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Arushi Nath. Grade 8 Student.

Observation Updates:

25 September 2022

24 September 2022

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28 August 2022

Didymos Orbit Path:

Here is the actual path being traced by asteroid Didymos across the night sky based on 20 observations taken. (Asteroid is passing at the centre of image)

Didymos Light Curve:

A light curve is the measurement of a celestial body’s brightness at certain intervals and over a given period of time (hours, days, months…). Asteroids shine due to the Sun’s light reflecting off their surface, and their brightness might vary due to one or more of the following factors: The asteroid’s distance to us is changing (closer objects appear brighter). The asteroid’s phase, just like the Moon’s, is changing as it orbits around the Sun (the larger the area of the asteroid that is illuminated, as seen from Earth, the brighter it will appear). The asteroid, because of its irregular shape, reflects light differently as it spins. If the orbit of an asteroid is well known, the first two effects can be numerically calculated and their contributions removed from the measured light curve. We are then left with a light curve whose changes are due solely to the spinning of the asteroid. https://www.nasa.gov/content/asteroid-grand-challenge/characterize/light-curve-analysis

Based on differential photometry, the light curve of asteroid based on 20 observations on 28 August 2022 has been graphed below:

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The Double Asteroid Redirection Test (DART) mission is NASA’s full-scale planetary defense test mission to defend Earth against potentially hazardous asteroids or comets on collision-path with Earth. The DART mission is led by NASA Johns Hopkins University Applied Physics Laboratory while NASA’s Planetary Defense Coordination Office is the lead for planetary defense activities.

This mission interests me a lot as it relates to near earth asteroids. Earlier in 2022, my project. “Strengthening Planetary Defense: Detecting Unknown Asteroids using Open Data, Math, and Python” won the Best Project Award (Innovation) at the 2022 Canada Wide Science Fair 2022 along with Gold Medal, Best in Category Award, Youth Can Innovate Award, and the Excellence in Astronomy Award from the Royal Astronomical Society of Canada.

The Binary Asteroid: Didymos-Dimorphos System

The destination of the DART mission is the Didymos (65803) – Dimorphos asteroid system that is not a threat to Earth. Didymos system is an eclipsing binary S-type Near Earth Object (NEO) and a member of the Amor group of asteroids.

Dimorphos is the moonlet of Didymos (diameter around 160m) and orbits Didymos, (diameter around 780m). As the system orbits the Sun, the asteroid pair comes close to Earth’s orbit. In 2003, it passed only 0.048 AU from Earth. At its farthest, when Didymos is on the opposite side of the Sun from Earth, a bit beyond the orbit of Mars, it is about 3 AU away. Didymos spins rapidly – rotating about once every 2.26 hours. The moonlet revolves around Didymos about once every 11.9 hours. The distance between the pair is about 1 km.

The DART Mission

DART is a kinetic impactor mission. The mission was launched on 24 November 2021 and will impact moonlet Dimorphos on 26 September 2022 . The spacecraft will act as a fast-moving projective and intentionally impact the moonlet Dimorphos on 26 September 2022 at a speed of 6 km/s. The goal is to slightly change the orbit of the moonlet through momentum transfer. The orbital period of Didymos would change and this and ground-based telescopes would be used to measure this change.

The observations will demonstrate the effectiveness of kinetic impact missions and test technologies to modify the trajectory of an asteroid should one threaten Earth in the future It will also yield information about physical properties of Didymos and Dimorphos, the size of crater made by the DART mission, and the momentum transfer efficiency resulting from DART’s impact.

The DART Payload

DART has only one onboard instrument: DRACO or the Didymos Reconnaissance and Asteroid Camera for Optical navigation. It is a high-resolution imager to help DART navigate to the Didymos system. In the final minutes before the impact, DRACO will stream images back to Earth at the rate of one per second, enabling the DART team to measure the moonlet’s size and shape to determine the impact site.

LICIACube, a CubeSat riding with DART provided by the Italian Space Agency (ASI), will be released, prior to DART’s impact to capture images of the impact and the resulting cloud of ejected matter.

Roughly four years after DART’s impact, ESA’s (European Space Agency) Hera Mission project will conduct detailed surveys of both asteroids, with particular focus on the crater left by DART’s collision and a precise determination of Dimorphos’ mass.

Post-Collision Expectations

Didymos is an eclipsing binary system. It means Didymos and its moonlet Dimorphos move in an orbit so placed in space in relation to Earth that the light reflected from one of them can at times be hidden behind the other (occultation). This property allows astronomers to accurately measure small period changes by ground-based optical light curve measurements.

As Dimorphos orbits Didymos at much a slower relative speed than the pair orbits the Sun, the result of DART’s kinetic impact within the binary system can be measured more easily than a change in the orbit of a the asteroid system around the Sun (heliocentric orbit). Scientists think the collision will change the speed of Dimorphos by a fraction of one percent. It should alter the moonlet’s orbital period around the larger asteroid by several minutes – enough to be observed and measured by telescopes on Earth.

An international campaign coordinated by Northern Arizona University’s Cristina Thomas – DART’s Observations Working Group Lead – uses powerful Earth-based telescopes to study the asteroid system.

My Research Goal

To measure changes in the Didymos asteroid system after the kinetic impact to its moonlet Dimorphos, namely in terms of brightness of the asteroid and the orbital period of Dimorphos. To do so, I will take measurements of the system before the collision and analyse changes that could be explained by a successful kinetic impact on Dimorphos.

I plan to create light curves of the system and carry out measurements. The light curves are created using photometry or measuring changes in the apparent brigthness of the asteroid. I plan to combine writing my own algorthms and using existing software to carry out photometric measurements and analysis. Through these light curves I hope to assess the changes in rotational period of Dimorphos.

The success of DART Mission lies in coordinated and joint efforts of global community of astronomers to make observations, prior to, during, and after the impact period. My research would be a small contribution to these efforts.

Analysing Changes with Phase Angle

Asteroids orbit around the Sun. As they do not have their own light, we see them because of sunlight reflected from their surface. The phase angle is the angle between the Sun, the asteroid, and the observer. The asteroid would appear the brightest when phase angle is zero and its apparent brightness decreases as phase angle increases.

Thus observing the asteroid over different phase angles, one could get more information about its surface, including albedo (reflectivity), composition, grain size, changes in its orbit, and even if it has been bombarded by any projectile!

In case of Didymos, analysing changes in its light curves with phase angle could reveal impact of the DART Mission.

Light Curve Analysis
There are three components in the light curve of a binary asteroid:

1. Primary rotation light curve
2. Secondary rotation light curve
3. Mutual event (orbital) light curve

The primary rotation light curve is always apparent (with observations of sufficient accuracy), while the secondary rotation light curve may or may not be resolved depending on the secondary-to-primary size ratio, elongation of the secondary, and accuracy of the photometric observations. When the binary asteroid is in a mutual occultation or eclipse geometry, i.e., when Earth or the Sun, respectively, is close to the mutual orbit plane of the two bodies, then there are superimposed brightness attenuations due to the occultations or eclipses (collectively called “mutual events”) that occur between the two bodies as they orbit one another. Didymos system will be over 60x brighter near impact than it was in 2020-2021 observations. https://aas.org/sites/default/files/2021-10/DPS53_Tues_CristinaThomas.pdf

What to observe and annotate in the Light Curve?

Primary Occultation – The secondary passing in front of the Primary (obscuring a part of it) for the observer.
Primary Eclipse – The secondary casting a shadow on the Primary.
Secondary Occultation – The primary passing in front of the Secondary for the observer. https://aas.org/sites/default/files/2021-10/DPS53_Tues_CristinaThomas.pdf

DataSets:

Asteroid Terrestrial-impact Last Alert System (ATLAS): All-sky survey aimed at detecting potentially hazardous near-Earth asteroids and undertaking their astrometry (to obtain precise position and orbital parameters). The survey also produces photometric measurements of asteroids to get information about their spinning rate, shape and composition.

Asteroid Light Curve Photometry Database (ALCDEF): Stores raw asteroid time-series photometry. More than 32 million observations for over 180 thousand distinct asteroids are currently available in the database. https://alcdef.org/php/alcdef_GenerateALCDEFPage.php

Asteroid Lightcurve Data Base (LCDB): The database contains photometry results such as asteroid spin axis rates, sizes, pole orientations, and/or taxonomic class for 34967 targets . Each object has one to several dozen detailed records that contain results obtained by reviewing the literature. https://sbn.psi.edu/pds/resource/lc.html

Existing Light Curves Dataset of Asteroid DIdymos: Available from 2003 – 2019 and December 2020 to March 2021. In 2003-2004, Didymos had a close approach to Earth at 0.05 AU. During this period, a number of ground-based observations were carrried out using optical and radio telescopes. https://iopscience.iop.org/article/10.3847/PSJ/ac7be1#psjac7be1f2

Software Used:

PlanetOrb, Tycho, Astrometry.net, Astrometrica, MPO Canopus/PhotoRed (for eclipsing binaries), Photometry Pipeline Didymos Test Targets

Community:

Minor Planet Mailing List

Planning and Testing
Generating light curves for an asteroid require long hours of observations (3- 6 hours) using remote and robotic telescopes. This means careful advance planning. Many factors play a role in making an observation plan to make use of limited (and expensive) telescope time, It is one of the most difficult (and uncertain) part of the observations.

Magnitude of the asteroid (what telescope aperture would be suitable and the exposure time), Current RA and Dec (which location it would be visible, from), Elevation above the horizon (at what time to observe), angular speed/non-sidereal rates of motion (what should be the field of view), celestial background brightness (which filter to use to keep signal to noise ratio high). In addition, there are many other conditions including lunar cycle (illumination of the moon), lunar elongation (angular distance from the moon), galactic latitude, weather (cloudy or not), telescope availability, and finally telescope use credits available.

Thus a lot of test runs need to be made to ensure that long hours of observations yield the results aimed for. The DART Mission also provides a set of test targets for astronomers to practice working under analogous conditions they may find during the observation cycles.

My Observations from Robotic Telescopes

I am taking several observations of Didymos using the 2-meter Faulkes Telescope South located at the LCO Siding Spring node in Australia to take my observations. I am particularly interested to see if there will be any changes in the rotation and brightness of the Didymos asteroid before and after the DART missions

Here are some of my current observations of DIdymos:

TABLE: List of observations for Didymos

DART Mission Requirements

For data to be included in analyses and publications undertaken by the DART Mission, all data must meet its nominal photometric guidelines.

Pre Impact

-Pre-impact observations will start in late June/early July 2022 and continue until impact. These will primarily be from southern hemisphere telescopes.

• Pre-Impact L1 (June 22 – July 6) V= 19.5 – 18.8
• Pre-Impact L2 (July 21 – August 8, extended) V= 17.9 – 16.7
• Pre-Impact L3 (August 20 – September 3) V= 16.0 – 15.2
• Pre-Impact L4 (September 18 – Impact) V= 14.6 – 14.5

Post Impact

-Post-impact observations will start (primarily) once the ejecta clears (approx. a few days to a week after impact, see later section). Some observers may choose to observe through the ejecta, but there the data will likely not be of sufficient quality to contribute to the orbital determination. (Although other interesting science may result.)

Data Accuracy

For investigations of the orbital period (main goal of the observing effort), RMS < 0.01 mag. For investigations of the secondary, RMS < 0.005 mag. 

This RMS value refers to consistency over the nightly run and will put a floor on the SNR of individual exposures. This can be challenging for several lunations when Didymos is moving quickly and in October when we are using brighter time and Didymos crosses the galactic plane. 

Tracking

The choice of how to track Didymos will be connected to your reduction methods and the resulting RMS. Options include sidereal tracking, full rate non-sidereal tracking, and half rate non-sidereal tracking. If necessary, test the method before the Didymos observations. 

Cadence

Assuming no loss in data points, we are targeting 20 frames per hour. This suggests a cadence between the start of subsequent exposures of ≤ 3 minutes.

Time CoverageIn order to determine precise event timings and shapes on a given epoch, we will need data with sufficient time coverage taken over a relatively short interval (typically within one or a few adjacent nights). A minimum of 6 hours of lightcurve coverage is required. An observer not able to get sufficient coverage should coordinate their observations with another observer/telescope so that the combined data from the two stations provide the needed coverage. Otherwise, the data may not be usable for lightcurve decomposition.

Filter

Under the assumption that there is no surface color variation, it does not strongly matter which filter is used. Select the filter to best control the systematics and optimize the resulting signal-to-noise from your telescope. Many past observations have used VR, R, I. For magnitude calibration against field stars, the SDSS ugriz filter set may be preferred as those filters map well to several key catalogs (e.g. PanSTARRS, SkyMapper, Gaia). Generally, observers should consider using red filters (e.g. SDSS r or i) in bright moon conditions, and bluer filters (e.g. SDSS g) at low galactic latitudes. The choice of filters in each of these circumstances will depend on the specifics of your instrument.

Due dates for photometry to be included in: 

27 July 2022: Final pre-impact position

Data due by 10 July 2022

25 October 2022: First observational results to Investigation Team

Data due by 12 October 2022

14 November 2022: Updated Observational Results to Investigation Team

Data due by 1 November 2022

21 December 2022- Updated Observational Results to IT with period change for initial publications

Data due by 8 December 2022

21 April 2023: Final determination of orbital period change

Pre-impact data due by 8 December 2022

Post-impact data due by 29 March 2023

Inputs from DART Mission Team and other Astronomers

It is wonderful to receive encouragement and inputs on citizen science that can be done from data being gathered on DIdymos from the Astronomy community (Nancy Chabot, Michael Busch, Andy Rivkin)

Several links have been suggested including:

Pre-arrival Studies: https://iopscience.iop.org/journal/2632-3338/page/DART_and_Hera_missions

AIDA DART asteroid deflection test: Planetary defense and science objectives https://www.astro.umd.edu/~dcr/reprints/cheng_pss157,104.pdf

Photometric Observations of the Binary Near-Earth Asteroid (65803) Didymos in 2015–2021 Prior to DART Impact https://iopscience.iop.org/article/10.3847/PSJ/ac7be1

Heliocentric Effects of the DART Mission on the (65803) Didymos Binary Asteroid System: https://iopscience.iop.org/article/10.3847/PSJ/ac7de7/pdf

Changing the heliocentric orbit of the Didymos system with DART: https://trs.jpl.nasa.gov/bitstream/handle/2014/54865/CL%2321-1722.pdf?sequence=1

DART Team Confirms Orbit of Targeted Asteroid: https://www.nasa.gov/feature/dart-team-confirms-orbit-of-targeted-asteroid

Resources:

Lightcurve Observations in Support of the DART Mission: Understanding the Orbit of the Didymos-Dimorphos System Cristina Thomas

Useful Information

Impact Date: September 26, 2022

Impact Time: 7:15 p.m. EDT, 23:15 UTC

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