Sky & Telescope - November 2021 - 36

80 YEARS: WHERE WE'RE GOING by Monica Bobra
To Boldly Go
By mid-century, we may have sent a craft to look
back at ourselves from beyond the solar system.
I
n the last decade, the notion of interstellar travel went from
fiction to reality. In 2012, after 35 years of interplanetary
travel across 122 astronomical units, Voyager 1 left the solar
system and crossed into interstellar space. In 2018, Voyager 2
did the same.
But the only other transmitting spacecraft on a trajectory
to interstellar space, New Horizons, is not even halfway
there. The scientific community's tenuous connection to
this unexplored region therefore depends on the 44-year-old
Voyagers staying alive or the New Horizons craft surviving
another couple decades.
Unless we send a dedicated mission to interstellar space.
U.S. astronomers have proposed interstellar missions since
1960; since then, NASA and the European Space Agency
(ESA) have entertained a couple dozen interstellar mission
concepts, none coming to fruition. But a few ideas persist
today. Interstellar Probe, a mission proposed by the Johns
Hopkins University Applied Physics Laboratory and funded by
NASA as a concept study, is one of them. Interstellar Probe
could travel to 200 a.u. in 30 to 40 years - beyond where
the Voyagers are now - and answer some of the biggest open
questions about our star's relationship with the galaxy.
THE HELIOSPHERE The Voyager spacecraft
have passed the heliosphere's
boundary, but proposed missions could
reach much farther - even perhaps to
the Alpha Centauri star system.
Seeing the Heliosphere
The Sun produces a wind, full of particles and magnetic fields,
that carves out a magnetic bubble in space. This heliosphere
envelops the solar system and protects us from high-energy
galactic particles. But data from the Voyager spacecraft as they
left the heliosphere raised more questions than answers (S&T:
Sept. 2020, p. 16). What does the heliosphere look like? What
is the material composition of interstellar space? And what
happens at the boundary where these two meet?
Interstellar Probe could answer these questions by turning
around and taking a picture of our home star's heliosphere,
like the dusty Pale Blue Dot image of Earth taken by Voyager
1. In fact, it could take three.
The first picture would show the heliosphere's shape.
Astronomers know that the heliosphere shelters us from
harmful, high-energy particles, and similarly protected
environments may harbor life elsewhere. But what it actually
looks like remains unclear.
To solve this mystery, astronomers can use energetic neutral
atoms, particles that arise when the solar wind bumps into
the interstellar medium, to map the boundary of the heliosphere.
While the Interstellar Boundary Explorer (IBEX)
is making these maps from vantage points near Earth, it
measures atoms at energies too low to probe the entire
heliosphere. The future Interstellar Mapping and Acceleration
Probe (IMAP) will make similar maps with an even larger
energy range and thus reach farther distances.
However, both these missions only measure energetic
neutral atoms that travel within the heliosphere, not the ones
HELIOSPHERE
KUIPER BELT
TERMINATION SHOCK
HELIOPAUSE
EARTH
BOW
WAVE
10 a.u.
102
(83 light-minutes)
1 a.u.
*Not to scale
(8.3 light-minutes)
36 NOVEMBER 2021 * SKY & TELESCOPE
a.u.
(831 light-minutes)
JOHNS HOPKINS APL
Sky & Telescope - November 2021

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