Sky & Telescope - January 2022 - 17

ralo s
Only about a third of Centaurs will
make it through the entire giant planet
region and past Jupiter to become
JFCs in the inner solar system.
many of the objects in today's Kuiper Belt were placed onto
their orbits when the giant planets formed and then migrated
to their current positions (S&T: Mar. 2021, p. 22). A variety of
gravitational effects can change KBOs' orbits. On long timescales,
the chaotic combination of orbital alignments with
Neptune and the gravitational pull from all the giant planets
can dramatically tilt or elongate a KBO's orbit, allowing it
to make closer and closer approaches to Neptune. Smaller
gravitational tugs from the most massive objects embedded
within the Kuiper Belt might even contribute to changes in
neighboring objects' orbits, too. All of these factors slowly
feed KBOs onto Neptune-crossing orbits over many millions
or even billions of years.
Close encounters with Neptune will then scatter these
objects, sending some of them to more distant parts of the
outer solar system and others onto smaller orbits in the Centaur
region. Many Centaurs will interact with just our outer
ice giants, Neptune and Uranus, before being tossed back into
the Kuiper Belt. But about half will continue their journey
inward and become Saturn-crossing Centaurs.
Every time a Centaur encounters a giant planet, its orbit
either gets smaller or larger. The exact outcome depends
on how closely the Centaur approaches the planet and the
direction of that approach. Only about a third of Centaurs
will make it through the entire giant-planet region and past
Jupiter to become JFCs in the inner solar system. The time
it takes an object to either traverse the giant-planet region
or be kicked back out into the Kuiper Belt is typically a few
million years.
Charting the Territory
Given that Centaurs represent a middle state in the continuum
between the outer solar system's Kuiper Belt populations
and the inner solar system's JFC population, it is perhaps
unsurprising that there isn't one single agreed-upon definition
for what a Centaur is. The commonly used definitions
place Centaurs as spending some or all of their time in the
giant-planet region, with their closest approach to the Sun
lying exterior to Jupiter's orbit but interior to Neptune's orbit.
Most definitions also require that their average distance from
the Sun be in the giant-planet region; the strictest require
that the object's orbit lies entirely inside that of Neptune.
Depending on which definition you use, the current
census of observed Centaurs ranges from about 250 to 350
objects. On this list are objects discovered before Chiron,
including 29P/Schwassmann-Wachmann 1, which was discovered
in 1927 and classified as a comet due to its bursts of
activity (S&T: Sept. 2021, p. 57). It wasn't until we had obserof
pl
na
e
wo
e
o
f
s o l a r
s y s t e
486958 Arrokoth
m
2060 Chiron
Neptune
el
Uranus
Neptune
Saturn
Neptune
Comet NEOWISE (C/2020 F3)
ENTRY Many Kuiper Belt objects such as Arrokoth (top) follow paths
minimally tilted with respect to the planets' orbits, as do Centaurs like
Chiron (center). But Oort Cloud comets such as C/2020 F3 (bottom)
often zoom through on highly elongated orbits at steep angles.
vations of more distant Centaurs and KBOs that we were better
able to recognize the Centaurs as a distinct population.
The Centaurs provide a critical link between the nearly
primordial icy bodies in the Kuiper Belt and the active, highly
evolved comets in the inner solar system. Like their namesakes,
Centaurs are hybrid creatures: part active comet, part
frozen planetesimal. They offer us an opportunity to watch
as objects experience their first significant thawing since they
were tossed into outer-solar-system cold storage.
The partly cometary nature of Centaurs became apparent
within a decade of Chiron's discovery. Large variations in its
brightness over time suggested it was venting icy material
from its surface; follow-up observations confirmed the presence
of a coma. We now have many more examples of activity
amongst Centaurs, ranging from relatively quiescent outgassing
to extremely bright outburst events throwing off large
amounts of dust and gas. Approximately 10% of the observed
Centaurs have confirmed cometary behavior - and this is
likely a lower limit, because not all Centaurs are amenable to
a search for activity!
The sublimation of water ice largely drives activity on comets
in the inner solar system, but for objects beyond Jupiter,
the Sun is too far away to heat up the water ice they contain.
Other ices that sublimate at lower temperatures, such as carbon
dioxide and carbon monoxide, instead must contribute to
Centaur activity - although some of the observed activity is
sk yand tele scope .o r g * JANUARY 2022 17
m
ets
y s
v
B
e
p l a n
A
b
o
GREGG DINDERMAN / S&T
https://skyandtelescope.org/

Sky & Telescope - January 2022

Table of Contents for the Digital Edition of Sky & Telescope - January 2022

Contents
Sky & Telescope - January 2022 - Cover1
Sky & Telescope - January 2022 - Cover2
Sky & Telescope - January 2022 - 1
Sky & Telescope - January 2022 - Contents
Sky & Telescope - January 2022 - 3
Sky & Telescope - January 2022 - 4
Sky & Telescope - January 2022 - 5
Sky & Telescope - January 2022 - 6
Sky & Telescope - January 2022 - 7
Sky & Telescope - January 2022 - 8
Sky & Telescope - January 2022 - 9
Sky & Telescope - January 2022 - 10
Sky & Telescope - January 2022 - 11
Sky & Telescope - January 2022 - 12
Sky & Telescope - January 2022 - 13
Sky & Telescope - January 2022 - 14
Sky & Telescope - January 2022 - 15
Sky & Telescope - January 2022 - 16
Sky & Telescope - January 2022 - 17
Sky & Telescope - January 2022 - 18
Sky & Telescope - January 2022 - 19
Sky & Telescope - January 2022 - 20
Sky & Telescope - January 2022 - 21
Sky & Telescope - January 2022 - 22
Sky & Telescope - January 2022 - 23
Sky & Telescope - January 2022 - 24
Sky & Telescope - January 2022 - 25
Sky & Telescope - January 2022 - 26
Sky & Telescope - January 2022 - 27
Sky & Telescope - January 2022 - 28
Sky & Telescope - January 2022 - 29
Sky & Telescope - January 2022 - 30
Sky & Telescope - January 2022 - 31
Sky & Telescope - January 2022 - 32
Sky & Telescope - January 2022 - 33
Sky & Telescope - January 2022 - 34
Sky & Telescope - January 2022 - 35
Sky & Telescope - January 2022 - 36
Sky & Telescope - January 2022 - 37
Sky & Telescope - January 2022 - 38
Sky & Telescope - January 2022 - 39
Sky & Telescope - January 2022 - 40
Sky & Telescope - January 2022 - 41
Sky & Telescope - January 2022 - 42
Sky & Telescope - January 2022 - 43
Sky & Telescope - January 2022 - 44
Sky & Telescope - January 2022 - 45
Sky & Telescope - January 2022 - 46
Sky & Telescope - January 2022 - 47
Sky & Telescope - January 2022 - 48
Sky & Telescope - January 2022 - 49
Sky & Telescope - January 2022 - 50
Sky & Telescope - January 2022 - 51
Sky & Telescope - January 2022 - 52
Sky & Telescope - January 2022 - 53
Sky & Telescope - January 2022 - 54
Sky & Telescope - January 2022 - 55
Sky & Telescope - January 2022 - 56
Sky & Telescope - January 2022 - 57
Sky & Telescope - January 2022 - 58
Sky & Telescope - January 2022 - 59
Sky & Telescope - January 2022 - 60
Sky & Telescope - January 2022 - 61
Sky & Telescope - January 2022 - 62
Sky & Telescope - January 2022 - 63
Sky & Telescope - January 2022 - 64
Sky & Telescope - January 2022 - 65
Sky & Telescope - January 2022 - 66
Sky & Telescope - January 2022 - 67
Sky & Telescope - January 2022 - 68
Sky & Telescope - January 2022 - 69
Sky & Telescope - January 2022 - 70
Sky & Telescope - January 2022 - 71
Sky & Telescope - January 2022 - 72
Sky & Telescope - January 2022 - 73
Sky & Telescope - January 2022 - 74
Sky & Telescope - January 2022 - 75
Sky & Telescope - January 2022 - 76
Sky & Telescope - January 2022 - 77
Sky & Telescope - January 2022 - 78
Sky & Telescope - January 2022 - 79
Sky & Telescope - January 2022 - 80
Sky & Telescope - January 2022 - 81
Sky & Telescope - January 2022 - 82
Sky & Telescope - January 2022 - 83
Sky & Telescope - January 2022 - 84
Sky & Telescope - January 2022 - Cover3
Sky & Telescope - January 2022 - Cover4
Sky & Telescope - January 2022 - SA1
Sky & Telescope - January 2022 - SA2
Sky & Telescope - January 2022 - SA3
Sky & Telescope - January 2022 - SA4
Sky & Telescope - January 2022 - SA5
Sky & Telescope - January 2022 - SA6
Sky & Telescope - January 2022 - SA7
Sky & Telescope - January 2022 - SA8
Sky & Telescope - January 2022 - SA9
https://www.nxtbook.com/nxtbooks/aas/st_202207
https://www.nxtbook.com/nxtbooks/aas/st_202206
https://www.nxtbook.com/nxtbooks/aas/st_202205
https://www.nxtbook.com/nxtbooks/aas/st_202204
https://www.nxtbook.com/nxtbooks/aas/st_202203
https://www.nxtbook.com/nxtbooks/aas/st_202202
https://www.nxtbook.com/nxtbooks/aas/st_202201
https://www.nxtbook.com/nxtbooks/aas/st_202112
https://www.nxtbook.com/nxtbooks/aas/st_202111
https://www.nxtbook.com/nxtbooks/aas/st_202110
https://www.nxtbook.com/nxtbooks/aas/st_202109
https://www.nxtbook.com/nxtbooks/aas/st_202108
https://www.nxtbook.com/nxtbooks/aas/st_202107
https://www.nxtbook.com/nxtbooks/aas/st_202106
https://www.nxtbook.com/nxtbooks/aas/st_202105
https://www.nxtbook.com/nxtbooks/aas/st_202104
https://www.nxtbook.com/nxtbooks/aas/st_202103
https://www.nxtbook.com/nxtbooks/aas/st_202102
https://www.nxtbook.com/nxtbooks/aas/st_202101
https://www.nxtbook.com/nxtbooks/aas/skywatch_2021
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com