Sky and Telescope - February 2018 - 10

NEWS NOTES

A Visitor from
Interstellar Space

Mars

Mercury

Venus
Sun

Earth

Path of
'Oumuamua
(1I/2017 U1)

Orbit of a
typical comet
within solar system
Neptune
Uranus

Saturn
Jupiter

WHEN THE PAN-STARRS 1 telescope
atop Haleakala on Maui swept up a
20th-magnitude object on October
19th, observer Robert Weryk (University of Hawai'i) noticed right away that
its motion didn't make sense. Orbital
computations soon showed that it had
passed within 0.26 astronomical unit
(38,200,000 km) of the Sun on September 9th. But its extreme orbital eccentricity, 1.19, meant that it's following a
hyperbolic trajectory that's not bound
to the Sun. For the ﬁrst time, observers had spotted an object visiting from
interstellar space.
At ﬁrst astronomers suspected it
was a comet, so the International
Astronomical Union's Minor Planet
Center (MPC) applied the designation
C/2017 U1. However, when even the

p 'Oumuamua appeared as a faint fuzz (in
cross-hairs) as recorded by Tenagra Observatory in Arizona on October 21st.

FE B RUA RY 2 018 * SK Y & TELESCOPE

Sun

p The track of the first-known interstellar
object, now named 'Oumuamua, as it passed
through the inner solar system. The inset shows
its location when found on October 19th.

deepest telescopic images revealed no
hint of a coma or tail, they decided it
must be asteroidal, so the designation
morphed to A/2017 U1.
Behind the scenes, IAU and MPC
ofﬁcials mounted an unusual effort to
name the new object quickly. The chosen name, 'Oumuamua, is a Hawai'ian
construct combining 'ou (to reach out)
and mua (meaning ﬁrst or in advance
of); the second mua is for emphasis.
But asteroids aren't cataloged by name
alone, and this is the ﬁrst of an entirely
new class of object. So, at the suggestion of MPC associate director Gareth
Williams, the IAU adopted the identiﬁer "I," for interstellar. As noted in the
naming announcement, "Correct forms
for referring to this object are therefore:
1I; 1I/2017 U1; 1I/'Oumuamua; and
1I/2017 U1 ('Oumuamua)."
Although the arrival of a faint object
from beyond the solar system caught
astronomers by surprise, they quickly
mobilized for follow-up observations
with powerful ground-based telescopes.
One lucky break came immediately
after the MPC's discovery announcement. Observer Joseph Masiero (Jet
Propulsion Laboratory) just happened to

be in the middle of a run with the 5-m
Hale Telescope at Palomar Observatory,
and he quickly obtained a spectrum at
visible and near-infrared wavelengths.
'Oumuamua is slightly reddish overall,
with the kind of unremarkable spectrum that a rocky surface would exhibit
after being "weathered" by long-term
exposure to space radiation.
While its spectrum seems reasonable, its shape borders on bizarre. Based
on pooled, rapid-response observations
from ﬁve big telescopes in Hawai'i and
Chile, the light curve for 1I/2017 U1
shows a 2½-magnitude range. As Karen
Meech (University of Hawai'i) and
others explain in the November 20th
Nature, this wide swing implies that the
object has an extremely elongated shape
- maybe 10 times longer than it is wide.
Moreover, the spin rate is relatively
fast, about 7.34 hours, so 'Oumuamua
must be made of rocky or metallic compounds with signiﬁcant tensile strength.
As the Nature researchers note, "It raises
the question of why the ﬁrst known
[interstellar object] is so unusual."
The Hubble Space Telescope might
have the ﬁnal word on the character of
1I/2017 U1. A team led by Meech was
granted enough Hubble time to study
the quickly disappearing interloper
three times before the end of 2017.
This object entered the solar system moving at 26 km per second
(58,000 mph). At that speed, in 10
million years it would traverse nearly
850 light-years. Eric Mamajek (Jet
Propulsion Laboratory) points out that
the object's incoming velocity is close
to that of the mean galactic velocity of stars that lie within 25 parsecs
(80 light-years) of the Sun, but it does
not match the relative velocity of any of
the dozen nearest systems. These characteristics all suggest that 'Oumuamua
has been drifting among the stars for a
very long time, perhaps billions of years.
The object entered from the direction
of the constellation Lyra, close to right
ascension 18h 50m, declination +35° 13′.
Now it's headed out of the solar system,
never to return, toward the Great
Square of Pegasus at 23h 51m, +24° 44′.
■ J. KELLY BEATTY

ORBIT DIAG R A M: LE A H TISCIONE, SOURCE: BROOKS BAYS / SOEST PUBLICATION SERVICES / UNIV. OF H AWAI'I;
'OU M UA M UA: PAULO HOLVORCEM & MICH A EL SCHWA RT Z (N ASA G R A N T #NN X15A E89 G)

Orbit of Jupiter

SOLAR SYSTEM

Sky and Telescope - February 2018

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