous space navigation that could see human-made spacecraft heading
into the far reaches of the Solar System, and even farther - by using pulsars
as guide stars. It's called Station Explorer for X-ray Timing and Navigation
Technology, or SEXTANT (named after an 18th century nautical navigation
instrument), and it uses X-ray technology to see millisecond pulsars, using
them much like a GPS uses satellites.
"This demonstration is
a breakthrough for future deep space exploration," said SEXTANT project
manager Jason Mitchell of NASA's Goddard Space Flight Center. "As the
first to demonstrate X-ray navigation fully autonomously and in real-time in
space, we are now leading the way."
 |
| A pulsar imaged in X-ray. (NASA/CXC/SAO/F.Seward et al.) |
Pulsars are highly
magnetised, rapidly rotating neutron stars - the result of a massive star's
core collapsing and subsequently exploding. As they spin, they emit
electromagnetic radiation. If an observer is in the right position, they can
appear as sweeping beams, like a cosmic lighthouse.
They're also extraordinarily
regular - in the case of some millisecond pulsars, which can spin hundreds of
times a second, their regularity can rival that of atomic clocks. This is what
led to the idea behind SEXTANT. Because these pulsars are so regular, and
because they're fixed in position in the cosmos, they can be used in the same
way that a global positioning system uses atomic clocks.
SEXTANT actually works like
a GPS receiver and it gets signals from at least three GPS satellites, all of
which are equipped with atomic clocks. The receiver measures the time delay
from each satellite and converts this into spatial coordinates. The
electromagnetic radiation beaming from pulsars is most visible in the X-ray
spectrum, which is why NASA's engineers chose to employ X-ray detection in
SEXTANT.
 |
| An illustration of NICER attached to the ISS. (NASA's Goddard Space Flight Center) |
To do so, they used a
washing machine-sized observatory attached to the International Space Station.
Called Neutron-star Interior Composition Explorer, or NICER, it contains 52
X-ray telescopes and silicon-drift detectors for studying neutron stars,
including pulsars.
They directed NICER to latch
onto four pulsars, J0218+4232, B1821-24, J0030+0451, and J0437-4715 - pulsars
so precise that their pulses can be accurately predicted for years into the
future.
Over two days, NICER took 78
measurements of these pulsars, which were fed into SEXTANT. SEXTANT then used
these measurements to calculate the position of NICER in its orbit around Earth
on the International Space Station.
This information was
compared to GPS data, with the goal being to locate NICER within a 10-mile (16
km) radius. Within eight hours, the system had calculated NICER's position, and
it remained below the 10-mile threshold for the remainder of the experiment. It
could take a few years for the technology to be developed into a navigation
system.
"This was much faster
than the two weeks we allotted for the experiment," said SEXTANT system
Architect Luke Winternitz. "We had indications that our system would work,
but the weekend experiment finally demonstrated the system's ability to work
autonomously."
This navigation system will
be suitable for deep-space vessels, but the concept has been proven. Now the
team is rolling up their sleeves to refine it. They will be updating and
fine-tuning its software in preparation for another experiment in the second
half of 2018. They also hope to reduce the size, weight, and power requirements
of the hardware.
Eventually, SEXTANT could be
used to calculate the location of planetary satellites far from the range of
Earth's GPS satellites, and assist on human spaceflight missions, such as the
space agency's planned Mars mission.
"This successful
demonstration firmly establishes the viability of X-ray pulsar navigation as a
new autonomous navigation capability," Mitchell said. "We have shown
that a mature version of this technology could enhance deep-space exploration
anywhere within the solar system and beyond."
This article was initially published on Sciencealert. You
can read the article here.
No comments