Sky & Telescope - February 2021 - 8

NEWS NOTES

Spacecraft Takes a Bite of Asteroid Bennu

IN BRIEF

CHEERS ERUPTED in the control room

In preparation for OSIRIS-REX's sampling
maneuver on Bennu, the mission team
mapped the surface at a resolution down
to 5 cm (2 inches) per pixel. Besides enabling a safe landing, the images revealed
unexpected properties of Bennu's surface.
Daniella DellaGiustina (University of Arizona) and colleagues report September 21st
in Nature Astronomy six anomalously
bright, large boulders that stand out from
the rest of the much darker surface. The
rocks' compositions indicate their likely origin on another asteroid: 4 Vesta. Perhaps
Bennu picked them up as it wandered
through the asteroid belt to its present
trajectory inside Mars's orbit. The images
also showed reflective patches on some
otherwise-dark boulders, which caught the
eye of Hannah Kaplan (NASA Goddard)
and colleagues. They found that the bright
spots are rich in carbonates, particularly
calcite, a highly reflective form of calcium
carbonate. In the October 8th Science,
Kaplan and colleagues make the case that
these veins of minerals precipitated out of
hot water flowing through cracks deep inside a young planetesimal. Bennu is likely
a chip off that planetesimal block, formed
from the debris of a catastrophic collision
about 1 billion years ago.
JEFF HECHT

on October 20th as NASA's OSIRISREX mission successfully " tagged " the
diamond-shape asteroid 101955 Bennu.
The spacecraft's touch-and-go sample acquisition mechanism (TAGSAM)
dug its head up to 48 cm (19 inches)
into the regolith, puffed nitrogen gas
to stir up and capture surface material,
and reared away.
Just approaching the boulder-strewn
surface was a feat for the OSIRIS-REX
team: The surface was much rougher
than expected, and the team took extra
time to select a landing site. As it ﬂew
in, the spacecraft ﬁrst had to slip past
a two-story-high boulder nicknamed
" Mount Doom " before targeting a clear
slot about the width of two parking
spots. The mission was 18.5 lightminutes from Earth, so the spacecraft
navigated the approach on its own.
Though the ﬁnal landing target was
much smaller than the mission's initial
25-meter-radius requirement, OSIRISREX touched the surface within a meter
of where it was supposed to land.
T OSIRIS-REX's sampling arm contacts Bennu.

The day after collection, the mission team imaged the TAGSAM to
conﬁrm success. In fact, the maneuver was almost too successful: The
sample overﬂowed the head as multiple large rocks wedged open a Mylar
ﬂap and allowed some smaller grains
to escape. The team acted quickly,
using the TAGSAM arm to place the
precious cargo in the return capsule
on October 27th.
Immediately stowing the sample
meant forgoing a spin test, which
would have " weighed " the collection by measuring the spacecraft's
moment of inertia and comparing
it to a baseline obtained before the
touch-and-go maneuver. Nevertheless, calculations showed the collected mass exceeded expectations:
The team estimates the asteroid
encounter ﬁlled the TAGSAM head
with 258 to 575 grams (0.56 to
1.26 lb) of material - well above the
minimum target of 60 grams set for
the mission. It appears only a small
fraction of this was lost due to the
open Mylar ﬂap.
Although the sampling phase is
complete, OSIRIS-REX won't begin
its return to Earth until March 2021,
and the return trip will take some
time: The ﬂyby and sample drop-off
is expected on September 24, 2023.
Once collected from the landing site
in the Utah Test and Training Range,
the sample will be divided: a quarter
will go to the mission's science team
and 4% to partners at the Canadian Space Agency. Another 0.5%
will go to the Japanese Aerospace
Exploration Agency in exchange for
pieces of the asteroid 162173 Ryugu,
which JAXA's Hayabusa 2 mission
will drop off during an Earth ﬂyby
in December 2020 (S&T: June 2019,
p. 10). The remainder of the sample
will be preserved, available to future
researchers worldwide as investigative technologies advance.
DAVID DICKINSON

FEBRUARY 2 021 * SK Y & TELESCOPE

Bennu's Surface Surprises

Read more about ﬁndings on Bennu's
surface at https://is.gd/BennuVesta.

COSMOLOGY

Astronomers Chart History
of Star Formation
A HISTORICAL RECORD of molecular
hydrogen gas - the stuff that makes
stars - provides astronomers with a
sweeping survey of star formation, as
well as a prediction for when the universe's stars will go dark.
Astronomers have already charted
the rise and fall of galaxies' star formation over time, revealing a stellar baby
boom that peaked about 10 billion
years ago; rates have been decreasing
ever since. To explore these trends,
astronomers went a step earlier in the
process, tracking not just the stars born
but the material used to make them.

N ASA / GODDA RD / UNIV ERSIT Y OF A RIZON A

ASTEROIDS

https://www.is.gd/BennuVesta

Sky & Telescope - February 2021

Table of Contents for the Digital Edition of Sky & Telescope - February 2021