WEBVTT FILE

1
00:00:01.001 --> 00:00:12.079
[music]

2
00:00:12.079 --> 00:00:15.115
NASA’s OSIRIS-REx spacecraft is
equipped with a suite of

3
00:00:15.115 --> 00:00:18.285
instruments designed to study
its target: a near-earth

4
00:00:18.285 --> 00:00:23.357
asteroid called Bennu. One such
instrument is called REXIS – The

5
00:00:23.357 --> 00:00:27.060
Regolith X-Ray Imaging
Spectrometer. That complex name

6
00:00:27.060 --> 00:00:31.398
matches its complex task. "REXIS
is something completely

7
00:00:31.398 --> 00:00:34.935
different than the rest of the
instruments aboard OSIRIS-REx.

8
00:00:34.935 --> 00:00:38.939
And that’s because REXIS will
detect elements – the individual

9
00:00:38.939 --> 00:00:42.743
atomic elements that make up
this asteroid. Everything else

10
00:00:42.743 --> 00:00:47.314
is mapping the geology or the
interior or even the mineral

11
00:00:47.314 --> 00:00:51.852
structure, and we are at the
most fundamental level looking

12
00:00:51.852 --> 00:00:55.589
at what this asteroid is made
out of." REXIS works off of how

13
00:00:55.589 --> 00:00:58.725
the Sun’s X-ray light interacts
with the surface material

14
00:00:58.725 --> 00:01:02.729
covering Bennu. Atoms on the
asteroid absorb these X-rays,

15
00:01:02.729 --> 00:01:06.066
causing their electron levels to
temporarily change and emit

16
00:01:06.066 --> 00:01:09.436
their own X-rays. These
re-emitted X-rays have a

17
00:01:09.436 --> 00:01:12.239
specific energy that tells us
about the atom that they came

18
00:01:12.239 --> 00:01:16.376
from. This process is called
fluorescence, but you can think

19
00:01:16.376 --> 00:01:20.514
of it like a glow. REXIS is a
telescope that images that X-ray

20
00:01:20.514 --> 00:01:23.583
glow which allows scientists to
create a map of the different

21
00:01:23.583 --> 00:01:26.653
elements present on Bennu’s
surface. And REXIS has the

22
00:01:26.653 --> 00:01:30.424
ability to do all this based on
its design and its position on

23
00:01:30.424 --> 00:01:34.594
the spacecraft. "REXIS is about
the size of a shoebox. It’s

24
00:01:34.594 --> 00:01:37.731
located on the main instrument
deck of the OSIRIS-REx

25
00:01:37.731 --> 00:01:40.634
spacecraft – basically this is
the side that’s always facing

26
00:01:40.634 --> 00:01:44.104
the asteroid." "The pieces of
the REXIS instrument are
composed

27
00:01:44.104 --> 00:01:48.909
of two primary parts. We have
the main REXIS telescope, which

28
00:01:48.909 --> 00:01:53.380
views the asteroid Bennu – it’s
consisting of four CCDs that

29
00:01:53.380 --> 00:01:57.250
look through a coded aperture
mask. The coded aperture mask is

30
00:01:57.250 --> 00:02:01.121
just a stainless steel plate
with a number of holes that are

31
00:02:01.121 --> 00:02:04.458
known to us. The second part of
the X-ray instrument is called

32
00:02:04.458 --> 00:02:09.196
the Solar X-ray Monitor, or the
SXM. The SXM is viewing the Sun

33
00:02:09.196 --> 00:02:12.165
at all times so that we can
monitor the X-ray spectrum, the

34
00:02:12.165 --> 00:02:15.869
solar x-ray spectrum incident on
the surface of the asteroid."

35
00:02:15.869 --> 00:02:20.040
"The Solar X-ray Monitor let’s
us calibrate out the day-to-day,

36
00:02:20.040 --> 00:02:23.410
even the hour-to-hour,
variations in the X-ray flux

37
00:02:23.410 --> 00:02:27.647
that comes from the Sun." So
like your digital camera or

38
00:02:27.647 --> 00:02:31.218
camera phone, the main
spectrometer uses a light
sensor, known as a

39
00:02:31.218 --> 00:02:35.455
CCD or charge-coupled device. In
this case there are four of

40
00:02:35.455 --> 00:02:39.693
them, and they’re only sensitive
to X-ray light. Before the X-ray

41
00:02:39.693 --> 00:02:43.530
photons from Bennu are detected
by the CCDs, however, they pass

42
00:02:43.530 --> 00:02:46.133
through a mask with a random
pattern of open and closed

43
00:02:46.133 --> 00:02:50.370
holes. By analyzing how the
shadow of the mask pattern is

44
00:02:50.370 --> 00:02:53.907
shifted on the CCDs, scientists
can determine the location of

45
00:02:53.907 --> 00:02:57.477
the X-rays coming from the
asteroid’s surface. This is how

46
00:02:57.477 --> 00:03:01.281
REXIS images Bennu. And one
surprising aspect of the REXIS

47
00:03:01.281 --> 00:03:05.318
instrument is the team that
built it: students from MIT and

48
00:03:05.318 --> 00:03:08.188
Harvard. "The students are
fantastic to work with, they’re

49
00:03:08.188 --> 00:03:11.258
so eager to learn. It’s been
great sort of watching them as

50
00:03:11.258 --> 00:03:14.294
they grow in this experience.
And I think they’ve come with

51
00:03:14.294 --> 00:03:17.364
some interesting ideas and some
ways of looking at it that we

52
00:03:17.364 --> 00:03:20.400
may not have had had we been
with someone who’s been doing

53
00:03:20.400 --> 00:03:23.370
this for, you know, years and
years and years." "Other
missions

54
00:03:23.370 --> 00:03:26.973
have had x-ray spectrometers
onboard, but not nearly the

55
00:03:26.973 --> 00:03:30.077
level of sensitivity and
sophistication of REXIS. And

56
00:03:30.077 --> 00:03:34.181
this really is a new generation,
a next-generation in X-ray

57
00:03:34.181 --> 00:03:37.918
imaging for planetary science,
and it’s all being done by

58
00:03:37.918 --> 00:03:39.553
students.

59
00:03:42.022 --> 00:03:49.196
[beeping]