NASA’s Voyager 2 has just entered deep space &
it has sent back this message to scientists. Today, we take a look at what the Voyager
2 just sent back to scientists.
There are mysteries all around us, and we are
fascinated by what we cannot understand. Inexplicable phenomena are discovered
every day, keeping our curiosity piqued and the prospect of actually knowing
the world around us a foreign concept.
Every-day it is possible to find something new and
exciting, big or small, in the most extreme places on Earth. Some take us to
different planets, and tell us about the cosmos, some tell us about our
histories. All of them leave us eager to learn more.
Twelve billion miles from Earth, there is an elusive
boundary that marks the edge of the sun’s realm and the start of interstellar
space. When Voyager 2, the longest-running space mission, crossed that frontier
more than 40 years after its launch it sent a faint signal from the other side
that scientists have now decoded.
The NASA craft is the second ever to travel beyond
the heliosphere, the bubble of supersonic charged particles streaming outwards
from the sun. Despite setting off a month ahead of its twin, Voyager 1, it
crossed the threshold into interstellar space more than six years behind, after
taking the scenic route across the solar system and providing what remain the
only close-up images of Uranus and Neptune.
Now Voyager 2 has sent back the most detailed look
yet at the edge of our solar system – despite Nasa scientists having no idea at
the outset that it would survive to see this landmark.
“We didn’t know how large the bubble was and we
certainly didn’t know that the spacecraft could live long enough to reach the
edge of the bubble and enter interstellar space,” said Prof Ed Stone, of the
California Institute of Technology, who has been working on the mission since
before its launch in 1977.
The heliosphere can be thought of as a cosmic
weather front: a distinct boundary where charged particles rushing outwards
from the sun at supersonic speed meet a cooler, interstellar wind blowing in
from supernovae that exploded millions of years ago. It was once thought that the
solar wind faded away gradually with distance, but Voyager 1 confirmed there
was a boundary, defined by a sudden drop in temperature and an increase in the
density of charged particles, known as plasma.
The second set of measurements, by Voyager 2, give
new insights into the nature of the heliosphere’s limits because on Voyager 1 a
crucial instrument designed to directly measure the properties of plasma had
broken in 1980.
Measurements published in five separate papers in
Nature Astronomy reveal that Voyager 2 encountered a much sharper, thinner
heliosphere boundary than Voyager 1. This could be due to Voyager 1 crossing
during a solar maximum (activity is currently at a low) or the craft itself
might have crossed through on a less perpendicular trajectory that meant it
ended up spending longer at the edge.
The second data point also gives some insight into
the shape of the heliosphere, tracing out a leading edge something like a blunt
bullet.
“It implies that the heliosphere is symmetric, at
least at the two points where the Voyager spacecraft crossed,” said Bill Kurth,
a University of Iowa research scientist and a co-author on one of the studies.
“That says that these two points on the surface are almost at the same
distance.”
Voyager 2 also gives additional clues to the
thickness of the heliosheath, the outer region of the heliosphere and the point
where the solar wind piles up against the approaching wind in interstellar
space, like the bow wave sent out ahead of a ship in the ocean.
The data also feeds into a debate about the overall
shape of the heliosphere, which some models predict ought to be spherical and
others more like a wind sock, with a long tail floating out behind as the solar
system moves through the galaxy at high speed.
The shape depends, in a complex way, on the relative
strengths of the magnetic fields inside and outside of the heliosphere, and the
latest measurements are suggestive of a more spherical form.
There are limits to how much can be gleaned from two
data points, however.
“It’s kind of like looking at an elephant with a
microscope,” Kurth said. “Two people go up to an elephant with a microscope,
and they come up with two different measurements. You have no idea what’s going
on in between.”
From beyond the heliosphere, the signal from Voyager
2 is still beaming back, taking more than 16 hours to reach Earth. Its
22.4-watt transmitter has a power equivalent to a fridge light, which is more
than a billion billion times dimmer by the time it reaches Earth and is picked
up by Nasa’s largest antenna, a 70-metre dish.
The two Voyager probes, powered by steadily decaying
plutonium, are projected to drop below critical energy levels in the mid-2020s.
But they will continue on their trajectories long after they fall silent. “The
two Voyagers will outlast Earth,” said Kurth. “They’re in their own orbits
around the galaxy for 5bn years or longer. And the probability of them running
into anything is almost zero.”
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