WEBVTT FILE

1
00:00:00.020 --> 00:00:04.060
Bell Tone

2
00:00:04.080 --> 00:00:08.190
Bell Tone

3
00:00:08.210 --> 00:00:12.340
Narrator: All the comets that we can see from Earth are orbiting the sun, but some belong to

4
00:00:12.360 --> 00:00:16.520
a special group called sungrazing comets. Sungrazers are comets that

5
00:00:16.540 --> 00:00:20.560
come very close to the sun at their nearest approach, a point called perihelion.

6
00:00:20.580 --> 00:00:24.690
To be considered a sungrazer, a comet needs to get within about 850,000

7
00:00:24.710 --> 00:00:28.860
miles from the sun at perihelion. Many come even closer,

8
00:00:28.880 --> 00:00:32.940
even to within a few thousand miles. Being so close to the sun is very hard

9
00:00:32.960 --> 00:00:37.100
on comets for many reasons. They are subjected to a lot of solar radiation

10
00:00:37.120 --> 00:00:41.280
which boils off their water or other volatiles. The physical push

11
00:00:41.300 --> 00:00:45.450
of the radiation and the solar wind also helps form the tails. And as they get closer

12
00:00:45.470 --> 00:00:49.620
to the sun, the comets experience extremely strong tidal forces, or

13
00:00:49.640 --> 00:00:53.710
gravitational stress. In this hostile environment, many

14
00:00:53.730 --> 00:00:57.870
sungrazers do not survive their trip around the sun. They don't actually crash into the solar

15
00:00:57.890 --> 00:01:01.890
surface, but the sun destroys them anyway. Many sungrazing

16
00:01:01.910 --> 00:01:05.970
comets follow a similar orbit, called the Kreutz Path, and collectively belong to

17
00:01:05.990 --> 00:01:10.000
a population called the Kreutz Group. In fact, close to

18
00:01:10.020 --> 00:01:14.030
85% of the sungrazers seen by the SOHO satellite are on this orbital highway.

19
00:01:14.050 --> 00:01:18.140
Scientists think one extremely large sungrazing comet broke up

20
00:01:18.160 --> 00:01:22.250
hundreds, or even thousands, of years ago, and the current comets on the Kreutz Path

21
00:01:22.270 --> 00:01:26.370
are the leftover fragments of it. As clumps of remnants make their way back around

22
00:01:26.390 --> 00:01:30.560
the sun, we experience a sharp increase in sungrazing comets, which appears

23
00:01:30.580 --> 00:01:34.590
to be going on now. Comet Lovejoy, which reached

24
00:01:34.610 --> 00:01:38.630
perihelion on December 15, 2011 is the best known recent

25
00:01:38.650 --> 00:01:42.820
Kreutz-group sungrazer. And so far, it is the only one that NASA's

26
00:01:42.840 --> 00:01:46.870
solar-observing fleet has seen survive its trip around the sun. Comet

27
00:01:46.890 --> 00:01:50.990
ISON, an upcoming sungrazer with perihelion on November 28, 2013,

28
00:01:51.010 --> 00:01:55.180
is not on the Kreutz Path. In fact, ISON's orbit

29
00:01:55.200 --> 00:01:59.220
suggests that it may gain enough momentum to escape the solar system entirely, and never

30
00:01:59.240 --> 00:02:03.370
return. Before it does so, it will pass within about 40

31
00:02:03.390 --> 00:02:07.400
million miles from Earth on December 26th. Assuming it survives its trip

32
00:02:07.420 --> 00:02:11.460
around the sun. All comets are great laboratories for

33
00:02:11.480 --> 00:02:15.580
scientists to learn more about our solar system, but sungrazing comets can also help us learn

34
00:02:15.600 --> 00:02:19.620
about the sun. Their tails of ionized gas illuminate invisible

35
00:02:19.640 --> 00:02:23.690
magnetic fields, so they can act as a tracer, helping scientists observe these normally

36
00:02:23.710 --> 00:02:27.890
unseeable features. Such fields have even ripped off comet tails,

37
00:02:27.910 --> 00:02:32.030
allowing astronomers to watch them blowing in the solar wind. A wind that

38
00:02:32.050 --> 00:02:36.210
abruptly accelerates between one and five million miles from the sun.

39
00:02:36.230 --> 00:02:40.270
Because of this, researchers will be watching ISON, and other sungrazing comets

40
00:02:40.290 --> 00:02:44.360
very closely. And since we are in a period of high sungrazing comet activity,

41
00:02:44.380 --> 00:02:48.400
scientists can expect many more chances to watch these beautiful,

42
00:02:48.420 --> 00:02:52.510
natural research satellites in the coming years.

43
00:02:52.530 --> 00:02:56.720
Beeping

44
00:02:56.740 --> 00:03:05.832
Beeping